Where should we go, on Mars, to look for droplets and streaks of present day liquid water? You may have heard of the "warm seasonal flows", and the recent "swimming pools of bacteria". 

However,  there are several other promising ideas for habitats such as the "Flow like features", the advancing sand dunes bioreactor, and possibilities for life using the humidity of the night time air on Mars. It's an exciting field with many new discoveries and ideas every year, and it is hard to keep up with the developments.

Let's start with the Flow Like Features. These are less well known than some of the other candidates but rather interesting, as the best explanation involves melt water at 0°C, warm for liquid water on Mars, also (at its source) pure fresh water, unusually for Mars.

One of the Flow Like Features which form briefly in spring in the south polar region around dark dune spots inside Richardson crater. These may be caused by liquid fresh water trapped under ice and melted by the solid state greenhouse effect, details below.

Animation Credits: Collegium Budapest, Mars Astrobiology Group 

This article is based on Nilton Renno's survey article from 2013,  Water and Brines on Mars: Current Evidence and Implications for MSL updated with more recent results. It covers all the main habitats I know of, for the surface of Mars, and the shallow sub surface (top two or thee cms down to just below the permafrost layer)

FLOW LIKE FEATURES ASSOCIATED WITH THE DARK DUNE SPOTS

These features are associated with the dark dune spots on the polar dunes, so let's introduce those first. 

These look quite a bit like trees and vegetation don't they! Famously, Arthur C. Clarke called them "Banyan trees" and wrote, only half joking

"I'm now convinced that Mars is inhabited by a race of demented landscape gardeners," 

See Popular Science email interview 2001 "The Banyan Trees of Mars"

However most of this is due to non life processes almost certainly. Just about everything in this image is probably the result of dry ice effects. See On Mars, Dry Ice 'Smoke' Carves Up Sand Dunes

The interesting features for life here are tiny details just a few tens of meters across - and you only get them at certain times of year, in the spring around the dark dune spots as they begin to decay. 

How this works differs depending on whether you are in the warmer North polar region or the far colder southern polar region (the difference here is due to Mars's highly elliptical orbit).

HOW THE DARK DUNE SPOTS AND LOW LIKE FEATURES WORK IN THE SOUTHERN POLAR REGION

We are all familiar nowadays with CO2 as a greenhouse gas in the atmosphere. Well, when it's in the form of dry ice, as it is in the Southern polar cap, it creates a "solid state greenhouse" as well, and that seems to be what happens here on Mars.

The sun shining through these thick layers of translucent dry ice heats up lower layers which turn directly into gas (because that's what dry ice does as it warms up in these conditiobs). It's trapped under the layers of dry ice, so forms a gas pocket.

Pressure builds up until eventually the sub surface gas pocket "explodes" to the surface as a geyser.

Here is an artist's impression

And this is what we see from orbit:


Dark dune spot in Richardson crater, S. polar region of Mars.

This happens at temperatures far too low for water. So is no doubt, this is a dry ice effect.

However, as it continues through to spring, as the surface gets warmer, in the second, third and fourth images, then that's when it's possible that water may begin to flow.

What we see from orbit are "Flow Like Features" or FLF for short, which you can see here, gradual progression from left to right in the sequence of images. This is in Richardson Crater again, in the South polar region.

They are small features only a few tens of meters in scale, and grow at a rate of around 1.4 meters per Martian sol.

Here is a high resolution animation of one of them through a Martian spring.

Credits: Collegium Budapest, Mars Astrobiology Group 

All the models to date involve some form of water.

The most interesting one for us though, is the one that involves melt water.

Möhlmann models these using a solid state greenhouse effect, but this time one that involves translucent ice rather than dry ice as the solid state greenhouse. 

The way it works is, according to his model - that the ice forms a thin translucent layer - and as it warms up in summer - then the ice melts a layer of water beneath it, a process that is reasonably well known on the Earth.

This picture shows ice melt in Antarctica by solid state greenhouse effect - the surface stays frozen, and deeper down, it melts, in a layer about 0.5 m deep - because the translucent ice above it acts as a greenhouse. Taken from: Melting, runoff and the formation of frozen lakes in a mixed snow and blue-ice field in Dronning Maud Land, Antarctica

In his model the ice, even in the polar regions, reaches 0°C, enough to melt ice to water. Then as it flows down the slope, and cools down, it mixes with salts to form salty brines.

On Mars this process is especially interesting, as it is a way for the water to stay liquid under pressure. After all, the dry ice geysers have CO2 gas sublimate under enough pressure to escape from the surface as geysers.

Another model for these features involves ULI water (undercooled liquid interface water) which then feeds brines that form the feature. This is a thin layer of water over the surface of something, which can melt at a lower temperature than the melting point of ice.

If it is indeed subsurface melt water, the source of the water is particularly warm at 0°C  - making the flows quite a promising habitat for life on Mars.

HOW THE FLOW LIKE FEATURES FORM IN THE NORTHERN POLAR REGION

Then - a slightly different process seems to form flow like features in the Northern polar region, this time using the solid state greenhouse effect in dry ice to create the brines themselves:

The flows increase at between 0.3 meters and 7 meters a day. The feature marked with the arrows grows at a rate of about 5 meters a day.

Here, the features form at around -90°C for the surface temperatures. You might think that is too cold for liquid water, even as brines. But in their models they did get liquid water beneath a layer of dry ice - as the thermal infra red penetrates the dry ice. The orbiting satellites are not able to see this increase in temperature because their resolution is too low at this wavelength to pick out such small features amongst the much colder surrounding dry ice landscape.

One of the research papers on these features postulates dry ice and sand cascading down the slope but most of the models involve liquid brines.

For details see the Dark Dune Spots section of Nilton Renno's paper Water and Brines on Mars: Current Evidence and Implications for MSL

WARM SEASONAL FLOWS

These, again, are exceedingly rare. Each asterisk on this map shows one of the sites



Sixteen sites in total marked, over entire surface of Mars. See Water seems to flow freely on Mars

So there aren't many of them. And the habitats themselves consist of just thin streaks on the slopes like this

There are many other features discovered on Mars such as the dry gullies (dry ice phenomenon most likely). So is important not to confuse them, as often similar looking features have very different explanations.

The warm seasonal flows are easily confused for instance with the linear gullies 


Not to be confused with the Warm Seasonal Flows - these "linear gullies" are probably caused by tobogganing blocks of dry ice, see Grooves on Mars may be result of blocks of dry ice sliding down slopes

Here is a movie of the warm seasonal flows - notice how they are much flatter - no 3D structure - just a darkening of the slope basically, also they change slowly and gradually through the season - this video goes through a Martian year.

They get gradually longer in the spring. In autumn they fade away. 

Why this is, nobody knows. It's not likely to be damp sand directly, because there wouldn't be enough water to make it as dark as this. So some secondary effect of some sort.

Still, seems it must be caused by water indirectly, almost certainly.

That's because of the temperature, far too hot for dry ice, and because there is no correlation at all with wind, or dust - and the seasonal nature of it. Researchers have simply run out of other plausible hypotheses. The only one that makes much sense is water - though that one is also quite hard to fit to the phenomenon.

The problem with water is that it needs to be replenished. Even tiny amounts of water like this - if it's been going on for millions of years, then the reservoir of ice or water would get exhausted, at the top of the slope. There is no rain - no mists here either (those occur at lower elevation). There seems to be no way for the water to get back up to the top of the slope. So how did it get there in the first place?

Perhaps it could have got there tens or hundreds of millions of years ago during one of the periodic changes of the Mars climate (this happens because of changing axial tilt and changing orbital eccentricity etc). But that's so long ago - is surprising there is any water left.

Perhaps it's something to do with deliquescing salts? Perhaps it comes, originally, from the high humidity in the night time atmosphere on Mars. After all you don't need that much water to feed the streaks, probably.

Or perhaps it comes from deep below the surface? Maybe these are places where the land is somewhat warmer, a geological hot spot? In that theory, the ice rises, sublimates, freezes, sublimates, and freezes, perhaps all the way to the surface from the deeper hydrosphere, over a geological hot spot.

Whatever the explanation, another puzzling thing is - that you get apparently geologically identical spots, and some have these streaks and some don't. Again, nobody knows why that is.

It could be perhaps because of subsurface salt deposits - at the head of the streaks (just a short way below the surface) - or geological hot spots - or some other unusual local phenomenon, which somehow feeds new water (in tiny quantities) to the heads of the streaks each spring. Our orbiters can't distinguish between these hypotheses at present.

Nobody really knows - one of many puzzles about Mars :). 

There's another rather similar phenomenon, the dark slope streaks


Again, not to be confused with the Warm Seasonal Flows - these "dark slope streaks" are probably caused by avalanches of dry sand, maybe combined with some ice and water effects

They look quite similar to warm seasonal flows, don't they? But they are much broader, and to experts in the field, only superficially similar.

They may be caused by avalanches of dry sand, perhaps in combination with some ice / water effects.

GULLIES

Mentioning these for completeness as, though probably dry ice phenomena, for a long time these were thought to be liquid water, and perhaps it is not yet totally settled, that some of them could be due to liquid water.

This is an image which suggested ground water because the gully seems to be associated with a fault, a likely place for ground water to seep out. NASA - Groundwater May be Source for Erosion in Martian Gullies

However recent observations of newly formed gullies strongly suggest dry ice, as probably the only explanation:

This pictures shows a newly formed gully on Mars. It's hard to see how dry ice can form gullies like this, but new gullies form in winter at temperatures that seem too low for water to melt. So - dry ice seems to be the preferred explanation. Possibly some kind of flow of the soil mixed with gas and dry ice.

DROPLETS OF WATER ON INTERFACE OF SALT AND ICE

This is a new idea for a habitat suggested earlier this year. But its origins trace back to the discovery of the drops on the legs of Phoenix back in 2008. It's especially exciting for habitability as we'll see.

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They seem to be drops of water because the drops grew at a rate depending on their initial volume as expected for droplets of liquid (similarly to the way droplets form in a cloud), they darkened before disappearing, and after they disappear then no more drops formed. Though they weren't observed to fall off, the assumption is that they did and took the salts with them.

So - originally the model used here was of deliquescing salts - salts taking in the moisture from the atmosphere. Though Mars' atmosphere is almost a vacuum, still it has some water vapour, and because of the huge temperature fluctuations, it reaches 100% humidity at night. 

But Nilton Renno and his team later tried another idea, droplets forming on the interface between perchlorate salts and ice. That's what lead to his "swimming pools for bacteria" announcement recently.

""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)"

See also Martian salts must touch ice to make liquid water, study shows

For his paper see Experimental evidence for the formation of liquidsaline water on Mars 

This is the Mars simulation chamber they used for their experiments

These ideas are really exciting for potential for life on Mars. Both perchlorates and ice are abundant on Mars. So there may, as he says in his video, be Phoenix type droplets scattered over large areas of Mars. Just a few droplets of water here and there, and for a few hours a year when the conditions are right - but each one would, as he said, be a swimming pool for bacteria.

TRANSGRESSING SAND DUNES

This is another idea, not mentioned in Nilton Renno's survey, published around the same time last year, HABITABILITY OF TRANGRESSING MARS DUNES which suggests that the sand dunes churn up salt layers and bring them to the surface - where they might then provide habitats for life by deliquescence.

Here is an image of dunes moving on Mars.

Rippling Dune Front in Herschel Crater on Mars

This rippled dune front on Herschel Crater on Mars movedf about one meter between March 3, 2007 and December 1, 2010 - and the pattern of ripples changes completely between the two images. Herschel Crater is just south of the equator in the cratered highlands. (More animations of moving sand dunes on Mars)

This habitat is unusual as it is one of the few suggestions for ways you could have life on Mars even in the dry equatorial regions where Curiosity is right now.

VIKING EVIDENCE AND GILBERT LEVIN'S EXPERIMENT

Gilbert Levin of course remains sure that Viking discovered life. And, he is no longer a "lone voice" saying this, a few others have started to wonder if just possibly they did.

There are intriguing signs - the recently discovered diurnal variations in the signal. See my Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know

If that is life - then that means life at Vikings location - which is surprising as it is very dry - but does have its morning frosts. And does have night time humidity 100% obviously.

