Where life could survive to the present day
Copyright © Robert Walker (UK). "Some rights reserved". First Publishing on Kindle: September 2015.
Cover shows one of the flow like features in Richardson's crater. Though not so well known as the Recursive Slope Lineae, an interesting thing about this habitat is that one of the two leading models for it involves fresh water, trapped under clear ice in a solid state greenhouse effect. If that's right, this may be the only habitat on or near the Mars surface with present day liquid fresh water, and of course, also above 0 °C.
You can also read this online (free) - published August 2013 on my Science20 blog. (Online version is identical except for formatting and table of contents). See my Science20 blog for more posts on Mars and other space related topics.
Originates as a draft article I wrote for wikipedia, in my user space, from January to May 2015, so released under CC by SA. It never got into wikipedia, as they thought that, though well written, it was more suitable for a journal article or essay than wikipedia.
You can also get this on kindle. For my other kindle books, see my author page on Amazon.com.
The present day habitability of Mars is an area of research that has exploded hugely in the last decade, to the extent that it's often hard to keep track of everything that's going on. Here you can read in detail about many new ideas in this rapidly expanding field.
From salty seeps (for the warm seasonal flows) to melted layers of water in clear polar ice (for the flow like features). Or, from ice fumaroles on Mars masking the heat of a volcanic vent, to advancing sand dunes bioreactors churning salts to the surface in equatorial regions, or temporary lakes after asteroid impacts. So many ideas are suggested every year for possible habitats on Mars, it is hard to keep track of them all.
This is an attempt to survey the entire field, as it is in 2015, and I have gone to the original sources to try to cover it in depth. It has many links to the original papers if you want to follow it up in even more detail.
Artist's impression of the Phoenix Lander landing on Mars.
Phoenix's atmospheric measurements of isotope ratios of carbon and oxygen gave evidence for liquid water on the surface now or in the recent geological past. Also its 2008 observations of possible droplets on its legs suggested new ways that water could be stable temporarily on Mars. These observations lead many scientists to reassess the present habitability of Mars
Life would meet many challenges on present day Mars. Liquid water boils at 0C, over much of its surface. Even at the depths of the Hellas basin, any water is close to boiling point and will dry out quickly. Ice also evaporates into the atmosphere over geological timescales - and most of the equatorial regions are thought to be dry to depths of tens of meters. As its axial tilt varies, Mars atmosphere is sometimes thicker, and liquid water may then form on the surface - but any dormant life in the top few meters of soil would be destroyed over periods of millions of years by cosmic radiation.
However, in 2008, droplets were observed on the landing legs of Phoenix, possibly droplets of salty water. Phoenix also made isotopic measurements which show that the Mars atmosphere has exchanged oxygen molecules with liquid on the surface in the recent geological past. This could indicate either recent episodic occurrences of liquid water (for instance after a meteorite strike) or water present every year, in contact with the atmosphere.
Most present day seasonal features on Mars are now thought to be caused by dry ice or wind effects. However, the "recursive slope lineae", and some of the "flow like features" form in conditions that suggest the occasional presence of small quantities of water on Mars. Also recent Mars surface simulations by Nilton Renno and his team have shown that small droplets of water can form on salt / ice interfaces for a few hours per day almost anywhere on the surface of Mars.
In a separate development, research by the German aerospace company DLR in Mars simulation chambers and on the ISS show that some Earth life can survive simulated Mars surface conditions without any water at all, and photosynthesize and metabolize, slowly. It can do this using the high relative humidity of the Mars atmosphere at night.
Other potential habitats include lakes formed in the higher latitudes after cometary or meteorite impacts, or as a result of volcanism. Covered by ice, these may remain liquid for centuries, or up to a few thousand years for the largest impacts. The planet may also have underground trapped layers of water heated by geothermal hotspots. Also there are suggestions that Mars may have a deep hydrosphere, a liquid layer below its cryosphere, a few kilometers below the surface. Deep rock habitats on Earth are inhabited by life so if this layer exists, it may also be habitable on Mars.
The main questions are
Carl Sagan with a model of the Viking Lander in Death Valley California. Viking 1 and II were the first spacecraft to search for present day life on Mars.
The Viking landers (operating on Mars from 1976-1982), are the only spacecraft so far to search directly for life on Mars. They landed in the equatorial regions of Mars. This would be a surprising location to find life, on modern understanding of Mars, as the soil there is thought to be completely ice free to a depth of at least hundred meters, and possibly for a kilometer or more. However some scientists have suggested ways that life could exist even in such arid conditions, using the night time humidity of the atmosphere, and possibly in some way utilizing the frosts that form frequently in the mornings in equatorial regions . .
The Viking results were intriguing, and inconclusive. There has been much debate since then between a small number of scientists who think that the Viking missions did detect life, and the majority of scientists who think that it did not.
The Viking lander had three main biological experiments, but only one of these experiments produced positive results.
The conclusion at the time, for most scientists, was that the Labeled release experiment had to have some non biological explanation involving the unusual chemistry on Mars. The other two experiments seemed to rule out any possibility of a biological explanation.
However, work since then has suggested re-evaluation of those results.
First, some have suggested that the gas chromatograph may not have been sensitive enough to detect the organics. Though other scientists have suggested that they could have detected low levels of organics.
Then in 2002, Joseph Miller, a specialist in circadian rhythms thought he spotted these in the Viking data. He was able to get hold of the original Viking raw data (using printouts kept by Levin's co-researcher Pat Straat) and on re-analysis this seemed to confirm his conclusions.
Another paper published in 2012 uses cluster analysis and suggested once more that they may have detected biological activity.
On the other hand, a paper published in 2013 by Quinn has refined the chemical explanations suggested for the labeled release observations, using radiation damaged perchlorates. By simulating the radiation environment on Mars, he was able to duplicate radioactive 14CO2emission from the sample. 
In short, the findings are intriguing but there is no consensus yet on whether the correct interpretation is biological or chemical. Most scientists favour the chemical explanation.
Until 2008, most scientists thought that there was no possibility of liquid water on Mars for any length of time in the current conditions there. However, in 2008 through to 2009, droplets were observed on the landing legs of Phoenix.
Click for three step animation (online)
Unfortunately, it wasn't equipped to analyse them but the leading theory is that these were droplets of salty water.  They were observed to grow, merge, and then disappear, presumably as a result of falling off the legs.
These may have formed on mixtures of salt and ice that were thrown up onto its legs when it landed. Experiments by Nilton Renno's team in 2014 in Mars simulation chambers show that water can form droplets readily in Mars conditions on the interface between ice and calcium perchlorate salts. The droplets can form within minutes in Mars simulation conditions. This is the easiest way they have found to explain the observations.
The deck of the Phoenix lander, photographed on Mars. The mass spectrometer used to make the atmosphere isotope measurements is at bottom right. Its observations showed that liquid water on the surface of Mars has exchanged oxygen atoms chemically with the carbon dioxide in the atmosphere in the recent geological past. Though it wasn't able to distinguish between water that is present on the surface intermittently (e.g. after a meteorite impact or volcanic eruption) or continuously (e.g. as deliquescing subsurface brines).
Phoenix also made isotopic measurements of the carbon and oxygen atoms in the atmospheric CO2 in the atmosphere. These measurements show that the oxygen has exchanged chemically with some liquid on the surface, probably water, in the recent geological past. This gives indirect but strong evidence that liquid water exists on the surface or has existed, in the very recent geological past.
In detail, first they found that the ratio of isotopes for 13C to 12C in the atmosphere is similar to Earth. Mars should be enriched in 13C because the lighter 12C is lost to space, but isn't. So this shows that the CO2 must be continually replenished. So Mars must be geologically active at least from time to time in the recent geological past.
Then with the oxygen, their findings were the other way around. The CO2 is enriched in 18O compared with the 16O compared with CO2 as emitted from volcanic activity. They can make this deduction using information from meteorites from Mars, one of which was formed as recently as 160 million years ago. This shows that the oxygen in the CO2 in the atmosphere must have reacted chemically with water on the surface in order to take up heavier oxygen-18.
This research wasn't able to determine if this liquid water is episodic (e.g. after a meteorite strike) or continuously present. However their findings suggested that the exchange with the liquid water happened primarily at temperatures near freezing, which may rule out some hypotheses, particularly hydrothermal vent systems, as the primary source for the water.
This image illustrates possible ways methane might be added to Mars' atmosphere (sources) and removed from the atmosphere (sinks). NASA's Curiosity Mars rover has detected fluctuations in methane concentration in the atmosphere, implying both types of activity occur on modern Mars.
Methane was detected in the Mars atmosphere for the first time in 2004. This stimulated follow up measurements, and research into possible biological or geological origins for methane on Mars. ..
If these measurements are valid (they were confirmed by three independent teams at the time), then there must be some source continually producing methane. Methane dissociates in the atmosphere through photochemical reactions - for instance it reacts with hydroxyl ions forming water and CO2 in the presence of sunlight. It can only survive for a few hundred years in the Mars atmosphere.
There are three main hypotheses for sources for the methane
The original remote observations from Earth needed confirmation by close up inspection on Mars. When Curiosity first landed, no methane was detected to the limits of its sensitivity (implying none is present at levels of the order of parts per billion).
However around eight months later, in November 2013, Curiosity detected Methane spikes up to 9 ppb. These spikes were observed in only one measurement (the measurements were taken roughly every month) and then dropped down to 0.7 ppb again. This happened again in early 2014.
This suggests a localized source to the researchers, since there is no mechanism known that could boost the global atmospheric levels of methane so quickly for such a short time. The leading hypothesis therefore is that a plume of methane gas escaped from some location not far from Curiosity and drifted over the rover, where it detected it.
However the nature of that source is currently unknown. It could as easily be due to inorganic sources as due to life.
The ExoMars Trace Gas Orbiter may help to answer this question, as it will be able to detect trace gases such as methane in the Mars atmosphere using techniques that are about a thousand times more sensitive than any previous measurements. It is due for launch in 2016 (it is part of the same mission that will land the first ExoMars static lander technology demo prior to the main 2018 rover mission).
Once it does these measurements, then the hope is that the results would have the resolution necessary to pinpoint the geographical locations of the sources on the ground. This could then be used to target rovers for later surface missions.
One way to distinguish between biogenic and abiogenic sources of methane might be to measure the carbon-12 to carbon-14 ratio. Methanogens produce a gas which is much richer in the lighter carbon-12 than the products of serpentization.
The dry gullies on Mars were first thought by many scientists to be formed by activity of water. Nowadays, it is thought that recent gullies are formed by dry ice processes, but that many of the older dry ice gullies result from the action of water.
The dry ice hypothesis for recent gullies was confirmed, reasonably conclusively, when new sections of gullies were seen to form at temperatures too low for water activity.
The hypothesis that many older gullies (but still geologically recent) were formed by action of water got strong support in January 2015. This research, while continuing to support the conclusion that the new features are formed by CO2 processes at present, suggests that the older gullies may well have been formed by floods of melt water associated with glacial melting of glaciers that form when the Mars axis tilts beyond 30 degrees. This could have happened within the last two million years (between 400,000 and two million years ago).
Sharp-featured recent gullies (blue arrows) and older degraded gullies (gold) in the same location on Mars. These suggest cyclical climate change within the last two million years
Many dark streaks form seasonally on Mars. Most of these are thought to be due to dry ice and wind effects. This image shows an example, probably the result of avalanche slides and not thought to have anything to do with water:
Slope Streaks in Acheron Fossae on Mars - these streaks are thought to be possibly due to avalanches of dark sand flowing down the slope
However a few of the streaks form in conditions that rule out all the usual mechanisms. These are the Warm Seasonal Flows, also known as Recurrent Slope Lineae.
Warm Season Flows on Slope in Horowitz Crater (animated version - online)
The leading hypotheses for these is that they are correlated in some way with the seasonal presence of liquid water - probably salty brines.
Dark Flows in Newton Crater Extending During Summer (animated version)
Warm Season Flows on Slope in Newton Crater (animated)
The dark streaks resemble damp patches, but spectral measurements from orbit don't detect water. One suggestion is that the water re-arranges the sand grains so causing a darkening, for instance by removing fine dust from the surface. The images were all taken in the afternoons, so it's also possible that the water flows in the early morning and that this water has evaporated when the Mars Reconnaissance Orbiter is able to take the images and do spectroscopic imaging. The streaks are also much narrower than the resolution of the spectroscopic imaging from orbit, so water could be missed for that reason also.
Slopes with the streaks are enriched in the more oxidized ferrous and ferric oxides compared with other similar slopes without the streaks, which could be the result of water. The strength of the spectral signatures of the ferrous and ferric oxides also varies according to the season like the streaks themselves. The leading hypothesis for these streaks is that they are caused by water, kept liquid by salts which reduce the freezing point of the water.
Most of them occur at higher latitudes, but in 2013 a few were also discovered in the Valles Marineres area, surprisingly close to the equator. This research turned up 12 new sites within 25 degrees of the equator, each with hundreds, or thousands of streaks. 
