Potentially habitable flow-like features from Martian dry ice geyser dune spots



These features near the Martian southern polar region are associated with the Martian Geysers. Before these geysers were well understood, there was a lot of speculation about what they might be. The seasonal patterns they form resemble trees and vegetation, and in 2001 looking at the Mars Global Surveyor images, Arthur C.  Clarke called them "Banyan trees", saying, only half joking "I'm now convinced that Mars is inhabited by a race of demented landscape gardeners,". At around the same time, a team of Hungarian scientists proposed that they might be the result of spreading colonies of overwintering photosynthetic microbial life.

Most of these patterns are now thought to be due to dry ice effects. Subsurface layers of dry ice are heated by the sun through the solid state greenhouse effect, and erupt as CO2 gas. The dark streaks, and spots are thought to be debris from the geysers, blown by the CO2 outgassing. These streaks are the "flow-like features", or FLF. What's interesting for the search for habitable brines is what happenes next. As Renno and Martinez put it

"There is mounting evidence that while dark spots and FLF form by “dry” gas venting, liquid brines form temporarily on them."

This would happen later in the spring and through to the summer. The dark streaks from the geysers begin to extend further down the slopes, sometimes at a rate of meters per day. There are streaks in both hemispheres but the details of how they form differ.

In the Southern hemisphere, they form in the debris of the geysers, and both of the current models for this part of the process involve liquid water. In one of these modelsfresh water that forms as subsurface meltwater, insulated from the surface temperatures and pressures at 0°C below snow-ice packs. These are optically thin in visible light but opaque to thermal infrared, so trapping heat from one day to the next in a solid state greenhouse effect familiar in similar situations in Antarctica. . The other model involves thin layers of ULI water (undercooled liquid water) which form on the surface of solar heated grains, then flows downwards, supplying several litres of water per day to the features. In both cases they then pick up salts from the debris from the geysers, which let them remain liquid in the cold near surface conditions as they flow down the slopes.

The northern hemisphere flow like features begin as wind-blown features on steep slopes. They start to extend later in the year, similarly to the southern hemisphere features. However, if they involve brines, the temperatures are far lower, with surface temperatures around -90 °C, though in the models that involve water, the brines themselves would be at warmer temperatures than the surrounding dry ice. Also, though most of the models for the northern hemisphere features involve water, they can also be explained with dry ice and cascading dust.

The southern hemisphere Richardson crater flow-like features are the ones of most interest for brines at temperatures within the range of habitability for Earth life (life based on novel biochemistry based on perchlorates or hydrogen peroxide in the place of the chloride salts of Earth life might tolerate or prefer lower temperatures .).

Southern hemisphere flow-like features
The process starts with the dark dune spots which form in early spring. Here are some examples in 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.

Solid state greenhouse effect model
Möhlmann uses a solid state greenhouse effect in his model, similarly to the process that forms the geysers, but with translucent ice or snow-ice packs, rather than dry ice as the solid state greenhouse layer.

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 cm 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.

This provides:


 * A way for pure water to be present on Mars, and to stay liquid under pressure, insulated from the surface conditions.
 * 5 to 10 cm below the surface, trapped by the ice above it
 * Depending on conditions, the liquid layer is at least centimeters in thickness, and could be tens of centimeters in thickness.
 * Initially of fresh water, at around 0 °C.

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 is 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.

Interfacial liquid layers model
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.

Northern Hemisphere flow like 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. They start as dark spots, with the flow like features 25 - 100 meters long and 2-10 meters wide emanating from the same slopes as the dark spots, thought to be wind-blown features - but then like the southern hemisphere features, they start to extend down the slopes. They do this at a rate of 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 both 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. For details see the Dark Dune Spots section of Nilton Renno's paper which also has images of the two types of feature as they progress through the season.

Earlier hypotheses
In 2003 a team of Hungarian scientists proposed that the dark dune spots and channels may be colonies of photosynthetic Martian microorganisms, which over-winter beneath the ice cap, and as the sunlight returns to the pole during early spring, light penetrates the ice, the microorganisms photosynthesise and heat their immediate surroundings. A pocket of liquid water, which would normally evaporate instantly in the thin Martian atmosphere, is trapped around them by the overlying ice. As this ice layer thins, the microorganisms show through grey. When it has completely melted, they rapidly desiccate and turn black surrounded by a grey aureole. The Hungarian scientists suggested that that even a complex sublimation process was insufficient to explain the formation and evolution of the dark dune spots in space and time. Science fiction writer Arthur C. Clarke promoted these formations as deserving of study from an astrobiological perspective.

In 2009 a multinational European team suggested that if liquid water is present in the spiders' channels during their annual defrost cycle, the structures might provide a niche where certain microscopic life forms could have retreated and adapted while sheltered from UV solar radiation. British and German teams also consider the possibility that organic matter, microbes, or even simple plants might co-exist with these inorganic formations, especially if the mechanism includes liquid water and a geothermal energy source. However, they also remarked that the majority of geological structures may be accounted for without invoking any organic "life on Mars" hypothesis (See also: Life on Mars.)