You can't say that any more. First there is the deep hydrosphere, below the cyrosphere, where our models suggest there is liquid water trapped under the pressure of the rocks above, and heated by geothermal heat. Since there is life in similar habitats on Earth, deep rocks with water and life using chemical energy, then is reasonably likely that if life did evolve on Mars, it is still there deep down.
Then has always been the idea that meteorite impacts on Mars, and also volcanic eruptions and geothermal heat could lead to liquid water on the surface temporarily. That was theoretical originally but now we have increasing evidence that Mars is volcanically active from time to time even right now. No eruptions or hot spots yet observed, but indirect but good evidence that this does happen on the timescale of millions of years.
In the case of a meteorite impact on the polar regions, it could create a lake that gets covered in ice quickly, but then due to the insulating effects of the ice, after a big impact, it would remain liquid for perhaps a thousand years. Long enough for a lot of life activity to go on if it can find a way to survive in between such events.
But then, more recently since Phoenix in 2008, first the droplets on the Phoenix lander legs - appear to be some form of liquid + isotope evidence from Phoenix's study of the atmosphere showing that the CO2 in the atmosphere has interacted with a liquid on the surface, probably water, and exchanged oxygen atoms with it.
And then increasingly, discovery of various potential habitats on the surface of Mars.
We aren't talking about lakes or rivers here. But increasing evidence that for a few hours a year you get droplets of liquid water forming, as well as seeps of liquid water. Just a few mms in thickness, but that is plenty of habitat for a microbe.
Latest idea is, droplets forming on the interface between perchlorate salts and ice.
As Nilton Remmo said,
Many other ideas also and indirect evidence for water seeps on Mars
Most features on Mars are due to dry ice or wind.
But these are amongst the strongest candidates for features caused by seepage of water:
In both those examples the dark patches we see from orbit aren't literally damp patches in the way you might expect from Earth - the Mars atmosphere is far too thin for that to work.
Rather some change in the appearance of the surface due to the seepage of a very thin film of water perhaps a mm or so in thickness. But is good evidence that water is in some way involved in the process because of the conditions in which they form.
There might also be life not as we understand it. For instance life that doesn't require water but has some other solvent.
Hypothetical types of biochemistry
But there is plenty of scope for life as we understand it also.
Also the life on Mars - if it evolved separately from Earth life - would be likely to have a different biochemistry. Different bases, maybe different backbone (PNA instead of DNA), life that is a mirror image of the DNA spiral, maybe not based on an analogue of DNA at all, lots of possibilities.
And there is also the chance of some kind of chemistry that is so complex it is almost life, and yet doesn't reproduce exactly or doesn't share all the characteristics we recognize as life currently.
I did several Science20 posts about this. see for instance:
Then has always been the idea that meteorite impacts on Mars, and also volcanic eruptions and geothermal heat could lead to liquid water on the surface temporarily. That was theoretical originally but now we have increasing evidence that Mars is volcanically active from time to time even right now. No eruptions or hot spots yet observed, but indirect but good evidence that this does happen on the timescale of millions of years.
In the case of a meteorite impact on the polar regions, it could create a lake that gets covered in ice quickly, but then due to the insulating effects of the ice, after a big impact, it would remain liquid for perhaps a thousand years. Long enough for a lot of life activity to go on if it can find a way to survive in between such events.
But then, more recently since Phoenix in 2008, first the droplets on the Phoenix lander legs - appear to be some form of liquid + isotope evidence from Phoenix's study of the atmosphere showing that the CO2 in the atmosphere has interacted with a liquid on the surface, probably water, and exchanged oxygen atoms with it.
And then increasingly, discovery of various potential habitats on the surface of Mars.
We aren't talking about lakes or rivers here. But increasing evidence that for a few hours a year you get droplets of liquid water forming, as well as seeps of liquid water. Just a few mms in thickness, but that is plenty of habitat for a microbe.
Latest idea is, droplets forming on the interface between perchlorate salts and ice.
As Nilton Remmo said,
""Based on the results of our experiment, we expect this soft ice that can liquify perhaps a few days per year, perhaps a few hours a day, almost anywhere on Mars. So going from mid lattitudes all the way to the polar regions. This is a small amount of liquid water. But for a bacteria, that would be a huge swimming pool - a little droplet of water is a huge amount of water for a bacteria. So, a small amount of water is enough for you to be able to create conditions for Mars to be habitable today'. And we believe this is possible in the shallow subsurface, and even the surface of the Mars polar region for a few hours per day during the spring." (transcript from 2 minutes into the video onwards)"
Many other ideas also and indirect evidence for water seeps on Mars
Most features on Mars are due to dry ice or wind.
But these are amongst the strongest candidates for features caused by seepage of water:
These are Flow Like Features, found in a few spots in the South polar region, thought to be possibly due to melt water beneath a covering "solid state greenhouse" layer of transparent ice. If so they are particularly habitable for life.
These are the "Warm Seasonal Flows" - not to be confused with the Dark Dune Streaks that are superficially a bit similar. So far only hypothesis is - due to liquid water also because they only form when temperatures are far too high for dry ice, occur even in equatorial regions on sun facing slopes, and are seasonal, grow in spring, fade away in winter, and no correlation with the wind and dust storms.
In both those examples the dark patches we see from orbit aren't literally damp patches in the way you might expect from Earth - the Mars atmosphere is far too thin for that to work.
Rather some change in the appearance of the surface due to the seepage of a very thin film of water perhaps a mm or so in thickness. But is good evidence that water is in some way involved in the process because of the conditions in which they form.
There might also be life not as we understand it. For instance life that doesn't require water but has some other solvent.
Hypothetical types of biochemistry
But there is plenty of scope for life as we understand it also.
Also the life on Mars - if it evolved separately from Earth life - would be likely to have a different biochemistry. Different bases, maybe different backbone (PNA instead of DNA), life that is a mirror image of the DNA spiral, maybe not based on an analogue of DNA at all, lots of possibilities.
And there is also the chance of some kind of chemistry that is so complex it is almost life, and yet doesn't reproduce exactly or doesn't share all the characteristics we recognize as life currently.
I did several Science20 posts about this. see for instance:
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
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