Since the temperatures are relatively warm throughout the year at these locations, then without a mechanism for replenishment, any subsurface ice would probably have sublimated long ago. McEwen, from the team who discovered the streaks at this new location, suggested that this may be evidence for water emerging from groundwater deep below the crust. He suggests this may have implications for searches for Martian life.
Quoting from Nature:
"The temperatures there are relatively warm throughout the year, says McEwen, and without a mechanism for replenishment, any subsurface ice would probably already have sublimated.
"He says that this suggests that water may come from groundwater deep in the crust, which could have implications for Martian life: "The subsurface is probably the best place to find present-day life if it exists at all because it is protected from the radiation and temperature extremes," he says. "Maybe some of that water occasionally leaks out onto the surface, where we could see evidence for that subsurface life." 
Upper map shows elevation, lower map shows albedo, and the black squares are confirmed sites of recurrent slope lineae.
"We observe the lineae to be most active in seasons when the slopes often face the sun. Expected peak temperatures suggest that activity may not depend solely on temperature. Although the origin of the recurring slope lineae remains an open question, our observations are consistent with intermittent flow of briny water. Such an origin suggests surprisingly abundant liquid water in some near-surface equatorial regions of Mars".
RSLs occur on very steep slopes. Typically the slope is 33 degrees, i.e. a 1 in 1.54 slope, and a concave slope with the runoff a slightly gentler 27 degrees, or 1 in 1.9626 or about 1 in 2. (see section 5.2 of this paper, "HiRISE Observations of Recurring Slope Lineae (RSL) during Southern Summer on Mars").
We now have evidence of hydrated salts associated with the RSLs. It was reported on September 28, 2015, Martian salt streaks 'painted by liquid water' (BBC). You can click through from that report to the full article in Nature: Ojha, Lujendra, et al. "Spectral evidence for hydrated salts in recurring slope lineae on Mars." Nature Geoscience (2015).
The salts were detected using CRISM, one of the instruments on Mars Reconnaissance Orbiter. It lets us do spectroscopic mapping of Mars. But - it's not got the same resolution as HiRISE, the optical imager.
They found the hydrated salts only towards the end of this cycle when the streaks were at their broadest. In the early spring when the streaks were short and narrow, they were unable to detect hydrated salts - either they weren't present or there in quantities too small for them to detect.
So something is hydrating the salts in these slopes seasonally at the same time as the streaks reach their broadest extent.
HiRISE spots the streaks at widths of 5 meters widest, right down to 30 centimeters - the optical resolution limit. It benefits from Mars's thin atmosphere - because it is so thin, we can spot fine details on the surface rather more easily than on Earth, with comparatively small satellites also.
CRISM however has a best resolution of 18 meters per pixel. So there is no way you can use it to distinguish the composition of the streaks from the composition of the slopes around them. And they didn't do that.
It's got another limitation as well. It can only look at the streaks at around 3 pm Mars time. That is, unfortunately, the time of day when they are likely to be at their driest. We'd dearly like to be able to observe them in the mornings, which is probably when they are actually flowing. Perhaps we might even get the spectral signature of water in that case. But sadly it's not possible.
MRO is in a slowly precessing sun synchronous orbit inclined at 93 degrees (orbital period 1 hr 52 minutes). Each time it crosses the Mars equator on the sunny side, South to North, the time is 3.00 pm, in the local solar time on the surface, and that is true all the year round. (See page 8 of Mars Reconnaissance Orbiter Communications).
So, we can't detect the flow of water directly. But, given that the only models for the RSLs involve flowing water, then detection of hydrated salts at times when the RSLs are at their broadest is strong circumstantial evidence for flowing water.
Alfred McEwan, project leader for HiRISE, says they made a rough estimate, for the amount of water in the streaks only in the Valles Marineres region. They assumed only 5% water, thickness of 10 mm which is about what you need for the material to flow at all - and they came up with 100,000 cubic meters of water flowing. That's a hundred thousand metric tons, so quite a lot of water. But distribtued over the entire year for many streaks, each one individually would be only a thin film flowing in the soil.
It's not yet known if the RSLs are habitable or not. Petchlorate rich brines can be liquid down to -60 °C. The water starts to flow when the surface temperature is between -23 °C to 0 °C. But that doesn't mean the water is at those temperatures, as it could be flowing below the surface. Mars has a steep temperature gradient in the top couple of cms of soil. Even when it is 20 °C at midday, then a couple of centimeters down, you'd hardly notice anything has happened.
Möhlmann originally suggested this process in 2011 as a possible way for liquid water to form on Mars, based on a mechanism that produces liquid water in similar
conditions in Antarctica. As the sunlight hits the ice, it would preferentially warm up any heat absorbing dust grains trapped inside. These grains would then store heat and form water by melting some of the ice, and the water, covered by ice, would be protected from the vacuum conditions of the atmosphere.
This process could melt the ice for a few hours per day in the warmest days of summer, and melt a few mms of ice around each grain. For instance, Losiak, et al, modeled dust grains of basalt (2-200 µm in diameter) if exposed to full sunlight on the surface of the ice on the warmest days in summer, on the Northern polar ice cap, and say this about their model, in 2014: "For example, for solar constant 350 W/m2, emissivity 0.80, grain size 2 um, and thermal conductivity 0.4 W/mK melting lasts for ~300 minutes [5 hours] and result in melting of 6 mm of ice." . They developed this model as a hypothesis to explain presence of extensive deposits of gypsum in the Northern polar ice cap and the dune fields around it, and concluded that, since the atmospheric pressure there is just above the triple point, this mms thin layer of liquid water could persist for a significant period of time there around grains of basalt in the middle of the day in summer.
This process has been been observed in Antarctica. On Mars, there could be enough water to create conditions for physical, chemical, and biological processes..
These intriguing high latitude features are associated with the Martian Geysers. The geysers themselves (if that is what they are) are thought to be results of dry ice turning to gas, and the dark spots and flow like features are thought to be debris from the geysers.
However, later in the year the flow like features extend further down the slopes. The details differ for the two hemispheres. In the Southern hemisphere, all current models for this part of the process involve liquid water. In the northern hemisphere then most of the models also involve water, although the northern hemisphere flow like features form at much lower surface temperatures.
This image shows the flow like features of the southern hemisphere.
(animated version online)
Flow-like features in Dunes on Richardson Crater, Mars. They form around the dark dune spots, in the debris of the hypothesized Martian Geysers. The dark material at the end of the flows moves at between 0.1 and 1.4 m/day in late spring / summer on Mars. All current models for it favour liquid water as a cause. Either interfacial layers, or else layers of water created through the solid state greenhouse effect.
The process starts with the dark dune spots which form in early spring. Here are some examples in Richardson Crater in the Martian southern hemisphere- one of the places where the Flow Like Features (FLFs) have been observed.
These are thought to result from the Martian Geysers.
The idea is that a semi-transparent solid such as dry ice or clear ice acts like a greenhouse to warm up a layer below the surface (the "solid state greenhouse effect"). When this lower layer consists of dry ice, then it turns into gas and as the pressure builds up, eventually escapes to the surface explosively as a Martian Geyser.
The debris from these geysers form the dark spots, and the "flow like features".
Then, as local summer approaches, the flow like features start to extend down the slope. These are small features only a few tens of meters in scale, and grow at a rate of a meter or a few meters per Martian sol through the late Martian spring and summer. This is the part of the process that is thought to be due to liquid water, in nearly all the models proposed for them so far.
A different mechanism is proposed for them in the Northern and in the Southern hemispheres.
The southern hemisphere features grow at a rate of around 1.4 meters per Martian sol.
Flow-like features on Dunes in Richardson Crater, Mars. - detail. This flow moves approximately 39 meters in 26 days between the last two frames in the sequence
All the models for these features, to date, involve some form of water.
Möhlmann uses a solid state greenhouse effect in his model, similarly to the process that forms the geysers, but with translucent ice rather than dry ice as the solid state greenhouse layer.
Blue wall of an Iceberg on Jökulsárlón, Iceland. On the Earth, Blue ice like this forms as a result of air bubbles squeezed out of glacier ice. This has the right optical and thermal properties to act as a solid state greenhouse, trapping a layer of liquid water that forms 0.1 to 1 meters below the surface. In Möhlmann's model, if ice with similar optical and thermal properties forms on Mars, it could form a layer of liquid water centimeters to decimeters thick, which would form 5 - 10 cms below the surface.
In his model, first the ice forms a translucent layer - then as summer approaches, the solid state greenhouse effect raises the temperature of a layer below the surface to 0°C, so melting it. This is a process familiar on the Earth for instance in Antarctica. On Earth, in similar conditions, the surface ice remains frozen, but a layer of liquid water forms from 0.1 to 1 meters below the surface. It forms preferentially in "blue ice". 
On Mars, in his model, the melting layer is 5 to 10 cms below the surface. The liquid water layer starts off millimeters thick in their model, and can develop to be centimeters thick as the season progresses. The effect of the warming is cumulative over successive sols. Once formed, the liquid layer can persist overnight. Subsurface liquid water layers like this can form with surface temperatures as low as -56°C.
If the ice covers a heat absorbing layer at the right depth, the melted layer can form more rapidly, within a single sol, and can evolve to be tens of centimeters in thickness. In their model this starts as fresh water, insulated from the surface conditions by the overlaying ice layers - and then mixes with any salts to produce salty brines which would then flow beyond the edges to form the extending dark edges of the flow like features.
Later in the year, pressure can build up and cause formation of mini water geysers which may possibly explain the "white collars" that form around the flow like features towards the end of the season - in their model this is the result of liquid water erupting in mini water geysers and then freezing as white pure water ice. 
If salt grains are present in the ice, then this gives conditions for brines to form, which would increase the melt volume and the duration of the melting. The brines then flow down the slope and extend the dark patch formed by the debris from the Geyser, so creating the extensions of the flow like features.
They mention a couple of caveats for their model, because the surface conditions on Mars at these locations is unknown. First it requires conditions for bare and optically transparent ice fields on Mars translucent to depths of several centimeters, and it's an open question whether this can happen, but there is nothing to rule it out either. Then, the other open question is whether their assumption of low thermal conductivity of the ice, preventing escape of the heat to the surface, is valid on Mars.  The process works with blue ice on Earth - but we can't say yet what forms the ice actually takes in these Martian conditions.
This solid state greenhouse effect process favours equator facing slopes. Also, somewhat paradoxically, it favours higher latitudes, close to the poles, over lower latitudes, because it needs conditions where surface ice can form on Mars to thicknesses of tens of centimeters. (The examples at Richardson crater are at latitude -72°, longitude 179.4°, so only 18° from the south pole.).
There is no in situ data yet for these locations, of course, to test the hypothesis. Though some of the predictions for their model could be confirmed by satellite observations.
Another model for these southern hemisphere features involves ULI water (undercooled liquid water) which forms as a thin layer over surfaces and can melt at well below the usual melting point of ice. In Mohlmann's sandwich model, then the interfacial water layer forms on the surfaces of solar heated grains in the ice, which then flows together down the slope. Calculations of downward flow of water shows that several litres a day of water could be supplied to the seepage flows in this way.
The idea then is that this ULI water would be the water source for liquid brines which then flow down the surface to form the features.
Seasonal processes in the Northern polar dunes with Flow Like Features. Time differences between the images are 22 days and 12 days. The final picture shows a long feature that formed new between the two images, and its length is 60 meters so it grew at a rate of at least 5 meters per day.
These features form at a much lower temperature than the southern hemisphere flow like features, at -90°C average surface temperature on kilometers scale - though the dark features are expected to be considerably warmer, and the subsurface is also expected to be heated by the solid state greenhouse effect through surface layers of dry ice (similarly to the proposed models for the Martian Geysers.
They progress through a sequence of changes, first wind blown, and then seepage features associated with the dune spots, and then finally, dark seepage features appear all along the dune crest as in this sequence. These images show the growth of the seepage features. 
The flow like features in the northern hemisphere polar ice cap form at average surface temperatures of around 150°K - 180°K, i.e. up to -90°C approximately.
The flows start as wind-blown features but then are followed by seepage features which increase at between 0.3 meters and 7 meters a day. 
"They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3 meters per day and 7 meters per day, first only from the spots, later from all along the dune crest." 
The seepage features first form at overall surface temperatures of 160°K (-110°C), as measured with the low resolution TES data. However this has a resolution of 3 km across track and only 9 km along the track of the observations. Also, much of the area is still covered in dry ice at this point, and it is opaque in the thermal infrared band so the orbital photographs measure the temperature of the surface of the dry ice rather than the small area of the dark spots and streaks.
Then, as with the model for the Martian geysers, shortwave radiation can penetrate translucent CO2 ice layer, and heat the subsurface through the solid state greenhouse effect.
The models suggest that subsurface melt water layers, and interfacial water could form with surface temperatures as low as 180°K (-90°C). Salts in contact with them could then form liquid brines. 
