Images sent back from the surface of Mars can make the planet look like the hot dry deserts of Earth. However, in reality Mars is incredibly cold. The average surface temperature is 210 kelvin (-63 °C or -82 °F) and the minimum temperature is 130 K (-143 °C or -226 °F). While many of the processes and landscapes we see on the planet have equivalents on Earth this extreme cold means that Mars also has some bizarre features that we have no direct comparison for. Dark spots and strange spider like channel networks are seen to form predominantly in the southern polar region of Mars year after year. They have been interpreted as the result of jets of carbon dioxide erupting out of the winter ice sheets as they start to defrost during the Martian spring. Such a dynamic and violent process is not something you would initially expect from what is generally viewed as currently being a quiet and well behaved planet. Kieffer et al. wrote a paper in 2006 which outlined the possible formation mechanism for these strange features. This article is a summary of their work and I’ve put a link at the end.
Because Mars’s atmosphere is carbon dioxide rich and the Martian winter reaches such low temperatures the polar regions become covered in a thin layer of carbon dioxide ice until the sun rises in the spring. The dark patches that appear as the sheet starts to warm were originally thought to be areas where ice had begun to defrost and the darker soil underneath exposed. This is a plausible idea but was disproven by observations from the Mars Odyssey satellite which showed the dark regions were only 3-5 °C warmer than the surrounding carbon dioxide ice sheet, a value far too low to be exposed soil.
Kieffer et al. developed an alternative model by studying a polar region from the end of the southern winter through to midsummer. The carbon dioxide ice was seen to disappear 160 days after sunrise at which point the soils rapidly warmed to 235 K (-38 °C or -37 °F). The authors state that this rate of ice melting is consistent with a winter slab of carbon dioxide ice about 1 m thick. Dark spots appeared from sunrise to up to 100 days later and slowy transformed into fan like shapes. At some locations large blotches were seen under the ice that were similar in appearance in the summer after the ice had melted. This suggests that in places the carbon dioxide ice is transparent or translucent and the rock surface below can be seen.
In the author’s model dark granular soil materials lie on the ice free surface during summer. As winter approaches and the carbon dioxide ice forms it creates a slab within which the dark particles are included. With sunrise in early spring the particles within the ice are heated and sink down through the ice over around 20 days. As the ice becomes translucent sunlight can penetrate to the subsurface which starts to warm and cause defrosting from the base of the slab. The pressure and temperature conditions on Mars mean that carbon dioxide transitions straight from a solid to a gas, a process known as sublimation. Therefore, as the ice defrosts carbon dioxide gas to start to build up under the slab. Eventually the gas pressure becomes great enough for the ice sheet to rupture and a jet of carbon dioxide to escape. As the trapped gas rushes to the rupture point it carves channels to form the spider like channel networks and then erupts onto the surface transporting some of the dark granular matter to form dark spots on the surface. The material is dumped near the vent and is slowly transported by wind to form fans. When the ice is completely melted the dark grains are returned to the surface and the cycle repeats year after year.
This is an intriguing model but it is not yet a definite explanation for how the dark spots and spider networks form. To know for sure we would have to send a mission to try and capture the geysers in action. Such a mission has already been proposed, known as the Mars Geyser Hopper, it would have the ability to study the geology of the region in the summer then overwinter and hopefully capture geysers as they form during the spring by ‘hopping’ to an appropriate viewing point. If the mission goes ahead it will launch in 2016.
Kieffer et al. 2006