If otherwise habitable, sand dunes seem interesting as a way to supply nutrients to life, because it gives an opportunity for materials to be churned to the surface from deeper deposits.

Then, Nilton Temmo's "swimming pools for bacteria" - they could be anywhere on Mars so including in the sand dunes, so long as it is far enough from equator to have ice, and so long as there are salt deposits as well. 

One thing I wondered, just a question, I don't know the answer - might they also happen on the interface between the Martian frosts and perchlorate salts on the surface? Those happen in equatorial regions too.

DLR EXPERIMENTS

There are the DLR experiments also, with lichens and cyanobacteria able to continue to metabolize almost anywhere on Mars - though not done very long experiments yet last time I read about it, months rather than years of Mars surface conditions simulation. And is a major challenge to simulate everything we know about surface of Mars in a laboratory. But - if they are right - then there could be life anywhere including equatorial regions and so in the sand-dunes - again depending on availabiity of nutrients.

phoenix-globs-02.jpg (650×258)
This polar lichen photosynthesized and metabolized normally through 34 days of simulated Mars - duration of experiment (credit DLR)

Remarkably the lichens they studied can metabolize and photosythesize right down to -40°C! See the end of this talk

They can survive partial shade right on the surface in simulated Mars environments.

Perhaps they might occur on Mars, as Endoliths - microbes living within rocks, similar to this, with partial sunlight filtering through the rocks:

image
Cryptoendolith inside rock in Antarctic

EXPOSE E EXPERIMENTS

This is an on going series of experiments on the ISS to test microbes under space and Martian conditions.

Expose E Experiment on the ISS. Used for some of the Mars UV and cosmic rays simulations.
image

In one experiment (2011), C was exposed to simulated space, and Martian conditions for 1.5 years. Survivors from a strain taken from the coastal desert in Chile (Chroococcidiopsis sp. Ccmee, 123) were able to form colonies, and repair DNA damage on rewetting after 1.5 years in space or martian conditions.

A new series of experiments launched in July of this year, Expose R2

"BIOMEX contains numerous chambers that are filled with biomolecules and organisms that include bacteria, archaea, algae, fungi, lichens and mosses. Replicate samples spread across the compartments are subjected to a range of environmental conditions. Some samples of each biomolecule or organism are embedded in a simulant Mars soil (ranging from just a single layer of soil to multiple layers), and other samples are left on their own to face the space environment without protection"
BIOMEX: Exploring Mars in Low Earth Orbit

MARS ANALOGUE LIFEFORMS

Some extremophiles on Earth can probably live on Mars just as it is. The DLR experiments used ChroococcidiopsisThis is especially interesting as it sometimes forms single species ecosystems beneath rocks in the Atacama desert and Antarctic McMurdo dry valleys. All it needs is water, carbon dioxide, sunlight, and trace elements and it creates everything else for itself. Has a wide range of different metabolic pathways it can use in different situations. It can fixate nitrogen - though on Mars there isn't much nitrogen in the atmosphere, so it probably needs a source of nitrates.

This is an extraordinarily ionizing resistant microbe, able to withstand tens of thousands of years of dormancy on the surface of Mars and repair the damage from ionizing radiation within a few hours. It can repair, 2.5 kGy of damage  within 3 hours given the opportunity to wake up for a few hours and metabolize.

Here a kGy is a thousand Grays, and a mGy is a thousandth of a gray. So 2.5 kGy corresponds to 2500/0.076 or over 32,000 years of radiation on the surface of Mars

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Chroococcidiopsis cyanobacteria photosynthetic primary producer, forms single species ecosystems on Earth and probably suitable for Mars 

Another single species ecosystem which could exist on Mars, is Desulforudis audaxviator the only known ecosystem to rely on radioactivity (indirectly) as a source of energy. All it needs is radioactivity, sulfide minerals, water, nitrates and and carbon dioxide. So it could find those on Mars.

Then another single species ecosystem, the red halobacteria (or haloarchaea as more properly called).


Halobacterium at a salt works near San Quentin, Baja California Norte, Mexico. Credit: University of California Museum of Paleontology.