An alternative mechanism for the Northern hemisphere involves dry ice and sand cascading down the slope but most of the models involve liquid brines for the seepage stages of the features. .
For details see the Dark Dune Spots section of Nilton Renno's paper Water and Brines on Mars: Current Evidence and Implications for MSL which also has images of the two types of feature as they progress through the season.
Martian conditions in miniature - In the Mars simulation chamber, DLR researchers recreated the atmospheric composition and pressure, the planet's surface, the temperature cycles and the solar radiation incident on the surface. The activity of polar and alpine lichen was investigated under these conditions.
A series of experiments by DLR (German aerospace company) in Mars simulation chambers and on the ISS show that some Earth life (Lichens and strains of chrooccocidiopsis, a green algae) can survive Mars surface conditions and photosynthesize and metabolize, slowly, in absence of any water at all. They could make use of the humidity of the Mars atmosphere.  Though the absolute humidity is low, the relative humidity at night reaches 100% because of the large day / night swings in atmospheric pressure and temperature.
The lichens studied in these experiments have protection from UV light due to special pigments only found in lichens, such as parietin and antioxidants such as b-carotene in epilithic lichens. This gives them enough protection to tolerate the light levels in conditions of partial shade in the simulation chambers and make use of the light to photosynthesize. Indeed UV protection pigments have been suggested as potential biomarkers to search for on Mars.
An experiment on the ISS as part of Expose-E in 2008-2009 showed that one lichen, Xanthoria elegans, retained a viability of 71% for the algae (photobiont) and 84% for the fungus (mycobiont) after 18 months in the ISS, in Mars surface simulation conditions, and the surviving cells returned to 99% photosynthetic capabilities on return to Earth. This was an experiment without the day night temperature cycles of Mars and the lichens were kept in a desiccated state so it didn't test their ability to survive in niche habitats on Mars. This greatly exceeded the post flight viability of any of the other organisms tested in the experiment. .
Another study in 2014 by German aerospace DLR in a Mars simulation chamber used the lichen Pleopsidium chlorophanum. This lives in the most Mars like environmental conditions on Earth, at up to 2000 meters in Antarctica. It is able to cope with high UV, low temperatures and dryness. It's mainly found in cracks, where just a small amount of scattered light reaches it. This is probably adaptive behaviour to protect it from UV light and desiccation. It remains metabolically active in temperatures down to -20 C, and can absorb small amounts of liquid water in an environment with ice and snow.
When exposed to full UV levels in a 34 day experiment in a Mars simulation chamber at DLR, the fungus component of the lichen Pleopsidium chlorophanum died, and it wasn't clear if the algae component was still photosynthesizing.
However, when partially shaded from the UV light, as for its natural habitats in Antarctica, both fungus and algae survived, and the algae remained photosynthetically active throughout. Also new growth of the lichen was observed. Photosynthetic activity continued to increase for the duration of the experiment, showing that the lichen adapted to the Mars conditions.
Pleopsidium chlorophanum collected at an altitude of 1492 m above sea level at "Black Ridge" in North Victoria Land, Antarctica. This lichen lives at altitudes of up to 2000 meters in Antarctica.
The photograph shows its semi-endolithic growth pattern in Antarctic conditions where it mainly occurs in fissures and cracks - here you can see the granite rock partly covering it.
The same species lichen occurs in warmer climates where it has a different growth habit - it then spreads out to cover flat rock surfaces.
This is remarkable as the fungus is an aerobe, growing in an atmosphere with no appreciable amount of oxygen and 95% CO2. It seems that the algae provides it with enough oxygen to survive. The lichen was grown in Sulfatic Mars Regolith Simulant - igneous rock with composition similar to Mars meteorites, consisting of gabbro and olivine, to which quartz and anhydrous iron oxide hematite (the only thermodynamically stable iron oxide under present day Mars conditions) were added. It also contains gypsum and geothite, and was crushed to simulate the martian regolith. This was an ice free environment. They found that photosynthetic activity was strongly correlated with the beginning and the end of the simulated Martian day. Those are times when atmospheric water vapour could condense on the soil and be absorbed by it, and could probably also form cold brines with the salts in the simulated martian regolith. The pressure used for the experiment was 700 - 800 Pa, above the triple point of pure water at 600 Pa and consistent with the conditions measured by Curiosity in Gale crater. 
The experimenters concluded that it is likely that some lichens and cyanobacteria can adapt to Mars conditions, taking advantage of the night time humidity, and that it is possible that life from early Mars could have adapted to these conditions and still survive today in microniches on the surface. 
In another experiment, by Kristina Zakharova et al, two species of microcolonial fungi – Cryomyces antarcticus and Knufia perforans - and a species of black yeasts–Exophiala jeanselmei were found to adapt and recover metabolic activity during exposure to a simulated Mars environment for 7 days. They depended on the temporary saturation of the atmosphere with water vapour like the lichens. The fungi didn't show any signs of stress reactions (such as creating unusual new proteins).
There Cryomyces antarcticus is an extremophile fungi, one of several from Antarctic dry deserts. Knufia perforans is a fungi from hot arid environments, and Exophiala jeanselmei is a black yeast endolith closely related to human pathogens.
The experimenters concluded that these black fungi can survive in a Mars environment. 
Mars is rich in perchlorates - a discovery made by Phoenix, and later confirmed by Curiosity and by analysis of Martian meteorites on Earth. It now seems that perchlorates probably occur over much of the surface of Mars. This is of especial interest since perchlorates deliquesce more easily than chlorides and at a lower temperature, so they could, potentially, take up water from the atmosphere more readily.
It is not yet clear how they formed. Sulfates, chlorides and nitrates can be made in sufficient quantities by atmospheric processes, but this mechanism doesn't seem sufficient to explain the observed abundances of perchlorates on Mars. .
Though there is little by way of water vapour in the Mars atmosphere, which is also a near vacuum - still it reaches 100% humidity at night due to the low nighttime temperatures. This effect creates the Martian morning frosts, which were observed by Viking in the extremely dry equatorial regions of Mars.
Ice on Mars Utopia Planitia. These frosts formed every morning for about 100 days a year at the Viking location. Scientists believe dust particles in the atmosphere pick up bits of solid water. That combination is not heavy enough to settle to the ground. But carbon dioxide, which makes up 95 percent of the Martian atmosphere, freezes and adheres to the particles and they become heavy enough to sink. Warmed by the Sun, the surface evaporates the carbon dioxide and returns it to the atmosphere, leaving behind the water and dust.The ice seen in this picture, is extremely thin, perhaps no more than one-thousandth of an inch thick. These frosts form due to the 100% night time humidity, which may also make it possible for perchlorate salt mixtures to capture humidity from the atmosphere, and this process could occur almost anywhere on Mars where suitable mixtures of salts exist.
The discovery of perchlorates raises the possibility of thin layers of salty brines that could form a short way below the surface by taking moisture from the atmosphere when the atmosphere is cooler. It's now thought that these could occur almost anywhere on Mars if the right mixtures of salts exist on the surface, even possibly in the hyper-arid equatorial regions. In the process of deliquescence, the humidity is taken directly from the atmosphere. It does not require the presence of ice on or near the surface.
Some microbes on the Earth are able to survive in dry habitats without any ice or water, using only liquid obtained by deliquescence. For instance this happens in salt pillars in the hyper arid core of the Atacama desert. They can do this at a remarkably low relative humidity, presumably making use of deliquescence of the salts.
Perchlorates are poisonous to many lifeforms. However, perchlorates are less hazardous at the low temperatures on Mars, and some Haloarchaea are able to tolerate them in these conditions, and some of them can use them as a source of energy as well. .
These layers are predicted to lie a few cms below the surface, and are likely to be thin films or droplets or patches of liquid brine. So,they probably won't be detected from orbit, at least not directly. Confirmation may have to wait until we can send landers to suitable locations with the capabilities to detect these layers. Some of the layers may form in equatorial regions, and analysis of results from Curiosity in early 2015 has returned indirect evidence for presence of subsurface deliquescing brines in Gale Crater. .
Whether any of these layers are habitable for life will depend on the temperatures and the water activity (how salty the brines are), which in turn depends on conditions and the composition of salts, whether they are mixed with soil, atmospheric conditions, and even the detailed structure of the microhabitats.
The possibility of liquid forming by deliquescence is improved hugely by the process of eutectic mixtures. The name comes from the Greek "ευ" (eu = easy) and "Τήξις" (tecsis = melting)
This is the process by which when you have a mixture of two salts, for example, a mixture of chloride with perchlorate, then the mixture is able to take up water at a lower relative humidity than either of the salts separately.
A similar process also occurs with temperature in place of humidity. A mixture of salts will remain liquid at a lower temperature than either separately. This is the way that Antifreeze works, and is also the mechanism by which salt keeps roads free from ice. See also Freezing-point depression.
The Deliquescing Relative Humidity for a mixture of salts is the humidity needed for the entire mixture to become liquid. This varies depending on the proportion of each salt in the mixture.
The relative proportions of two salts needed to remain liquid with the lowest level of humidity is known as the eutonic point.
Any mixture of two salts, even if the proportions are well away from the eutonic point, can still take up some water vapour at this lowest level of humidity. It will continue to do this until one of the salts is entirely used up to create this optimal mixture. If there is an excess of the other salt, it remains out of solution in the solid phase.
This diagram shows how it works - for a fictitious mixture A and B.
Here DRH = Deliquescing Relative Humidity, ERH = Eutonic Relative Humidity.
E(A+B) is the optimal or Eutectic mixture. And L here refers to the liquid phase. So, to the left we have a mixture of A with E(A+B) and, once it reaches the eutonic point, only part of it is liquid, and some of the salt A will remain in its solid phase. To the right, similarly, some of the salt B remains in its solid phase above the eutonic point.
So as the humidity is increased, for a given A / B mixture, first the lower horizontal line is reached, at which point some of the mixture of salts becomes liquid. This is known as the "eutonic relative humidity" - the point at which any mixture will start to take up some water vapour.
As humidity is raised further, more and more of the mixture becomes liquid. Eventually the upper, curved line is reached - and at that point, the entire mixture will be in its liquid phase.
Similarly if the axis is temperature - then as the temperature is raised, first part of the mixture will go liquid, at a temperature corresponding to the optimal mixture of the salts, and then when the upper curved line is reached, the entire mixture will be liquid.
Because of this eutectic mixture effect, if you add a tiny amount of perchlorates to the less deliquescent chlorides, this is enough to reduce the minimum relative humidity needed to deliquesce to the eutonic relative humidity for the mixture. This is not only lower than the deliquescence relative humidity of the chlorides, it is also lower than the deliquescence relative humidity for the perchlorates as well.
You can also get similar eutectic mixtures of three or more different types of salts. E.g. a mixture of perchlorates, chlorates, sulfates, and chlorides (or nitrates also if present) in the case of Mars, along with cations of sodium, potassium, calcium, and magnesium. So there are many possibilities to consider here.
In addition to this, once the salt mixtures take up water, they lose it less readily, so they can stay liquid even when the humidity is then reduced again below the eutonic point (delayed efflorescence). Similarly for eutectic freezing, they can be supercooled below the temperature where they would normally freeze, and may remain liquid for some time below the eutonic point.
You get a eutectic also for freezing of a single salt, with molar concentrations. If you have a mixture of salt and water then different mixtures will freeze at different temperatures. The eutectic is the optimal mix of water and salt with the lowest freezing temperature. As you freeze a mixture, then no matter what the original concentration, some of it will remain liquid down to the freezing point of the eutectic mixture.
However as you freeze further below that temperature, you may find that the salt continues to remain liquid. The reason for this is that for a salt to come out of solution through nucleation, it has to form a new interface between the crystal surface and the liquid, which requires energy. Once the nucleation starts, then crystallization is rapid, but the nucleation can be delayed often for many hours.
For instance, MgSO4 has a eutectic of -3.6°C but through supercooling can remain liquid for an extra -15.5°C below that. Here is a table of some salts likely to be found on Mars, showing the eutectic temperature for each one (with the molar concentration for the optimal eutectic concentration in brackets) and the amount of supercooling below that temperature that they found with experiments (adapted from table 2 of  - omitted some of the columns).Salt system Eutectic (°C) Amount of supercooling below eutectic (°C)
|MgSO4||-3.6°C (1.72 m)||15.5|
|MgCl2||-33°C (2.84 m)||13.8|
|NaCl||-21.3°C (5.17 m)||6.3|
|NaClO4||-34.3°C (9.2 m)||11.5|
As the salt / liquid solution cools in Mars simulation conditions, then the results can be complicated, because for instance MgSO4 releases heat in an exothermic reaction when it crystallizes. This keeps it liquid for longer than you'd expect. In their experiments, it remained liquid for twelve hours as it gradually cooled below the eutectic temperature before eventually it froze at 15.5 degrees below the eutectic temperature. In simulated Mars conditions you also have to take account of the effect of soil mixed in with the salts. Surprisingly, using Mars analogue soil, this does not reduce the supercooling and can in some cases permit more supercooling. 
