Nature, August 2006. CO2 Jets on Mars:

Scientists have now found a better explanation for the dark spots and fans found on the south polar region of Mars. These spots, which can cover tens of square kilometers, are visible in the cryptic region. Originally, it was suggested that the spots were soil, exposed after early defrosting of the polar ice. However, an intensive new study with the Mars Odyssey Thermal Emission Imaging System (THEMIS) shows that the temperatures of the spots are inconsistent with expected temperatures of soil. Instead, the dark spots are within a few degrees of solid CO2 (~ -130°C). Soil, even when bordered by CO2, is greater than ~ -50°C. This suggests that the dark spots exists either on top, underneath, or inside of a layer of CO2. The previous suggestion that the spots are defrosted, CO2 ice free regions, no longer seems plausible.

Artist's rendition of what the Cryptic region might look like
This is an artist's rendition of what the Cryptic region might look like if one was standing on the surface of the polar cap. The cryptic region is believed to be an impermeable, translucent layer that allows sunlight to shine all the way through to the underlying soil. The sunlight heats the soil under the ice, leading to sublimation of the dry ice from the base. The sublimation that occurs under the ice results in pressures at the base of the impermeable ice slab that lifts the ice. Eventually the ice slab cracks and allows the built up gas to escape as a jet. The gas coming out of these jets can reach speeds 30 feet per second (20mph). The jets carry dust and dirt from underneath the ice slab up into the atmosphere. The dirt and heavier dust fall back to the ground forming a large dark circle around the jet while the lighter dust is carried down wind. See the full size image (6.3 mb)
- - -
MRO THEMIS web site
Artistic credit to Arizona State University/Ron Miller

Both the north and south polar caps of Mars exhibit great change throughout the year. The southern cap though is privy to a spotting anomaly. These spots, which are often associated with fans and blotches, are dark regions that form in the southern spring and summer. Radial channels, called fans, which are much like the legs of a spider, are often collocated with the spots after the CO2 ice disappears. Oddly though, the fans remain at near CO2 ice temperatures into the spring. This suggests that the fans must actually be thinly deposited soil on top of the ice. Based on the thermal conduction of Martian soil, the deposits must be ~1mm thick. Any thicker and the soil would warm from sunlight faster than it can dissipate the heat into the underlying ice. These deposits are consistent with local wind patterns, suggesting that the soil is erupted from CO2 jets and is carried by the wind to form the fan pattern.

Artist's animation of the process of jets and spider formation
This animation is an artist's rendition of the process that probably creates the CO2 jets and spiders.

CO2 jets likely form in the cryptic region from sublimation of annealed CO2 ice on the border between the ice and the soil. The sublimed CO2 gas then builds in pressure, lifting the ice above. At some point, it manages to crack the ice, escaping in the form of a jet, while also taking along some of the soil for the boundary where it sublimed. The depression left from the excavated soil, works as a primer for the following year. This means, that because of the previous erosion, spots will typically form again in the same location.

More information is available at the following web sites: