“Spiders” on Mars

In this weeks guess the planet I showed you these spiderlike landforms.These features are actually called spiders or "araneiforms". They are found near the south pole of Mars, an area covered with carbon dioxide ice. On Earth Carbon Dioxide is only really found in its gaseous form, but because temperatures on Mars are so much colder the icecaps consist of large amounts of solid CO₂. This has the potential to produce landforms which are not found on earth at all. 

There is still a fair amount of debate surrounding the formation of these spider-like features, but it seems likely that they are created when geysers erupt from the ice. These eruptions erode the spiderweb like channels that can be seen in the image, and deposit dark material on the surface, making them stand out from the bright terrain on which they form. 

So what causes these geysers? There are a variety of theories for what is warming up the subsurface and causing the eruptions to occur. The leading theory is that these slabs of CO₂ are largely translucent, so sunlight can penetrate through the ice. It is possible that the ice thus acts like a greenhouse, trapping head and allowing the subsurface to warm up. Of course warm ice doesn’t stay in the solid phase very long, and as we know from Earth you have to keep CO₂ very cold if you want it to remain as ice. 

Phase changes such as boiling, melting, and freezing are controlled by two factors; temperature and pressure. Earth is very unusual, in that our mean temperature and pressure conditions lie close to what is called the “triple point” of water. This is the point at which it can easily exist in any of the three common states of matter, as liquid water, as ice, or as water vapour in the atmosphere. Earth’s temperature and pressure conditions are a long way from the triple point of carbon dioxide. It can only be stable as a gas on Earth. 

On Mars temperatures are much colder so CO₂ Ice can form much more readily. However liquid CO₂ still isn’t stable under these conditions. When CO₂ ice undergoes a phase change it goes straight from its solid state to a gaseous one, a process called sublimation. We can actually tell that the fluid which carved the spider channels was a gas rather than a liquid, because some of the channels appear to flow up hill. 

This theory explains several aspects of the spider features, but there are a wide range of other hypotheses as well, including that the heat is generated by biological processes under the ice. Many scientists think that a greenhouse effect is more likely, but until we visit the south pole of Mars and observe these features directly we won’t know for sure. This sort of uncertainty is very common in planetary science, and is why rover missions are so important. 

Further reading

NationalGeographic 

NASA

 Image credit: HiRISE image: ESP_011408_0930, NASA/JPL/University of Arizona