With some of the salt solutions, depending on chemical composition, then the supercooling produces a glassy state instead of crystallization, and this could help to protect supercooled microbes from damage.
In experimental studies of salt pillars in the Atacama desert, microbes are able to access liquid at extremely low relative humidities due to micropores in the salt structures. They do this through spontaneous capillary condensation, at relative humidities far lower than the deliquescence point of NaCl of 75%. 
'The Atacama desert hosts the closest analogue of what a real, live Martian might be like', in its salt rock formations.
Micro-environmental data measured simultaneously outside and inside halite pinnacles in the Yungay region (table 2 from )Variable Halite exterior Halite interior
|Mean annual RH, %||34.75||54.74|
|Maximum annual RH, %||74.20||86.10|
|Minimum annual RH, %||2.90||2.20|
The researchers, Wierzchos et al, did detailed studies with scanning electron microscopes. At 75% relative humidity then brine was abundant inside the salt pillars. As the humidity was reduced, even at 30% RH, the cyanobacteria aggregates shrinked due to water loss, but still there were small pockets of brine in the salt pillars.
"Endolithic communities inside halite pinnacles in the Atacama Desert take advantage of the moist conditions that are created by the halite substrate in the absence of rain, fog or dew. The tendency of the halite to condense and retain liquid water is enhanced by the presence of a nano-porous phase with a smooth surface skin, which covers large crystals and fills the larger pore spaces inside the pinnacles... Endolithic microbial communities were observed as intimately associated with this hypothetical nano-porous phase. While halite endoliths must still be adapted to stress conditions inside the pinnacles (i.e. low water activity due to high salinity), these observations show that hygroscopic salts such as halite become oasis for life in extremely dry environments, when all other survival strategies fail.
Our findings have implications for the habitability of extremely dry environments, as they suggest that salts with properties similar to halite could be the preferred habitat for life close to the dry limit on Earth and elsewhere. It is particularly tempting to speculate that the chloride-bearing evaporites recently identified on Mars may have been the last, and therefore most recently inhabited, substrate as this planet transitioned from relatively wet to extremely dry conditions"
Microbes also inhabit Gypsum deposits (CaSO4.2H2O), however Gypsum doesn't deliquesce. Researchers found that the regions of the desert that had microbial colonies within the gypsum correlated with regions with over 60% relative humidity for a significant part of the year. They also found that the microbes imbibed water whenever the humidity increased above 60% and gradually became desiccated when it was below that figure. 
The combination of all these effects means that mixtures of salts, including perchlorates in the mixture, can be liquid at lower temperatures than any of the salts separately, and also take up water from the atmosphere at lower relative humidity, and once liquid, can remain liquid for longer than you would predict if you didn't take account of these effects. And if there are micropores in the salt deposits, any life within them could also take advantage of an internal relative humidity higher than the external humidity of the atmosphere.
On Mars the relative humidity of the atmosphere goes through extremes. It reaches 100% humidity every night in the extreme cold, even in equatorial regions. In the daytime the relative humidity becomes much less, approaching 0%, and any exposed salts would lose their liquid.
The surface temperatures of the top few cms also change enormously from day to night (more stable but lower temperatures are encountered deeper below the surface) and over the entire surface of Mars, temperatures are tens of degrees below freezing every night.
But because of these other effects these liquid layers, may resist efflorescence and remain liquid longer than you'd expect as the air dries out in the daytime, and also stay liquid longer than you'd expect through supercooling as the temperatures plummet at night.
The result is that you could have layers of liquid, on Mars, quite some way below the surface 1 or 2 cms where liquid water in its pure state can form.
So this discovery of perchlorates on Mars has major implications for presence of liquid, and so habitability.
Given the presence of salts, and including perchlorates, widespread over Mars, it would seem that these liquid layers must surely exist, though not yet directly confirmed by observation.
However some of these liquid layers may be too cold for life (some are liquid at temperatures as low as -90C or lower), or too salty (not enough "water activity). The main focus of research here for habitability is to find out whether there are mixtures of salts that can deliquesce on Mars at the right temperature range and with sufficient water activity for life to be able to take advantage of the liquid. The consensus so far is that though many of these would be too cold, or too salty for life, it seems possible that some of these, in optimal conditions, with the right mixture of salts and at the right depth below the surface, may also be habitable for suitable haloarchaea. The lifeforms would need to be perchlorate tolerant, and ideally, able to use it as a source of energy as well..
The conditions for these liquid layers to form may include regions where there is no ice present on the surface such as the arid equatorial regions of Mars. 
Researchers using data from Curiosity in April 2015 have found indirect evidence that liquid brines form through deliquescence of perchlorates in equatorial regions, at various times, both at the surface, and down to depths up to 15 cms below the surface. When it leaves sandy areas, the humidity increases, suggesting that the sand takes up water vapour.
At night, the water activity is high enough for life, but it is too cold, and in the day time it is warm enough but too dry. The authors concluded that the conditions in the Curiosity region were probably beyond the habitability range for replication and metabolism of known terrestrial micro-organisms. 
The idea behind this proposal is that the constantly moving sand dunes of Mars may be able to create a potential environment for life. Raw materials can be replenished, and the chemical disequilibrium needed for life maintained through churning of the sand by the winds.
Advancing Dune in Nili Patera, Mars. Back-and-forth blinking of this two-image animation shows movement of a sand dune on Mars. This discovery shows that entire dunes as thick as 200 feet (61 meters) are moving as coherent units across the Martian landscape. The sand dunes move at about the same flux (volume per time) dunes in Antarctica. This was unexpected because of the thin air and the winds which are weaker than Earth winds. It may be due to "saltation" - ballistic movement of sand grains which travel further in the weaker Mars gravity.
The sources of carbon would come from space - it's supplied at a steady rate of 5 nanograms per square meter per sol from micrometeorites. At the equator it has a mean lifetime of 300 years - but lasts longer if buried.
On the leeward side of transgressing dunes, then the sand can be buried at the rate of centimeters per year. Since the UV light only penetrates the top centimeter of the soil, then the interplanetary carbon would be buried, beyond reach of UV, within a year.
Additionally, if there was photosynthetic life or similar in the sand dunes, this could fix CO2 from the atmosphere as an additional source (there is of course no evidence for this yet).
As for water, then their idea is that the frost that forms in the morning in the equatorial regions would also occur below the surface (is no reason for it to be confined to the surface). Then, in presence of salts, the day / night temperature cycles could force this water to migrate downwards and form potentially habitable layers of brine a few centimeters below the surface.
They suggest for instance, a eutectic mixture of Mg(ClO4)2 and Ca(ClO4)2 brines which have eutectics of -71°C and -77°C. This is well below the lowest known temperatures for growth for terrestrial microbes, of -20°C, but growth at lower temperatures may be possible on Mars so long as liquid is present.
Ferrous iron cold be the electron donor. And ferric iron or perchlorate could be the oxidant - electron acceptor.
The main nutrients (N, P, S) and trace nutrients (Mg, Ca, K, Fe, etc.) are all readily available with exception of N. They suggest that the dunes could have reduced nitrogen produced from the atmospheric N2 catalyzed by iron oxides in presence of UV radiation.
This is of special interest as a potential habitat that is accessible by MSL and other equatorial region rovers, as it doesn't require presence of surface ice.
In summary, their conclusion is that if MSL detects organic carbon, and reduced nitrogen compounds (which it has now done) then these sand dunes could be potential microbial habitats on present day Mars:
"Advancing martian dunes mix oxidants, reductants, water, nutrients, and possibly organic carbon in what could be considered bioreactors. Thus, martian dunes function as small scale analogues of the global geological cycles that are important in maintaining Earth's habitability. On Mars, carbon can be cycled from the surface of the dune to its subsurface where it may come in contact with moisture and oxidants. Compounds oxidized at the surface of dunes by UV radiation and oxygen are buried on the lee side of dunes and mixed with reductants, carbon, and ephemeral brines. In addition, reduced compounds will be exposed at the surface on the windward side of dunes where they can be oxidized and complete the cycle. ... Additional measurements by MSL such as detecting organic carbon and reduced nitrogen compounds would support the hypothesis that moving dunes are potential microbial habitats. The absence of these compounds would indicate that the today's dunes are unlikely to be habitable." 
This is the result of a research team led by Nilton Renno, professor of atmospheric, oceanic and space sciences at Michigan University. He is also project scientist for Curiosity in charge of the REMS weather station on Mars, was also a scientist on the Phoenix lander team.
He made the widely reported statement about "swimming pools for bacteria" on Mars. 
In the academic paper about this research he writes:
"The results of our experiments suggest that the spheroids observed on a strut of the Phoenix lander formed on water ice splashed during landing [Smith et al., 2009; Rennó et al., 2009]. They also support the hypothesis that “soft ice” found in one of the trenches dug by Phoenix was likely frozen brine that had been formed previously by perchlorates on icy soil. Finally, our results indicate that liquid water could form on the surface during the spring where snow has been deposited on saline soils [Martínez et al., 2012; Möhlmann, 2011]. 'These results have important implications for the understanding of the habitability of Mars because liquid water is essential for life as we know it, and halophilic terrestrial bacteria can thrive in brines'"
Ice and salt are both common in the higher latitudes of Mars, so these millimeter scale micro-habitats on salt / ice boundaries may likewise be a common feature on Mars.
These interfacial layers occur on boundaries between ice and rock due to intermolecular forces that depress the freezing point of the water. The water flows and acts as a solvent. These layers may be used by microbes in arctic permafrost, which have been found to metabolize at temperatures as low as -20°C. Life may be possible in interfacial layers as thin as three monolayers, and the model by Stephen Jepsen et al obtained 109 cells/g at -20°C, though the microbes would spend most of their time in survival mode.. Models show that interfacial water should form in some regions of Mars, for instance in Richardson crater.
This is a possibility that was highlighted recently with the close flyby of Mars by the comet Sliding Spring in 2014 C/2013 A1 Sliding Spring. Before its trajectory was known in detail, there remained a small chance that it could hit Mars. Calculations showed it could create a crater of many km in diameter and perhaps a couple of km deep. If a comet like that was to hit polar regions or higher latitudes of Mars, away from the equator, it would create a temporary lake, which life could survive in.
Models suggest that a crater 30 - 50 km in diameter formed by a comet of a few kilometers in diameter would result in an underground hydrothermal system that remains liquid for thousands of years. This happens even in cold conditions so is not limited to early Mars, so a similar impact based temporary underground hydrothermal system could be created today if there was a large enough impact like Sliding Spring. The lake is kept heated by the melted rock from the initial impact in hydrothermal systems fed by underground aquifers.
There is evidence that volcanism formed lakes 210 million years ago on one of the flanks of Arsia Mons, relatively recent in geological terms. This may have consisted of two lakes of around 40 cubic kilometers of water, and a third one of 20 cubic kilometers of water, which probably remained liquid for hundreds, or even of the order of thousands of years. 
Two views of Arsia Mons, based on Viking orbiter imagery and Mars Global Surveyor elevation data, from the south (top) and north (bottom).
There is clear evidence that Mars is not yet geologically inactive 
It seems likely that there are magma plumes at least deep underground, associated with the occasional surface volcanism on the geological timescale of millions of years. And given that there has been activity on Olympus Mons as recently as four million years ago, it seems unlikely that all activity has stopped permanently.
But so far no currently active volcanism has been observed, nor have any present day warm areas have ever been found on the surface, in extensive searches. The Mars Global Surveyor scanned most of the surface in infrared with its TES instrument. The Mars Odyssey's THEMIS, also imaged the surface in wavelengths that measure temperature.
Another way to search for volcanic activity is through searches of trace gases produced in volcanic eruptions. So far nothing has been observed from Earth but instruments are limited in their sensitivity and get only limited observing time for Mars as well. This is going to be a focus of future searches however. One of the instruments on the the 2016 ExoMars Trace Gas Orbiter is NOMAD (Nadir and Occultation for Mars Discovery), which will search for trace gases indicating current volcanic activity, as well as searching directly for organics that could result from life processes, and the methane plumes. 
If these hot spots exist, they could keep water liquid through geothermal heating. The water could be trapped under overlying deposits and kept at a pressure high enough to stay liquid. They could also be a source for intermittent surface or near surface water (for instance one of the hypotheses for the RSLs is that they may be occur over geological hot spots deep below the surface that indirectly supply them with water).
Another possibility is a volcanic ice tower - a column of ice that can form around volcanic vents, for instance on Mount Erebus, Ross Island, Antarctica. These would be only a few degrees higher in temperature than the surrounding landscape so easy to miss in thermal images from orbit. 
As well as the lava tube caves, Mars may have other caves also less visible from orbit. It has most of the same processes that form caves on the Earth, and also has processes unique to Mars that may also create caves, for instance through direct sublimation of ice or dry ice into the atmosphere. Caves are of especial interest on Mars for astrobiology, because they can give protection from some of the harsh surface conditions. If the caves are isolated from the surface, or almost isolated, they may have conditions similar to similarly isolated caves on the Earth.