Various salts (mainly sulfates, chlorates and perchlorates) are common on Mars, and it could cope with those. Photosynthesizing so can get energy from sunlight. Has an advantage that it often forms a thin crust of salt which would help filter out the UV light. Also the salt itself has deliquescing properties and in the Atacama desert, haloarchaea are able to survive without any liquid water at all, just using the deliquescing capabilities of the salt.

Then, some halite endoliths can manage even at less than 100% humidity in the extremely dry core of the Atacama desert.


Novel water source for endolithic life in the hyperarid core of the Atacama Desert. In this picture, the salts are show in context in the top photograph (a). In b, then the colonized layer is marked Co.. In c, then the life is highlighted through fluorescent microscopy (red autofluorescence). In d, staining of the DNA shows the bacteria that live by eating the cyanobacteria (this particular specimen isn't a single species ecosystem).

They seem to do it through micropores in the salt deposits. Theoretically, these could take in water from the atmosphere at relative humidities as low as 50-55% through capilliary water condensation. The researchers used miniature weather stations inside the pinnacles, and were able to observe this happening, that the deposits became damper as soon as the relative humidity of the atmosphere went above 50-55%.

I haven't seen studies like this for Mars, but the potential for micropores in the salts to assist with capture of water vapour from the atmosphere seems well worth studying..

PROBLEM OF NITRATES

Of the main elements needed for life on Earth, the one element in short supply on Mars is nitrogen. There is some in the atmosphere but only 0.2 mbar. Can life use that? Or is there some other source of nitrogen on Mars?

Nitrogen is essential to life on Earth. What's more it's likely to be needed by XNA based life also. Nitrogen is so vital for life that exobiologists have suggested it's as important for our rovers to "follow the nitrogen" as to follow the water. 

It's important because nitrogen mediated hydrogen 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 (see Alien life could use an endless array of building blocks) and perhaps use PNA or some other form of XNA (Xeno nucleic acid) with a different backbone from DNA, still it is likely to use nitrogen if it resembles Earth life, even remotely.


Diagram of helical structure f a protein, showing how the nitrogen mediated hydrogen bonds hold it together

How Nitrogen mediated hydrogen bonds are used in DNA

For more, see Searching for Organics in a Nibble of Soil. Diagrams credit J. Bada.

See also Seeking signs of life on mars: in situ investigations as prerequisites to sample return missions which goes into the importance of the search for nitrogen on Mars, as well as other considerations to guide our search there.

Nitrogen can be fixed by volcanic processes and lightning. It can also be delivered on meteorites, as some carbonaceous meterorites are rich in nitrogen

Another idea suggested recently is nitrogen fixation in pure water metastable thin liquid films (different from the salty brines habitat) on Mars (cooled below the point where they would normally freeze). See An active nitrogen cycle on Mars sufficient to support a subsurface biosphere

Another source of nitrogen - it might have deposits of nitrates from the early Mars. The early Mars atmosphere was probably nitrogen rich like Earth, and various processes such as meteorite impacts could have created deposits of nitrates. I've got a section about that in Where Should we Send our Rovers to Mars to Unravel Mystery of Origin of First Living Cells?

These might be deep underground. For instance some of the worlds richest deposits of nitrogen are in the Atacama desert, and for a long time supplied the global fertilizer trade - but they are deep underground. You can still find ruins of nitrate mines in the Atacama desert.


Abandoned nitrate factory (Saltpeter) in abandoned city of Humberstone (Chile) - before the widespread use of artificial fertilizer, the mines in the Atacama desert were an important source of fertilizer. 

The desert soils here are rich in nitrates below the surface, though there is little sign of them on the surface itself. Calculations show that the nitrates could have formed over a period of about 10 million years brought in on occasional fogs and rains

According to recent research, 
possibly they originated in groundwater from marine deposits instead - but whatever the origin of the Atacama deposits, the original  idea of atmospheric origin suggests the possibility of similarly thick deposits forming on early Mars through atmospheric processes in as short a time period as ten million years.

Deposits like that would be hard for our rovers to find on Mars, since they can't dig below the surface, to date anyway.

Nitrates can also be delivered by meteorites.

