Originally Answered: Is there life on Mars?
These intriguing features are very rare on Mars - not the same as the better known dry gullies (created by dry ice probably).
They form only on sun facing slopes and only when the temperatures go above zero centigrade. Far too warm for dry ice. Can't be a wind phenomenon as they aren't correlated with winds or dust storms.
They are seasonal - start off short early in the year, gradually get longer as the year progresses, and then in the autumn, slowly fade away.
All the attempts at modeling them so far assume that they are liquid water.
The problem is though - that they are very narrow streaks - and also - if there is water it is likely to be a thin layer, perhaps just a mm or so, and - no more than slightly damp regolith. Very hard to detect from orbit by isotope measurements.
Still, it would be plenty of liquid for life "living on the edge" such as microbes found in places such as the McMurdo dry valleys and the high Atacama desert.
There are several other possibilities for habitats for life on Mars. That's just the one that's hardest to dismiss as we so far have no other hypothesis for them.
Indirect evidence comes from isotope measurements by Phoenix, showing that the oxygen in the CO2 molecules in the atmosphere has interacted with liquid water in the geologically recent past. So we know, pretty sure that either there is a small amount of water on Mars all the time - or it is there episodically quite often (e.g. after asteroid impacts) - or both.
We've also detected salts on Mars - and on the Earth microbes in deserts can survive in salts that have no water at all except the water they take in from the atmosphere via deliquescence - and Mars because of the huge temperature swings between day and night has 100% relative humidity at night. Phoenix may have detected this effect accidentally, when salt deposits splashed on its legs during is landing formed dark droplet shaped objects - which then fell off - and didn't form again. It's indirect evidence as it couldn't access them to analyse what they are. But that's what started the modern search for ways that life could inhabit the surface of present day Mars.
DLR in Germany also has discovered some algae, and lichens, that are able to do the same trick and take up the night time humidity in a simulated Mars environment on their own, without need for salt deposits.
The main limiting factor - at least for Earth like life on Mars is absence of nitrogen - but Curiosity has found nitrates on Mars - and on Earth microbes can live in places with very low levels of nitrates. And nitrates should be delivered by meteorites - and most of our analyses of Mars can't see even a cm below the surface.
So seems not impossible that there are enough nitrates on Mars for life.
So - nowadays you get some researchers who are quite up beat about the prospect of finding present day life on Mars. Though others still remain highly skeptical of the idea.
As for past life -then there is increasing evidence that Mars had a global ocean - reached the point now I think where almost nobody would deny it - though it might have been frozen over quite early on and only melted sporadically. And as for Earth and Venus - it would have started off with a dense Venus like atmosphere, oceans well above boiling point kept liquid by the high atmospheric pressure - which slowly cooled down.
Depending how long evolution takes - there might well have been enough time for life to evolve before it cooled down. What's more the later snowball phase on Mars could itself have triggered evolutionary jumps - at least - the development of multi-cellular life on Earth happened immediately after its last snowball phase.
Mars as it is now is basically a "snowball" planet - it isn't white because it is also very dry and has lost most of its original water - either to space or underground or both.
Still - Earth obviously had life throughout its own snowball phases. So that doesn't mean there is no life on Mars. Just that it is very hard to detect.
Similarly if we saw an exoplanet in a snowball phase like Mars or snowball Earth - it would be just about impossible to tell whether it has life on it or not unless we can send a probe to it (not likely in near future).
So upshot is - that many scientists now think it is likely Mars had life in the past - and quite a few think it's likely it still has life.
But both types of life are likely to be hard to detect. Past life because it was habitable only for a short time, we don't yet know what to look for (what would Mars micro-fossils look like?), it might have occurred only in a few places on Mars especially right at the beginning - best chance of finding unambiguous evidence probably is through organics (unless Mars had multi-cellular life) and it's not easy to preserve organics for billions of years on Mars because of radiation in the rocks and cosmic radiation for surface deposits. Still - researchers think that we do have methods of measuring biosignatures that could detect ancient life on Mars - first instruments able to do this due to launch in 2018 on ExoMars.
For present day life - then - are all the present day possible habitats inhabited? They well might not be in such harsh conditions. And the levels of microbes are so small - it is extremely hard to detect life in the coldest driest deserts on Earth and probably even harder on Mars - and the life is likely to be hidden below surfaces of rocks or a cm or so beneath the surface of the soil.
And in both cases - we haven't yet started the search. Curiosity would not be able to detect life and distinguish it from meteorite organics except in unusual circumstances - e.g. obvious macro-fossils etc. It can find organics, but cannot easily tell if it was created by life processes, and organics (even with chirality signatures) can be delivered on comets and meteorites or formed by inorganic processes on Mars. And nothing else sent to Mars since the 1970s would be able to spot life on Mars in these low concentrations even if it looked straight at it - or put a sample containing the life into its analysis chambers. Nothing, since Viking, would be able to detect life in the McMurdo dry valleys or the Atacama desert. The Viking labelled release had that capability - but it was controversial for other reasons and for some reason, was never repeated in any later rover to address the issues discovered in the original experiment.
So the search has only just begun. We might find past life on Mars right away - even Curiosity might find it - if we are very lucky. But it might as easily take several decades.
And we have almost no chance of discovering present day life until ExoMars - and that's going to the wrong place to look for present day life so only if it is ubiquitous and easy to find. So - expect it to be some time before we start a proper search for present day life, and again that's likely to take some decades unless we are very lucky.
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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