In the "Workshop on Mars 2001", the main possibilities for cave formation listed are:
"(1) diversion of channel courses in underground conduits; (2) fractures of surface drainage patterns; chaotic terrain and collapsed areas in general; (4) seepage face in valley walls and/or headwaters; (5) inactive hydrothermal vents and lava tubes."
They remark that caves that formed at headwaters or where liquid seeped from the rocks may be of special interest for astrobiology, and these may be places where some ice would still be present. Of course research has moved on since 2001.
In 2014, Penelope Boston lists some of the main possible types of cave. She divides into the four main categories which she then divides into further subcategories.
She points out a few processes that may be unique to Mars. Amongst many other ideas she suggests:
Snottites in Cueva de Villa Luz in Southern Mexico. They live off H2S, and they create sulfuric acid which eats into the rock and enlarges the cave. The colony covers itself with a mucus like layer which protects it and helps it to create its own chemical microclimate inside. Some of the microbes involved are obligate aerobes so need a small amount of oxygen to survive, but some are capable of surviving as anaerobes and don't need oxygen at all.
Some cave habitats on Earth, if shielded from the surface, may be almost exact duplicates of similar habitats on Mars. For instance the Snottites in the toxic sulfur cave Cueva de Villa Luz flourish on Hydrogen Sulfide gas. Some of these species are aerobes (needing only small amounts of oxygen), and others are anaerobes and could survive anywhere on Mars where similar habitats exist. Mars has been shown to be geologically active in the recent geological past through the Phoenix isotope measurements. Although there are no currently known geological hotspots or activity is currently known, there may well be subsurface thermal systems where caves similar to the Cueva de Villa Luz could occur.
If these ice sheets exist, they may provide a source of water for surface life, for instance for the Recursive Slope Lineae in the equatorial regions on the flanks of Valles Marineres.
Changes in tilt of Mars's axis. At times it tilts so far that it has equatorial ice sheets instead of the more usual polar ice caps. The sub surface ice sheets in the equatorial regions, if they exist, may be remnants of these larger ice sheets from the past.
As the axial tilt of Mars changes, at times it tilts so far that it has equatorial ice sheets instead of polar caps.
Several lines of evidence suggest, that Mars may have remnant subsurface equatorial ice sheets today. The first evidence of this was based on radar measurements from the (MARSIS) instrument aboard the Mars Express Spacecraft in 2007. These detected subsurface deposits that had similar density and dialectric constant to a mixture with more dust and sand than the polar ice deposits, and similar in volume and extent. 
Other papers have provided additional, but not yet conclusive evidence that these may indeed be deposits of ice. For instance a 2014 paper reports observations of young ring-mold craters on tropical mountain glacier deposits on the flanks of Arsia and Pavonis Mons. Ring-mold craters are distinctive features that result from impact into debris covered ice. The observations suggest presence of remnant equatorial ice, over 16 meters below the surface. 
Ice in the equatorial regions would normally be lost through sublimation into the near vacuum of the Mars atmosphere, to a depth of a hundred meters or more, and this happens quite rapidly over geological timescales, over timescales of order of 100,000 years or so. So for remnant ice to survive there today, then special conditions are needed. For instance trapped ice beneath an impervious layer (capstone). Or replenished from below. This is a matter for active research with no established conclusions yet.
Deep rock habitats on Earth are inhabited by life so may also be on Mars. However they need liquid water to survive, which may possibly exist below the cyrosphere.
The Mars cryosphere is the layer of permanently frozen permafrost. In higher latitudes it starts a few cms below the surface, and may continue down for several kilometers. In equatorial regions the surface of Mars may be completely dry down to a kilometer or more, so the cryosphere starts at the base of that dry layer.
If the Mars hydrosphere exists, it lies below the cryosphere, and is a layer where the ice is kept liquid by geothermal heating, and prevented from evaporating by the overlying layers of ice.
We don't have any evidence yet of a hydrosphere, but do have evidence of a deep subsurface cryosphere. This evidence is in the form of hydrogen / deuterium isotope ratios in Martian meteorites, which give indirect evidence that Mars must have a subsurface reservoir of water, most likely in the form of ice.
Research in 2014 into the deuterium / hydrogen isotope ratios in the water in martian meteorites gives evidence of a subsurface reservoir with a ratio in between the composition of the mantle and the composition of the water mixing with its current atmosphere. This supports the hypothesis that Mars has a deep cryosphere which may contain much of the original water from Mars."
If the hydrosphere exists, estimates in a paper from 2013 put it's depth at around 5 kilometers below the surface. Whether this layer exists or not depends on the presence or otherwise of perchlorates, and clathrates, and it also depends on the total inventory of water on Mars, so there are many unknowns in the models. They used an estimate of the total inventory of less than 500m GEL (Global Equivalent Layer), and doubled the required thickness of the cryosphere, which leaves less water available for the hydrosphere than in previous models. There may still be groundwater in places where it is perchlorate rich, and isolated pockets.
But if the global inventory of water is larger than the amount they assumed for their study, there may be ground water under much of the surface of Mars.
If this hydrosphere exists, then it may be more habitable than similar depth zones on Earth because of the lower gravity, leading to larger pore size. Possible metabolisms at this depth could use hydrogen, carbon dioxide, and possibly abiotic hydrocarbons. The carbon for biomass could come from magmatic carbon in basalts which has been detected in Martian meteorites. It could also support methanogens feeding off methane released from serpentinization, and the alteration of basalt could also be a basis for iron respiration. 
Similar habitats on Earth are inhabited by microbes and even multi-cellular life. So this is a potential habitat of astrobiological interest on Mars. As well as that, if the habitat exists it is a possible reservoir that could replenish surface areas of Mars with life and permit lifeforms to transfer from one part of Mars to another subsurface - a process that is known to happen beneath arctic permafrost layers.
It's not feasible to drill down to sample it in the near future. However, liquid may be released to the surface as a result of impact fracturing and other events so making it possible to sample it via surface measurements.
One prime place to visit to search for evidence of the deep hydrosphere is McLaughlin Crater. The observations suggest it contained an ancient lake, with alteration minerals rich in Fe and Mg, and the detection of carbonates there suggests that the fluids were alkaline, and are consistent with the expected composition of fluids that emerged from the deep subsurface hydrosphere. The Nature article concludes "Lacustrine clay minerals and carbonates in McLaughlin Crater might be the best evidence for groundwater upwelling activity on Mars, and therefore should be considered a high-priority target for future exploration"
A few places on Earth, such as the hyper arid core of the high Atacama desert and the McMurdo dry valleys in Antarctica approach the dryness of current Mars conditions. In some parts of Antarctica, the only water available is in films of brine on salt / ice interfaces. There is life there, but it is rare, in low numbers, and often hidden below the surface of rocks (endoliths), making the life hard to detect. Indeed these sites are used for testing sensitivity of future life detection instruments for Mars.
Other analogues duplicate conditions that may occur in particular locations on Mars. These include ice caves, the icy fumaroles of Mount Erebus, hot springs, or the sulfur rich mineral deposits of the Rio Tinto region in Spain. Other analogues include regions of deep permafrost and high alpine regions with plants and microbes adapted to aridity, cold and UV radiation with similarities to Mars conditions. 
This section is for analogues of conditions that could prevail on present day Mars. This includes analogues of deep subsurface habitats, and temporary habitats that can form after volcanic eruptions and large meteorite impacts. However, it leaves out sites that are thought to be analogues only of conditions on early Mars. Also, it leaves out geological analogues, or analogues used only for testing engineering details for landing systems and rovers. For analogues more generally see Terrestrial Analogue Sites.
The Yungay area used to be considered the driest area, at the core of the Atacama desert. It can go centuries without rainfall, and parts of it have been hyper-arid for 150 million years. The older regions in this area have salts that are amongst the closest analogues of salts on Mars with nitrate deposits that contain not only the usual chlorides, but also sulfates, chlorates, chromates, iodates, and perchlorates.
There's hardly any life. No animals or plants. The infrared spectra are similar to the spectra of bright soil regions of Mars. 
In 2003, a group led by Chris McKay repeated the Viking Lander experiments in this region, and got the same results as those of the Viking landers on Mars, decomposition of the organics by non biological processes. The samples had trace elements of organics, no DNA was recovered, and extremely low levels of culturable bacteriaThis lead to increased interest in the site as a Mars analogue.
Yet it does have some microbial life, including cyanobacteria, both in salt pillars, as a green layer below the surface of rocks, and beneath translucent rocks such as quartz. The cyanobacteria in the salt pillars have the remarkable ability to take advantage of the moisture in the air at low relative humidities. They begin to photosynthesize when the relative humidity rises above the deliquescence relative humidity of salt, at 75%, presumably making use of deliquescence of the salts.. It's also been found that cyanobacteria in these salt pillars can photosynthesize when the external relative humidity is well below this level, taking advantage of micropores in the salt pillars which raise the internal relative humidity above the external levels.
It's been used for testing instruments intended for future life detection missions on Mars, such as the Sample Analysis at Mars instruments for Curiosity, the Mars Organic Analyzer for ExoMars, and Solid3 for Icebreaker Life, which in 2011, in a test of its capabilities, was able to find a new "microbial oasis" for life two meters below the surface of the Atacama desert. 
This site is even drier than the Yungay area. It was found through a systematic search for drier regions than Yungay in the Atacama desert.
In a paper about the results published in March 2015 they report  an average atmospheric relative humidity 17.3%, and soils relative humidity a constant 14% at depth of 1 meter, which corresponds to the lowest humidity measured by Curiosity on Mars. Maximum atmospheric relative humidity 54.7% (compared with 86.8% for Yungay).
Remarkably, they found living organisms there also. And there was no decrease in the numbers of species with soil depth, down to a depth of one meter, although different microbes inhabited different soil depths.
They couldn't find any archaea in this region, though using methods that detected archaea in other regions of the Atacama desert. Depending on future results, this may mean it is beyond the dry limit of this domain of life.
Species found were
There was no colonization of Gypsum, showing the extreme dryness of the site.
Researchers scout out field sites in Antarctica's Beacon Valley, one of the most Mars-like places on Earth. Image credit: NASA
These valleys lie on the edge of the Antarctic plateau. They are kept clear of ice and snow by fast katabatic winds that blow from the plateau down through the valleys. As a result they are amongst the coldest and driest areas in the world.
The central region of Beacon Valley is considered to be one of the best terrestrial analogues for the current conditions on Mars. There is snowdrift and limited melting around the edges and occasionally in the central region, but for the most part, moisture is only found as thin films of brine around permafrost structures. It has slightly alkaline salt rich soil. 
This small pond in Antarctica, 100 meters by 300 meters, and 10 cm deep, is of great interest for studying the limits of habitability for present day life on Mars.
Research using a time lapse camera shows that it is partly fed by deliquescing salts revealing dark tracks that resemble the Recurrent Slope Lineae on Mars. The salts absorb water by deliquescence only, at times of high humidity, then flows down the slope as salty brines. These then mix with snow melt, which feeds the lake. The first part of this process may be related to the processes that form the RSLs on Mars.
It has an exceptionally low water activity of 0.3 to 0.6. Though microbes have been cultivated from it, they have not been shown to be able to reproduce in the salty conditions present in the lake, and it is possible that they only got there through being washed in by the rare occasions of snow melt feeding the lake. If this turns out to be the case, it may possibly be the only natural water body of any size without indigenous life on the Earth. For details, see #Lowest water activity level for life on Mars.
Blood Falls seeps from the end of the Taylor Glacier into Lake Bonney. The tent at left provides a sense of scale
A schematic cross-section of Blood Falls showing how subglacial microbial communities have survived in cold, darkness, and absence of oxygen for a million years in brine water below Taylor Glacier.
This unusual flow of melt water from below the glacier gives scientists access to an environment they could otherwise only explore by drilling (which would also risk contaminating it). It's source is a subglacial pool, of unknown size, which sometimes overflows. Biogeochemical analysis shows that the water is marine in source originally. One hypothesis is that its source may be the remains of an ancient fjord that occupied the Taylor valley in the tertiary period. The ferrous iron dissolved in the water oxidizes as the water reaches the surface, turning the water blood red.
Its autotrophic bacteria metabolize sulfate and ferric ions. According to geomicrobiologist Jill Mikucki at the University of Tennessee, water samples from Blood Falls contained at least 17 different types of microbes, and almost no oxygen. An explanation may be that the microbes use sulfate as a catalyst to respire with ferric ions and metabolize the trace levels of organic matter trapped with them. Such a metabolic process had never before been observed in nature.. It's of astrobiological importance as an analogue for environments below the Glaciers on Mars, if there is any liquid water there, for instance through hydrothermal melting (though none such has been discovered yet). . It's also an analogue for cryovolcanism in icy moons such as Enceladus. Subglacial environments in Antarctica need similar protection protocols to interplanetary missions.
"7. Exploration protocols should also assume that the subglacial aquatic environments contain living organisms, and precautions should be adopted to prevent any permanent alteration of the biology (including introduction of alien species) or habitat properties of these environments.
28. Drilling fluids and equipment that will enter the subglacial aquatic environment should be cleaned to the extent practicable, and records should be maintained of sterility tests (e.g., bacterial counts by fluorescence microscopy at the drilling site). As a provisional guideline for general cleanliness, these objects should not contain more microbes than are present in an equivalent volume of the ice that is being drilled through to reach the subglacial environment. This standard should be re-evaluated when new data on subglacial aquatic microbial populations become available"
Blood Falls was used as the target for testing IceMole in November 2014. This is being developed in connection with the Enceladus Explorer (EnEx) project by a team from the FH Aachen in Germany. The test returned a clean subglacial sample from the outflow channel from Blood Falls. Ice Mole navigates through the ice by melting it, also using a driving ice screw, and using differential melting to navigate and for hazard avoidance. It is designed for autonomous navigation to avoid obstacles such as cavities and embedded meteorites, so that it can be deployed remotely on Encladus. It uses no drilling fluids, and can be sterilized to suit the planetary protection requirements as well as the requirements for subglacial exploration. The probe was sterilized to these protocols using hydrogen peroxide and UV sterilization. Also, only the tip of the probe samples the liquid water directly. 
"Geologist, sedimentation expert and Mars Science Laboratory team member David Rubin of the USGS Pacific Coastal and Marine Science Center investigates longitudinal dunes in China's Qaidam Basin."
At 4500 meters (nearly 15,000 feet), it's the plateau with highest average elevation on the Earth. The atmospheric pressure is 50% - 60% of sea level pressures, and as a result of the thin atmosphere it has high levels of UV radiation, and large temperature swings from day to night. Also, the Himalayas to the South block humid air from India, making it hyper arid.
In the most ancient playas (Da Langtang) at the north west corner of the plateau, the evaporate salts are magnesium sulfates (sulfates are common on Mars). This combined with the Mars like conditions make it an interesting analogue of the Martian salts and salty regolith. An expedition found eight strains of halobacteria inhabiting the salts, similar to some species of Virgibacillus, Oceanobacillus, Halobacillus, and Ter-ribacillus.
Two sites of special interest: Colour Peak and Gypsum Hill, two sets of cold saline springs that flow with almost constant temperature and flow rate throughout the year. The air temperatures are comparable to the McMurdo dry valleys, range -15°C to -20°C (for the McMurdo dry valleys -15°C to -40°C). It's an area of thick permafrost with low precipitation, leading to desert conditions. The water from the springs has a temperature of between -4°C and 7°. A variety of minerals precipitate out of the springs including gypsum, and at Colour Peak crystals of the metastable mineral ikaite (CaCO3· 6H2O) which decomposes rapidly when removed from freezing water. 
"At these sites permafrost, frigid winter temperatures, and arid atmospheric conditions approximate conditions of present-day, as well as past, Mars. Mineralogy of the three springs is dominated by halite (NaCl), calcite (CaCO3), gypsum (CaSO4·2H2O), thenardite (Na2SO4), mirabilite (Na2SO4·10H2O), and elemental sulfur (S°).
Some of the extremophiles from these two sites have been cultured in simulated Martian environment, and it is thought that they may be able to survive in a Martian cold saline spring, if such exist.
The frozen soil and permafrost hosts many microbial communities that are tolerant of anoxic, acid, saline and cold conditions. Most are in survival rather than colony forming mode. It's a good terrestrial analogue of the saline acidic brines that once existed in the Meridani Planum region of Mars and may possibly still exist on the martian surface. Some of the microbes found there are able to survive in Mars-like conditions.
"A martian soil survey in the Meridiani Planum region found minerals indicative of saline acidic brines. Therefore acidic cryosol/permafrost habitats may have once existed and are perhaps still extant on the martian surface. This site comprises a terrestrial analogue for these environments and hosts microbes capable of survival under these Mars-like conditions"
This is the largest known sulfide deposit in the world, the Iberian Pyrite Belt (IPB).
Many of the extremophiles that live in these deposits are thought to survive independently of the sun. It's rich in iron and sulfur minerals such as
Jarosite on quartz Potassium iron sulfate Arabia District, Pershing County, Nevada 2779
This makes it an excellent analogue of a Mars subsurface habitat.
Much of the water on Mars is permanently frozen, mixed with the rocks. So terrestrial permafrosts are a good analogue. And some of the Carnobacterium species isolated from permafrosts have the ability to survive under the conditions of the low atmospheric pressures, low temperatures and CO2 dominated anoxic atmosphere of Mars. 
One of the numerous Ice Fumaroles near the summit of Mount Erebus in Antarctica. If these also occur on Mars, they could provide a habitat for life, and would be extremely hard to spot from orbit due to the low external temperatures. Image credit Mount Erebus Volcano Observatory
These ice fumaroles occur near the summit of Mount Erebus. If these occur on Mars, then they would provide a warm environment, high water vapor saturation, and partial UV shielding. The examples on Earth don't have significant amounts of liquid water. However, they do have close to 100% humidity inside, and are able to sustain microbial communities of oligotrophs, i.e. micro-organisms that can survive in nutrient poor environments.
For photographs of ice fumaroles see "Ice Towers and Caves of Mount Erebus". Similar habitats may possibly exist on Mars and the ice would make them hard to detect from orbit.
The caves on Erebus are of especial interest for astrobiology as most surface caves are influenced by human activities, or by organics from the surface brought in by animals (e.g. bats) or ground water. The caves at at Erebus. are high altitude, yet accessible for study. There is almost no chance of photosynthetic based organics, or of animals in a food chain based on photosynthetic life, and no overlying soil to wash down into them.
They are dynamical systems that collapse and rebuild, but persist over decades. The air inside the caves has 80% to 100% humidity, and up to 3% CO2, and some CO and H2, but almost no CH4 or H2S. Many of them are completely dark, so can't support photosynthesis. Organics can only come from the atmosphere, or from ice algae that grow on the surface in summer, which may eventually find their way into the caves through burial and melting. As a result most micro-organisms there are chemolithoautotrophic i.e. microbes that get all of their energy from chemical reactions with the rocks, and that don't depend on any other lifeforms to survive. The organisms survive using CO2 fixation and it's hypothesized that some use CO oxidization for the metabolism. The main types of microbe found there are Chloroflexi and Acidobacteria. .   
They may be of astrobiological significance for Mars. If Mars is currently active, in icy regions, then it may form similar ice fumaroles, and if so they would be only a few degrees higher in temperature than the surrounding landscape and hard to spot from orbit. 
Home Plate on Mars, explored by Spirit is thought to be an example of an ancient fumarole on Mars.
It's thought that ice caves may exist on Mars - ice preserved under the surface in cave systems protected from the surface conditions. 
The ice caves near the summit of Mt. Erebus (Antarctica) associated with the fumaroles are dark, in a polar alpine environments starved in organics and with oxygenated hydrothermal circulation in highly reducing host rock..
As well as analogues of ice fumaroles on Mars, if they exist, they may also be useful analogues for hydrothermally heated ice caves on Mars more generally if they exist.
There are several almost enclosed ecosystems below the surface of the Earth, in cave habitats, which are especially good analogues for Mars as similar environments might exist there. And the mines give access to deep subsurface environments which turn out to be inhabited on the Earth, and may possibly exist on Mars.
The only caves found so far on Mars are lava tube caves. These are insulated to some extent from surface conditions and may retain ice also when there is none left on the surface, and may have access to chemicals such as hydrogen from serpentization to fuel chemosynthetic life. Lava tubes on Earth have microbial mats, and mineral deposits inhabited by microbes. These are being studied to help with identification of life on Mars if any of the lava tube caves there are inhabited.
First of the terrestrial sulfur caves to be investigated as a Mars analogue for sulfur based ecosystems that could possibly exist underground also on Mars. On Earth, these form when hydrogen sulfide from below the cave meets the surface oxygenated zone. As it does so, sulfuric acid forms, and microbes accelerate the process. 
The high abundance of sulfur on Mars combined with presence of ice, and trace detection of methane suggest the possibility of sulfur caves below the surface of Mars like this.
The Snottites in the toxic sulfur cave Cueva de Villa Luz flourish on Hydrogen Sulfide gas and though some are aerobes (though only needing low levels of oxygen), some of these species (e.g. Acidianus), like those that live around hydrothermal vents, are able to survive independent of a source of oxygen. So the caves may give insight into subsurface thermal systems on Mars, where caves similar to the Cueva de Villa Luz could occur.
Crystals of Meridianiite, formula Magnesium sulfate 11 hydrate MgSO4•11H2O. Evidence from orbital measurements show that this is the phase of Magnesium sulfate which would be in equilibrium with the ice in the Martian polar and sub polar regions It also occurs on the Earth, for instance in Basque Lake 2 in Western Columbia, which may give an analogue for Mars habitats
Vugs on Mars which may be voids left by Meridianiite when it dissolved or dehydrated
Opportunity found evidence for magnesium sulfates on Mars (one form of it is epsomite, or "Epsom salts"), in 2004.  Curiosity has detected calcium sulfates on Mars. (Curiosity is currently exploring ancient deposits at the base of Mount Sharp. It won't reach the youngest Magnesium Sulfate deposits towards the summit of Mt. Sharp until towards the end of its extended mission if that goes ahead. ). Orbital maps also suggest that hydrated sulfates may be common on Mars. The orbital observations are consistent with iron sulfate or a mixture of calcium and magnesium sulfate..
So magnesium sulfate is a likely component of cold brines on the planet, especially with the limited availability of subsurface ice. Terrestrial magnesium sulfate lakes have similar chemical and physical properties. They also have a wide range of halophilic organisms, in all the three Kingdoms of life (Archaea, Bacteria and Eukaryota), in the surface and near subsurface.. With the abundance of algae and bacteria, in alkaline hypersaline conditions, they are of astrobiological interest for both past and present life on Mars.
These lakes are most common in Western Canada, and the northern part of Washington state, USA. One of the examples, is Basque Lake 2 in Western Columbia, which is highly concentrated in magnesium sulfate. In summer it deposits epsomite ("Epsom salts"). In winter, it deposits meridianiite. This is named after Meridiani Planum where Opportunity rover found crystal molds in sulfate deposits (Vugs) which are thought to be remains of this mineral which have since been dissolved or dehydrated. It is preferentially formed at subzero temperatures, and is only stable below 2°C , while Epsomite (MgSO 4·7H 2O) is favored at higher temperatures . 
Another example is Spotted Lake, which shows a wide variety of minerals, most of them sulfates, with sodium, magnesium and calcium as cations.
"Dominant minerals included blöedite Na2Mg(SO4)2·4H2O, konyaite Na2Mg(SO4)2·5H2O, epsomite MgSO4·7H2O, and gypsum CaSO4·2H2O, with minor eugsterite, picromerite, syngenite, halite, and sylvite", 
Spotted Lake in British Columbia in Canada. Its sulfate concentrations are amongst the highest in the world. Every summer the water evaporated to form this pattern of interconnected brine pools separated by salt crusts
Though many of the experiments use these lakes as an analogue for ancient Mars, some researchers also have investigated them as an analogue to explore the possibility of microbes that could inhabit cold magnesium sulfate rich brines in present day Mars. Some of the microbes isolated have been able to survive the high concentrations of magnesium sulfates found in martian soils, also at low temperatures that may be found on Mars. . 
Sulfates (for instance of sodium, magnesium and calcium) are also common in other continental evaporates (such as the salars of the Atacama desert), as distinct from salt beds associated with marine deposits which tend to consist mainly of halites (chlorides).
Lake Vostok drill 2011
Subglacial lakes such as Lake Vostok may give analogues of Mars habitats beneath ice sheets. In 2001, Duxbury et al suggested that processes similar to the ones that maintain Lake Vostok liquid could work on Mars.
Sub glacial lakes are kept liquid partly by the pressure of the depth of ice, but that contributes only a few degrees of temperature rise. The main effect that keeps them liquid is the insulation of the ice blocking escape of heat from the interior of the Earth, similarly to the insulating effect of deep layers of rock. As for deep rock layers, they don't require extra geothermal heating below a certain depth.
In the case of Mars, the depth needed for geothermal melting of the basal area of a sheet of ice is 4-6 kilometers. The ice layers are probably only 3.4 to 4.2 km in thickness for the north polar cap. However, Duzbury et al. showed that the situation is different if you consider a lake that is already melted. When they applied their model to Mars, they showed that a liquid layer, once melted, could remain stable at any depth over 900 meters even in absence of extra geothermal heating.
So, according to their model, if the polar regions had a subsurface lake perhaps formed originally through friction as a subglacial lake at times of favourable axial tilt, then supplied by accumulating layers of snow on top as the ice sheets thickened, they suggest that it could still be there. If so, it could be occupied by similar lifeforms to those that could survive in Lake Vostok. 