These ideas for nitrates on Mars were just a hypothesis until last year -when Curiosity discovered the first conclusive evidence of nitrates.

So - it does have nitrates - patchy surely, low levels - but are present. And remember Curiosity can't drill even cms below the surface.

The McMurdo dry valleys and the Atacama deserts are similarly challenging in this way also - not much at all by way of nitrates - unless a micro-organism is nitrogen fixing.

There is a small amount of nitrogen in the Mars atmosphere,  0.2 millibars. One study found that microbes could continue to do nitrogen fixation at 5 millibars, but they found no evidence of fixation below 1 mbar. This was done at Earth normal atmospheric pressure. This is a 1989 paper: Biological nitrogen fixation under primordial Martian partial pressures of dinitrogen. 

Can any extremophiles handle nitrogen fixation in the Mars atmospheric condtions? I've asked around but nobody seems to have studied this. 

An obvious follow up experiment would be to do the same tests, but with Antarctic nitrogen fixating extremophile micro-organisms. The authors of this paper propose the experiment to do that, but as far as I can see, haven't actually done it.

If anyone reading this knows of any studies of nitrate fixation in Mars analogue atmospheres (especially, not just low partial pressure of nitrogen, but of the CO2 also) - would be really interested to hear about it - either way - whether it is possible or likely to be impossible.

However one way or another seems at least possible that surface life on Mars may have access to enough nitrogen, at least in places, at levels similar to nitrogen poor dry deserts on Earth.

INDIRECT EVIDENCE FOR SURFACE OR NEAR SURFACE WATER IN RECENT GEOLOGICAL PAST

Of course, we haven't yet conclusively found water in any of these habitats. The features are too small to analyse from orbit with the instruments we have at present - especially since the water is likely to be present in small quantities. 

For instance, with the warm seasonal flows,  - you might think the dark patches are simply sand darkened with water. But - it's not likely that there is enough water for that. Rather is some other effect of the water interacting with the sand. So most of what you see there is still sand. Most probably salty water involved in some way as that's the only way to explain the dependence on temperature and seasonal lengthening. So - think of it more as a seasonal slight seep of a thin layer of salty water, with associated darkening effect on the sand. They are thin anyway. So obviously going to be a challenge to detect the spectral signature of water from orbit, and so far this has not yet been achieved.

But we do have indirect evidence that there is liquid water on the surface of Mars, in the geologically recent past.  The measurements don't tell us if it is there all the time, or episodically, but shows it is there at least sometimes.

This evidence came about thorugh a reinterpretation of the Phoenix lander result sin 2010 by Paul Niles, a space scientist at NASA's Johnson Space Center in Houston,. He found intriguing indirect evidence that CO2 in the atmosphere has interacted with water in the geologically recent past.

The low gravity of Mars favours loss of carbon-12 to space compared with carbon-13. The ratio of carbon-13 to carbon-12 measured by Phoenix is at a level that suggests that the atmosphere of Mars must have been replenished from volcanoes recently. The proportions of the isotopes of oxygen-18 and oxygen-16 in the CO2 however show a signature characteristic of oxygen that has had a longer period of time to get lost to space. This suggests that the CO2 must have reacted with liquid water in the geologically recent past.

" Niles and his team theorize this oxygen isotopic signature indicates liquid water has been present on the Martian surface recently enough and abundantly enough to affect the composition of the current atmosphere. The findings do not reveal specific locations or dates of liquid water and volcanic vents, but recent occurrences of those conditions provide the best explanations for the isotope proportions."

This may be an indirect observation of these habitats with liquid water on the surface of Mars

WHY WE NEED TO GET UP CLOSE TO SPOT ANY LIFE - NEGLIGIBLE EFFECT ON ATMOSPHERE

With the rarer types of habitat here, there may be no more than a few square kilometers of habitat over entire surface of Mars, for a few hours a year.

If it was all photosynthetic life, and as active as life in the ice covered ponds of the McMurdo Dry Valleys of Antarctica, the oxygen produced would contribute at most a few hundred kilograms of gases per surface square kilometer of habitat.

