The Polar Ice Caps

 

Mars may very well have had seas at some point, but those days are long gone. Toxic, salt-covered deserts are now where the seafloor once was and the lack of liquid precipitation makes permanent bodies of open water exceedingly rare. Most water is now either underground or bound in ice.

Like Earth, Mars has quite prominent ice caps on both its poles. Those of the Red Planet must be considerably older, however, as it has been cold throughout its history, while Earth’s poles only froze over in the last few million years. Mars’ ice caps are quite different from each other in size. The northern polar cap is smaller and does not extend past the 90 degrees latitude, while the southern cap is massive and can in some places extend as far north as 50 degrees S. In Aonia Terra it even connects with the massive ice shields of the Tharsis Plateau, creating a continuous tongue of ice all the way into the northern hemisphere. That so much ice is found in the South seems odd at first, for it is the North which is thought to have once housed an ocean, but it makes sense when looking at the topography and climate. As previously discussed, the southern hemisphere is much taller than the northern one and experiences more intense winters. Any water evaporating on the northern hemisphere is therefore more likely to adiabatically precipitate in the form of snow or ice in the southern hemisphere than to rain down again in the North. By this process, the southern ice shield stole away most of the North’s moisture over the eons as the planet cooled down.

 Fig. 2: Extent of the southern and Tharsis ice caps. The small northern ice cap is not shown

The surface of most of the ice caps is nigh uninhabitable, especially in their centres. Only the hardiest of microorganisms tend to survive inside the glacier crevasses, exposed rocks and wind-eroded valleys. Among them are algal scum and a few interesting types of flechtoids, a type of proteroareozoan that resembles a mix between Antarctic lichen and slime molds. Thanks to highly melanized cells they are able to survive even the worst days full of UV-radiation. A series of images captured in 2001 by the ‘98 Mars Polar Lander potentially shows flechtoids slowly creeping across the ice, similarly to slime molds, perhaps a coordinated escape in response to deteriorating conditions. This behaviour has never been observed by human explorers, however. 

 Fig. 3: Starbursts on the southern ice cap.

As one travels farther away from the poles, one comes to the “tidal zone” where seasonal changes finally start affecting the ice shields. A fascinating phenomenon observed here every spring thaw are the so-called “starbursts”, a form of geyser that erupts from the ice and paints it in radial patterns in reddish and brownish hues. These are a product of complex interactions between the atmosphere, the ice and the life inside. When it is summer in the South, it is dust storm season in the North and, as you already know, these storms can often engulf the whole planet. Thus, the ice caps are covered in layers of dust every year, which are consequently covered by layers of ice and snow in the freezing winter. Various organisms, such as phytoareonts, macroareonts and sub-glacial flechtoids, are buried alongside the dust and stay dormant during autumn and winter, but once spring arrives, the dark particles heat up and reawaken the thawed-out organisms. Using the dust as nutrition, they engage in photosynthesis and generate heat through their metabolic processes, creating pockets of liquid water and air inside the ice sheaths. Through rifts and cracks formed by the shifting glaciers, these may connect with each other, creating more complex, though ephemeral ecosystems. Once these pockets connect with the surface, a geyser forms due to the significant difference in air-pressure between the atmosphere inside and outside the ice. In quick bursts, dust, water and microbes are spread across the glacial surface, creating dark coverings that in summer might heat up enough to create small pools. Most organisms, however, either do not survive the ejection or enter a dormant state. All of them enter dormancy once the days grow shorter, for autumn and winter last 370 sols, longer than a year on Earth.

Most fascinating from a zoological perspective are of course the many sub-glacial lakes dotted beneath both ice caps. Most of them are inhospitable. Created by pressure-melting at the very sole of the ice, they are kept liquid at sub-zero temperatures only by their extremely salty perchlorate-mixture. In some places, however, topographical or geothermal features lead to the creation of lakes with amenable conditions for macroscopic life. The water is still cold, but liveable, and not much saltier than what one might find in the Arctic oceans of Earth. In some places they even consist of freshwater. The most productive lakes are those with only a thin ice sheet protecting them, as the flora here can engage in photosynthesis for part of the year, being fed by the nutritious dust that continuously erodes out of the overlying frost. In summer these may even thaw open, allowing for interactions between the surface and sub-glacial life. Many other lakes are under ice sheets too thick and layered with dust to let in much light. Here life makes do with chemotrophy and is adapted to the darkness, much like deep sea life on Earth. It may not sound like heaven, but at least it is not hell.

Most of the lifeforms so far discovered in the lakes resemble the phyla also known from the surface, though a few are entirely unique to this habitat. There is also a considerable difference in the faunal composition between the northern and southern ice cap. The debate is still ongoing as to whether the sub-glacial lifeforms represent the last remnants of marine ecosystems from Mars’ long-gone aqueous phase or if they are merely former surface or sub-terranean creatures that have invaded the ice lakes from above or below in more recent times. The former situation seems plausible for the northern cap, but not for the southern one.

Image Sources:

• Fig. 3: Jet Propulsion Laboratory