Investigations of life in deep mines, and drilling beneath the ocean depths may give an insight into possibilities for life in the Mars Hydrosphere and other deep subsurface habitats, if they exist.
In high alpine and polar regions, lichens have to cope with conditions of high UV fluxes low temperatures and arid environments. This is especially so when the two factors, polar regions and high altitudes are combined. These conditions occur in the high mountains of Antarctica, where lichens grow at altitudes up to 2,000 meters with no liquid water, just snow and ice. Researchers described this as the most Mars-like environment on the Earth. 
This section is organized around the listing of the main factors limiting surface and near surface life on Mars, according to Schuerger
These are thought to be (not in order of importance):
Other authors also cite:
Schuerger also mentions:
Other conditions that apply locally, rather than globally include:
Based on the capabilities of Earth microbes, the usually cited lowest temperature for life is -20°C. However, there is indirect evidence of continuing bacterial activity in glaciers down to -40°C, at a very low metabolic rate of ten turnovers of cellular carbon per billion years.
There can be some activity at even lower temperatures. In an experiment to test incorporation of the amino acid Leucine, Karen Junge et all used two controls at -80°C and -196°C, well below the eutectic freezing point of salt, and to their surprise, they found that the Colwellia psychrerythraea strain 34H was able to continue to incorporate low levels of Leucine right down to -196°C. They hypothesize that the Leucine enters the cell boundaries at higher temperatures in the first few seconds of the experiment, then gets incorporated into the cell at lower temperatures (it doesn't get incorporated right away as they proved through zero time controls).
Price et al did a review of the literature to date, in 2004, and came to the conclusion that there is no evidence of a fixed lowest temperature limit to metabolism, in the presence of impurities and thin films of water to supply liquid to microbes.
"Our results disprove the view that the lowest temperature at which life is possible is ≈-17°C in an aqueous environment, as well as the remark that “the lowest temperature at which terrestrial and presumably martian life can function is probably near -20°C. Our data show no evidence of a threshold or cutoff in metabolic rate at temperatures down to -40°C. A cell resists freezing, due to the “structured” water in its cytoplasm. Ionic impurities prevent freezing of veins in ice and thin films in permafrost and permit transport of nutrient to and products from microbes. The absence of a threshold temperature for metabolism should encourage those interested in searches for life on cold extraterrestrial bodies such as Mars and Europa."
The amount of water available for microbes to use in a salty or sugary solution is known as its water activity level which is normally expressed as the ratio of the partial vapour pressure of the water in the solution to the vapour pressure of pure water at the same temperature.
The tiny Don Juan pond in Antarctica, 100 meters by 300 meters, and 10 cm deep. This pond is about as salty as it could possibly be, with the CaCl2 levels approaching saturation at 60% w/v. It's so salty it stays liquid all the year round, at temperatures ranging from 0°C to -40°C. It has a eutectic of -51.8°C so is believed to be liquid all the year round. The water activity level measured is an exceptionally low 0.3 - 0.6. Though the temperature range is fine for life, it may be too salty for life to reproduce there. Microbes have been found, but they could only grow in less salty conditions. It might be that microbes sometimes can grow there when the water activity level is occasionally raised through influxes of water, and then die. Or they might be washed in from the surroundings.
Honey has a low water activity level of 0.6. That's why honey doesn't spoil - you don't need to keep honey in a fridge, because its water activity level is so low that though microbes would find plenty to eat, and though there is plenty of water there in the honey, the water is not available to the microbes because of the low water activity level.
The fungus Xeromyces bisporus can tolerate a water activity level of 0.605 in sugar - first discovered in a 1968 study of spoilage in prunes. It can divide at these low water activity levels, so can germinate, but needs higher levels of water to create fungal spores through asexual sporulation and even more for sexual sporulation..
Until recently, it was thought that that was an isolated case, as no other microbe was known able to tolerate such levels. The usually accepted lower limit was 0.755 for halophiles. However over the last few years there have been many reports of microbes at lower activity levels than that, comparable to Xeromyces bisporus, in salt solutions. Some papers have suggested the possibility of cellular reproduction at even lower levels.
In a recent 2014 survey paper of the literature on the subject "Multiplication of microbes below 0.690 water activity: implications for terrestrial and extraterrestrial life" , the authors came to the conclusion that the best consensus at present is that the lowest level of water activity needed for cell division is about 0.605, and that some halophiles are able to tolerate such low levels. They remark on the difference between the situation for water activity and the situation for temperatures, where there is much better evidence of microbes able to tolerate temperatures below the usually cited -20°C.
Radioresistant microbes are able to repair damage due to the equivalent of several hundred thousand years of Mars surface cosmic radiation within a few hours of revival from dormancy (they don't need to reproduce to do this, they repair their own DNA). For details: ionizing radiation resistance (radiodurans). This mechanism seems to be a byproduct of desiccation resistance, since the microbes have no need to tolerate high levels of cosmic radiation - shielded by the Earth's magnetosphere and atmosphere. Martian microbes are likely to have similar mechanisms, this time evolved in the presence of ionizing radiation. 
At times the Mars atmosphere is thicker than it is now depending on variations in its orbital eccentricity and axial tilt. At other times it has extensive ice sheets which then melt. Research in january 2015 by Dickson et al suggests that the Mars axial tilt has varied beyond the 30 degrees to the point where it has thin glacier like ice sheets at the mid attitudes within the 400,000 to 2 million years, and that this may have carved some of the older gully systems through melt water.
That's still challenging for life with a maximum of around 500,000 years dormancy on the surface. However the cosmic radiation only penetrates a few meters into the ground, with most of the effects shielded in the top 1.5 meters (400 grams per cm2 of material, at 2.6 grams per cm3 typical regolith density) and significant shielding at a depth of half a meter. Below that depth, there could be dormant microbes that have survived for longer periods. Depending on the depth below the surface they could remain dormant for millions of years. Some microbes on Earth have lasted for many millions of years in ice and salt, and have been revived. So some of these on Mars also may still be viable today. Such microbes could also survive in caves on Mars in dormancy, or in subsurface locations kept habitable by geothermal hot spots, until times when Mars is more habitable than it is today.
If there is present day life on the Mars surface, these effects of ionizing radiation suggest that it has to
In the 2014 MEPAG classification of special regions, ionizing radiation was not considered limiting for classifying the "Special regions" where present day surface life might survive.
It is a challenge for life to survive on the surface, or the near subsurface, because of the hyper arid conditions, combined with low temperatures. Often when the temperature is high enough for cellular division, the humidity is too low and vice versa. . Also in surface conditions, it's not possible for microbes to remain in dormancy through the changes in axial tilt when the Mars atmosphere becomes thicker and more habitable (as it does from time to time).
Authors in recent publications present a variety of views on the possibility of present day life on the surface of Mars or in the near subsurface.
There is greater agreement on deep subsurface habitats since conditions there may be similar to Earth conditions. They would be protected from UV, cosmic radiation, and the low pressure of the atmosphere, and water activity would be likely to be similar to Earth. For instance the deep hydrosphere (if it exists), or temporary lakes that form after impacts or volcanic eruptions, seem likely to be habitable, by analogy with similar habitats on Earth.
One way to examine the possibility for life on Mars is to look at the Redox pathways that the life could use as a source of energy. This involves a pairing of an electron donor and an electron acceptor. For details see Electron transport chain, and Microbial metabolism.
Here is a table of some of the available donors and acceptors in Mars conditions, table from  (added CO2).electron donors, any of: electron acceptors, any of:
|FeSO2+: available in Fe-rich silicates||Fe3+: available in numerous alteration
|H2: available in subsurface?||SO42- available in salts|
|CO: available in atmosphere||O2: partial pressure too low|
|organics: meteoritic likely to be present at surface||NO3-: presence or abundance unknown|
|organics: endogenous available in subsurface||ClO4-: available but not shown to support life|
|-||CO2: in the atmosphere|
A candidate metabolism would use one of the electron donors in the first column paired with one of the electron acceptors on the right as a source of energy. (The final dash on left hand side is there just because the list of electron donors is shorter than the list of electron acceptors).
See also the presentations in: Redox Potentials for Martian Life
This is a list of some of the proposed Mars analogue lifeforms, which may be capable of living on Mars (if the postulated liquid water habitats there exist).
Top candidates for life on Mars include
Most of these candidates are single cell microbes (or microbial films). The closest Mars analogue habitats on Earth such as the hyper arid core of the Atacama desert are inhabited by microbes, with no multicellular life. So even if multicellular life evolved on Mars, it seems that most life on Mars is likely to be microbial.
Because of the low levels of oxygen of 0.13% in the atmosphere, and (as far as we know) in any of the proposed habitats, all the candidate lifeforms are anaerobes or able to tolerate extremely low levels of oxygen. This also makes multicellular animal life unlikely, though not impossible as there are a few anaerobic multi-cellular creatures. Some multicellular plant life such as lichens, however, may be well adapted to Martian conditions (the algae supply oxygen for the fungus). Also some multicellular life such as Halicephalobus mephisto can survive using very low levels of oxygen which may perhaps be present in some Mars habitats.
Several lifeforms, including cyanobacteria Nostoc sp. Gloeocapsa Chroococcidiopsis sp., lichens Buelia frigida, Circinaria gyrosa, and fungi Cryomyces antarcticus, are currently being tested in the on-going year and a half Expose-R2 experiments in a small Mars simulation chambers on the exterior of the ISS (Expose R2) as part of BIOMEX (Biology and Mars Experiment).File:EXPOSE-R2.jpegInstalling the Expose-R2 facility on the International Space Station. Image taken with the helmet camera of Oleg Artemyev, on 18 August 2014, EVA-39 - Image credit:Roscosmos
Some of these simulation chambers are kept with atmosphere and filters to simulate Mars conditions of UV, and in some of the chambers they are in Mars simulation soil to simulate the Mars surface. Others are exposed to vacuum, e.g. to test panspermia hypotheses.
Test samples include bacteria and biofilms, cyanobactera, archaea, green algae, lichens, fungi, bryophytes, and yeast that have been found to be especially resistant in ground experiments and previous experiments on the ISS. They also include pigments and cell wall components.
The experimenters are studying the same organisms in Mars simulation chambers on the ground. The experiment has multiple goals - to find out what species could survive transfer to another planet on a meteorite (panspermia), to find out what detectable biosignatures would remain after exposure to space and to Mars surface conditions, and to find out their ability to survive in these conditions and possible genetic changes. 
Charles Cockell has analysed the possible trajectories for life on Mars using the idea of an "uninhabited habitat". On Earth these are exceedingly rare, but do occur sometimes. For instance, after a new lava flow, then the lava may initially be inhabitable but uninhabited.
It's possible to test the hypothesis that these habitats exist by finding environments on Mars with the elements needed for life, including an energy source and liquid water, with no active life. 
So then there are three states for Mars:
As Mars evolved, initially when it first formed in the early solar system, it was too hot for life, and so was uninhabitable. Then there are various trajectories it could follow after that, starting from the early Mars. In his paper "Trajectories of Martian Habitability" he identifies six main possible trajectories. T
He also suggests other more complex trajectories. For instance that it starts with uninhabited habitats and the life evolves there at a much later date, or is seeded from Earth at a later date. Or trajectories where life on Mars becomes extinct, and then reoriginates on Mars or is transferred to Mars from Earth. Or even, a logical possibility but seems unlikely, that it was for some reason uninhabitable in the early Noachian and became habitable later.
In his paper he discusses ways that this could be tested with observations. For instance, if you find that promising environments with water in present day and past Mars lacked some fundamental requirement for all known life, or the conditions were outside the range of physical and chemical tolerances of all known organisms, then that could be evidence for trajectory 1. If you find conditions for life but no life, past or present, that's evidence for trajectory 2, and so on.
He points out that if Mars does have uninhabited habitats, these would be a useful control to investigate the role of biology in planetary scale biological processes on Earth.
This is mentioned in "The NASA Astrobiology Roadmap". If we don't find life on Mars, we may still find evidence to fulfill this goal.
"GOAL 3—Understand how life emerges from cosmic and planetary precursors. Perform observational, experimental, and theoretical investigations to understand the general physical and chemical principles underlying the origins of life.
"Characterize the exogenous and endogenous sources of matter (organic and inorganic) for potentially habitable environments in the Solar System and in other planetary and protoplanetary systems."
"If life never developed elsewhere in our Solar System, is there a prebiotic chemical record preserved in ancient rocks that might contain clues about how life began on Earth?"
The search for life on Mars is of special interest for the search for a second example of life, which can help us to discover which of the many common shared features of the biochemistry of Earth organisms are essential for life, and which are accidents of evolution. Chris McKay puts it like this in his 2010 article "An origin of life on Mars.":
"The search for a second example of life is a key goal for astrobiology. All life on Earth shares common biochemistry and descends from a common ancestor. This prevents us from understanding which aspects of biochemistry and genetics are essential features of life and which are merely particular to the evolutionary history of life on this planet. To develop a more general understanding of life, we need more than one example. Hence, we hope that Mars may have been the site of an independent origin of life."