So, if the entire surface of Mars is habitat (which surely it isn't), that contributes less than 0.0002% of the Mars atmosphere if left to build up over a "residence time" of 4,500 years. That compares to a measured 0.145% of oxygen in the atmosphere - so any seasonal effect is likely to be hidden in the noise. For the calculation, see my How Life May Exist On Mars With Atmosphere Close To Equilibrium

Also other gases such as methane and ammonia - likely to be well below the parts per billion detection thresholds - unless the life is very abundant on the surface. 

The methane plumes, if they are confirmed, and are signs of life - they are more likely to come from a more prolific deep down habitat than these ones.

PRESENT DAY LIFE ON MARS LIKELY TO BE HARD FOR ROVERS TO LOCATE

One thing that has come out of the field expeditions to the Mars analogue habitats on Earth - that life is likely to be hard to find. Even once you've identified potentially habitable areas of Mars, then, if there is life there, it may take a long search to find the life itself within that area, and at first, and even after a fair bit of searching, it may seem totally lifeless.

Clearly this is a task for mobile rovers rather than for sample return (this leads into my next article which is going to be on the value of in situ exploration on Mars).

Quoting from Desert Cyanobacteria and the Terrestrial Analogues of Mars in the 2013 survey paper Cyanobacteria from Extreme Deserts to Space

In the McMurdo Dry Valleys in Antarctica and in the hyper-arid core of the Atacama Desert in Chile rockinhabiting communities are dominated by cyanobacteria, mostly belonging to the genus Chroococcidiopsis; the study of these communities is gathering appreciation in astrobiology since they are considered the closest terrestrial analogous of two Mars environmental extremes: cold and aridity [19]. Field expeditions have also taught us that life is not obvious at large scales but rather rare in isolated islands amidst a microbially depauperated bare soil, thus suggesting that if life ever existed or exists on Mars, microhabitats would probably be widely dispersed among virtually lifeless surroundings 

CONCLUSIONS?

Well, we can't draw any conclusions yet.

If there is water on the very surface of Mars or the shallow subsurface (the top two or three centimeters above the permafrost layer) - then it is hard to spot from orbit.

Our orbiters just don't have instruments able to detect the water signatures of a few drops of liquid on the interface between ice and salt, or the shallow seeps of a few mms of slightly damp sand that may form the Warm Seasonal Flows, or the Flow Like Features. 

The Flow Like Features at the South Pole - if they are indeed ice melt-water features due to the solid state greenhouse effect - would be dramatically hotter than the surrounding landscape. But again they are so small scale, and shallow, that our orbiters currently have no hope of detecting that change of temperature.

Closest to direct observation, the droplets on the legs of the Phoenix lander. If those did form on mixtures of salt and ice thrown up during the landing - surely this is a phenomenon that can't be unique to the landing of Phoenix. From time to time the salts and ice must come into proximity in a similar way and form droplets.

But we'll only know with enough surety to satisfy scientists if we send landers to visit these places and look at them close up. And if we do find and confirm liquid water on Mars, it may take some time after that before we find life there, if there is life. In Antarctica, many apparently equally habitable spots are uninhabited, with the life just in occasional patches here and there. Same may well be true on Mars.

ExoMars in 2018 will visit the equatorial regions. It just possibly might find present day life also, especially if it is as abundant as the Viking labelled release experiment suggests to some.

Apart from that, it's surely going to be well into the 2020s before we have a chance to get an answer to any of these questions. The successor to Curiosity will not carry any life detection instruments, and also, will not target any area of Mars likely to have these surface liquid habitats for present day life. It's not going to be sterilized sufficiently for that, apart from any other considerations.

Perhaps some successor to ExoMars? Or maybe one of the other countries interested in Mars space exploration will surprise us?

FUTURE ARTICLES

  • "What We Could learn from Microbial ETs on Mars" about how much we could learn from ETs there, even if they are only microbial.
  • "How does In Situ exploration of Mars compare with sample return?" - suggesting, that perhaps it is worth doing a proper comparision study of the value of in situ exploration compared with a sample return. The only one I know of came out strongly in support of in situ exploration for exobiology, so I'll talk a bit about why that is. Also about the safety issues that need to be considered if we ever do return a sample from Mars.

Not sure which of those articles will come first.