The South Polar Layered Deposits
While classifying on Planet Four: Terrains images you might have seen images like these:
These banded layers are part of what is known as the South Polar Layered Deposits (SPLD) (an equivalent version exists on the North Pole of Mars as well). There is dust and water ice composing these layers. The SPLD has been measured to have a height of ~4 km and covering a surface area of ~90,000 square kilometers. Unlike the temporary seasonal ice cap which is comprised of carbon dioxide, the 1.6 million cubic kilometer SPLD is mainly water ice mixed with dust comprising on the order of 10% volume of the entire structure. Above this SPLD sits the very thin temporary (1-10 m) cap of amount of carbon dioxide ice/frost that snows out in the winter and sublimates over the spring and summer seasons. In 2011, it was discovered by radar sounding that although the structure is mainly water ice and dust there is a buried reservoir of carbon dioxide ice within a section of the SPLD, with a volume of 9500 to 12,500 cubic kilometers. If this frozen gas was exposed and released in contact with the current Martian atmosphere, the atmosphere would almost double in its inventory of carbon dioxide by mass.
How the SPLD layers are exactly deposited and whether there are new layers actively forming are open questions. The exact age of the SPLD is unknown. We know there is a heavily cratered terrain that covers much of the Southern hemisphere, but there are relatively few craters on the actual surface of the SPLD. This suggests that the top of the SPLD is about 10 million years in age, relatively a youngster in terms of geological time scales. The fact there is banding indicates the existence of a repetitive cycle that built up the SPLD and its northern equivalent over time. The darkness and thickness of the banding is thought to be controlled by the amount of dust present in Mars’ atmosphere (and likely by the past frequency of global Martian dust storms). The SPLD then likely represents a locked away record of the Martian atmosphere at the time of deposition, showing how dusty the atmosphere was compared to the water vapor present. If we can understand the formation mechanism, then we would be able to read this natural historical record.
One other important thing about Mars is that without a large moon like the Earth has, Mars’ axial tilt swings significantly over time up to ~60 degree obliquity, which can put the poles in significant light and darkness. This giant swings in axial tilt (as well as other orbital properties occur in cycles known as Milankovitch cycles). The increased summer and longer winters likely impact dust storms and the formation of the SPLD, but what role it plays is still an active area of study.
The extent of the SPLD has been mapped both with ice penetrating radar and a laser altimeter aboard orbiting spacecraft , so that is why it is not list as of one of the categories to select on Planet Four: Terrains. While we’re not asking you to identify the Solar Polar Layered Deposits in the classification interface, if you’re interested in identifying them in the CTX images you can add a hashtag in Planet Four Terrains: Talk (we recommend #spld ) or you can make a collection of those images.