Until recently, it was assumed that any life on Mars would necessarily be a second genesis. But it is now understood that life could be transferred between planets on meteorites, so it is possible that life on Mars, if it exists, could be related to Earth life, or some of the life could be related to Earth life.
In order to decide whether the life is a second genesis or not, it's not enough to examine fossils. For one thing, microbes often don't form easily recognized convincing fossils, so the fossils may be hard to recognize, and rare in occurrence. But as well as that, fossils don't tell us what the chemical basis is for the life.
It's necessary to be able to study organics, and it preferably, viable cells. If life on Mars had same chirality, genetic code, choice of amino acids, lipids and so on, that would be evidence of a shared ancestry. If any of those differ, then it is likely to represent a second genesis.
Writing in 2010, Chris McKay says
"Possible targets include: (1) Life in the surface soil, (2) Life in subsurface liquid water, (3) Organisms, probably dead, but preserved in ancient salt or mineral deposits, and (4) Organisms, dead or alive, preserved in ancient ice."
Organics are common throughout the outer solar system, including meteorites, and comets. So when organics are found on Mars, the first thing to be decided is whether or not it is biological in origin. If it is related to Earth life, and sufficiently well preserved, this can be detected though search for DNA, RNA, ATP and other key molecules associated with life on Earth. But if it is not related to Earth life, then it may be harder to decide whether it is the result of biological processes.
One way to detect alien biology may be through the "Lego principle". This is the idea that chemicals used by life may be recognized because they use a wide range of chemicals with similar chemical structure, and chemicals very similar to each other (e.g. only differing in chirality) may have widely different concentrations. This is something that could be recognized even if the life has a different chemical basis from Earth life.
However, over time, the pattern degenerates as chemical bonds break and reform, especially in warmer conditions. So ideally we need to find life that is either alive, or has been preserved in cold conditions since it was deposited.
In his 2010 article, Chris McKay suggests targeting possibly still viable organisms preserved in ancient subsurface ice. This is also the main target for his proposed mission Icebreaker Life.
Even a null result in search for life on Mars would be of astrobiological significance. For instance it might tell us that the origins of life depend on particular conditions not present on Mars. For instance that it depends on a particular energy source, or material or on abundance of some particular nutrient (e.g. nitrogen).
Search for present day life on Mars requires more stringent planetary protection than the search for past life. For instance, if Curiosity were to discover traces of liquid water on Mars, some micro habitat in conditions that make it potentially habitable to Earth life, it would not be able to approach it to measure it to search for present day life as it is not sufficiently sterilized for this task.
Regions of Mars that may be habitable for present day life are classified as "Special regions" and any parts of a spacecraft that touch such regions have to be sterilized to Viking levels of sterilization or better. So far no modern spacecraft have yet been sterilized to these levels. It is a major challenge as the heat treatment used for Viking would destroy many modern instruments. However low vapour hydrogen peroxide sterilization may be able to take the place of heat treatment - it is already approved for spacecraft use. As well as that there are emerging new ideas for sterilization that may be more effective with less damage to the spacecraft, such as use of ionized gas in vacuum conditions. 
The best way to search for early life, as far as we can tell at present, is to search for organics. And the organics is easily confused with organics from non life processes and from space.
One of the main conclusions of Bada et al's white paper was that we should look for organics with nitrogen on Mars. Nitrogenous organics are likely to be rare because there are few sources of nitrogen on Mars.
This is important because nitrogen bonds are easily broken and are central to biology as we know it. So even if life on Mars is very different from Earth life, perhaps using different amino acids for instance with a different backbone from DNA, still it is likely to use nitrogen if it resembles Earth life.
Once we find these compounds, that's not enough as you also get nitrogenous organics from comets and meteorites and natural processes. We then need to search for biosignatures.
We also need to be able to drill below the surface (as ExoMars will be able to do) to the maximum depth possible. That's because our best chance of finding evidence of past life is to drill down below the surface layers damaged by ionizing radiation, ideally to ten meters depth or more (though the two meters depth of ExoMars is a good start here). Their main points are:
Curiosity has detected evidence of nitrates in both scooped wind drifted sand and samples drilled from sedimentary rocks. The results support 110–300 ppm of nitrate in the wind drifted samples, and 330–1,100 ppm nitrate in the mudstone deposits. The authors suggest that it is likely to be the result of fixation during meteorite impact or lightning associated with volcanoes in early Mars. 
Results from the Curiosity SAM instrument presented in March 2015 show presence of what may be a fatty acid molecule. Also confirm presence of chlorobenzene. Neither of these are biosignatures, for instance organisms use fatty acids to build cell membranes, but they can also have inorganic origins. But they show that complex organics can survive on the surface of Mars, so increasing the chance of later detecting microbial life on the surface if it is there. 
Viking 1 and 2 are the only successful missions to Mars to date designed to search for present day life.
The failed British mission, Beagle 2, had the search for present day life as an objective as well as past life.
Curiosity (rover) and Opportunity (rover) are currently searching for habitable conditions with the main focus on past habitability. However they are not equipped to detect biosignatures of life, either past or present, and also were sent to sites selected with past rather than present day life as the main target.
Curiosity does have some capabilities that could be of interest for life detection. It can detect isotope ratios in organics or in the methane plumes suggestive of life which could give indirect evidence of life processes as life preferentially incorporates lighter isotopes.
Also Curiosity has one experiment that can potentially be used to detect chirality if it finds a potentially interesting sample to test. It has a Chirasildex column which can be used to separate out entantiomers of astrobiological significance.
The ExoMars Trace Gas Orbiter will help with the search for trace levels of organics in the atmosphere, with sensitivity up to a thousand times greater than previous missions. Detection sensitivities are at levels of 100 parts per trillion, improved to 10 parts per trillion or better by averaging spectra which could be taken at several spectra per second . This would lead to global mapping of distribution of methane and other organics in the atmosphere which could help to pinpoint sources on the surface.
ExoMars is also designed to search for both present day and past life. It will have capabilities to test for biosignatures "in situ" on Mars, and to drill to depths of up to 2 meters. However the target has been selected with the search for past life as its prime objective, so it will only discover present day life if it is widespread on Mars.
Mars 2020 is designed for sample caching for a future sample return, and much of the mass used for instruments on Curiosity is used instead for sample return. As a result it has a reduced suite of instruments compared with Curiosity (rover). The main thing of note for exobiology is that it will have two Raman spectrometers, first to fly to the planet (except for ExoMars if they get there first) on Supercam and on SHERLOC which uses a spot of ultraviolet laser light to micro-map minerals and organics on the samples on the scale of 50 microns. On the other hand a Raman spectrometer gives information about arrangements of atoms such as a carbon atom double bonded to oxygen, but it can't detect specific molecules in the sample like SAM. . Again the target will be selected with past life as its prime objective.
Icebreaker Life is a mission suggested by Chris McKay to search for past life preserved in ice on Mars, and present day life on Mars.
ExoLance is an ingenious proposal that uses ground penetrating "lances". Curiosity carries ballast in the form of two 75 Kg tungsten weights, which it discards on arrival at Mars to help with the asymmetric trim of the aeroshell, to generate a lifting vector. That is how it manages to achieve higher precision than other missions to date. See MSL - guided entry. The idea is to put impact resistant instruments into these weights, and make them into ground penetrating "missiles" able to penetrate to a depth of a meter or so, where conditions may be more favourable for preservation of life..
NASA propose to return samples to test for biosignatures and signs of life back on Earth. Their Mars 2020 mission is designed to cache the sample, for later return to Earth by some future mission.
Instruments designed to search directly for this life include
These can detect life on Mars if it is DNA based so related to Earth life. As DNA sequencers, they can sequence the entire genome of any lifeform found.
These can detect life even if it doesn't use any recognized form of conventional life chemistry. But requires the life to be "cultivable" in vitro when it meets appropriate conditions for growth.
This is an article I worked on gradually for wikipedia for several months in Spring 2015. It's released under CC by SA, but I'm the copyright holder. Some parts of it started off as a deleted section from an old version of the Water on Mars article which was the result of previous work by many editors. However there is no substantial content left from that article except a few citations as I rewrote everything (as you can see by comparing the two).
I submitted it for review as a new article for wikipedia in June and simultaneously released it on my science20 blog on June 9 2015. However it never got into wikipedia. The reviewers said "This is well written as a journal article or essay, but it's not really a Wikipedia article".
The original draft is here. The draft in wikipedia also leaves out some of the images.There's a nice discussion here where a patient editor explained their policy to me, which is quite restrictive due to their aim to create free content that can be re-used by anyone, for any purpose, including all forms of commercial re-use (not just for educational use). When images don't have such permissive licenses, they are often rejected from wikipedia, although acceptable almost anywhere else.
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, Lasue, Jeremie, et al. "Quantitative assessments of the martian hydrosphere." Space Science Reviews 174.1-4 (2013): 155-212.
"Consequences for the Global Inventory of Water: Clifford and Parker (2001) calculated that a planetary inventory of 500 m GEL (Global Equivalent Layer] could have been cold trapped into the thickening cryosphere by the end of the Late Hesperian (∼3 Gy). With the nearly twofold increase in the potential maximum thickness of the cryosphere suggested here the prospects for longterm survival of subpermafrost groundwater are in greater doubt. [table and calculations]... Therefore, even assuming a high (30 mW m−2) heat flow and a eutectic solution of NaCl, the present-day occurrence of groundwater on Mars appears highly unlikely, given the smaller (<500 m GEL) estimates for the total planetary inventory given in Table 4. But even under these limited conditions, some subpermafrost groundwater may still survive in places where it is perchlorate-rich, although such occurrences are likely to be restricted to isolated pockets rather than any system of regional or global extent. If the planetary inventory of H2O is closer to the upper estimate of ∼1 km GEL from Table 4, then subpermafrost groundwater may still persist beneath much of the surface, but will generally reside at depths ≥5 km, or roughly twice as deep as previously thought . However, natural variations in crustal thermal conductivity, heat flow, and groundwater composition, may permit isolated occurrences of groundwater to exist at significantly shallower depths."
, Lasue, Jeremie, et al. "Quantitative assessments of the martian hydrosphere." Space Science Reviews 174.1-4 (2013): 155-212.
"Aquifer Habitability Finally, deep aquifers below the cryosphere may have provided a hydraulic connection between various subpermafrost habitats. If Mars were ever inhabited, these hydraulic connections would likely have provided a means for biota to be transported from one habitable environment to another. An analogous system is fracture networks within or under permafrost in the terrestrial arctic. These systems harbor sulfatereducing microorganisms and other anaerobic taxa that can grow within the cold, saline conditions of the permafrost. Analogous conditions may exist within the Martian deep-subsurface where impact-generated fractures may have allowed both microorganisms and nutrients to migrate from one habitat to another—even ones arising from recent impacts and their associated hydrothermal environments, if habitats on Mars were inhabited and life existed on that planet "
"The Mt. Erebus ice caves are at high altitude in one of the most remote and oligotrophic environments on Earth and represent an excellent accessible model system for understanding fundamental microbe-mineral interactions contributing to the subsurface biosphere. This environment ensures that they are highly oligotrophic with almost no potential for the introduction of photosynthesis-based organic matter from invading animals or wash-down of organics from overlying soils. Mt. Erebus ice caves are moist, relatively warm habitats (on average ~0°C,) that persist over decades even though they are dynamic systems with cycles including collapse and post-collapse re-building. Sub-glacial fumaroles issue air-dominated gasses with 80–100% humidity and up to 3% CO2. The volcano source gas emissions, some of which may be entrained in the fumaroles, contain CO and H2, but are essentially devoid of CH4 and H2S. Many of the caves are completely dark and therefore unable to support photosynthesis. In these DOVEs the only possible sources of organic carbon are from atmospheric deposition or ice algae that may grow on the surface of the ice during summer and subsequently be introduced into the caves through burial from above and melting from below. Thus, Mt. Erebus DOVEs provide an ideal ecosystem to study chemolithoautotrophic microorganisms that, in other cave and basaltic environments, would be masked by heterotrophic and photosynthetic organism biomass. Consequently they may shed new insights into the role of volcanoes and volcanic emissions in supporting life."
"Finally there are other harmful radiation sources reaching Mars: ionizing and neutron radiation caused by galactic cosmic radiation and solar particle events.
"Due to the lack of a magnetic field and the low shielding of the Martian atmosphere (the Martian overhead airmass is 16 g cm-2 instead of the terrestrial 1000 g cm-2) the doses of ionizing radiation at the surface of Mars reach values about 100 times higher than those on the Earth.
"However, since a great variety of microbes tolerate this type of radiation at similar or even greater doses than those found on Mars, ionizing radiation cannot be considered a limiting factor for microbial life on Mars and thus here we will limit our study to solar UV shielding and VIS radiation pentration."
(from RADIATIVE HABITABLE ZONES IN MARTIAN POLAR ENVIRONMENTS full text here pdf)