NASA’s Mars Reconnaissance Orbiter and Odyssey probes have discovered something quirky and highly unexpected on the surface of Mars. It appears that every Martian spring there are huge CO2 geysers erupting near the Martian south polar ice cap.

The teams working with the probes noticed that each spring, as the ice melts off the southern polar cap, a series of dark spots appear that are 15 to 46 metres across. Then a few days later dark fans appear next to these spots.

The team have now deduced that these are geysers of carbon dioxide gas, shooting at great speed out from under the surface of the polar cap.

The process discovered is unlike anything else seen in the Solar System to date and seems to be a regular feature of the Martian seasons.

During the summer the cap is just a water ice polar cap. Then over the colder months a three-foot layer of CO2 gas forms as ice over the top. As it does so, sand and other dust get trapped under neath. Then in spring as sunlight begins to pass through the cap again, the gas lower down sublimates and gradually forces its way up through weaknesses in the surface. Then as the light increases, more and more is released and shoots upward through the gaps at 100 miles per hour, taking the sand and dust along with it. This is what forms the dark fan shapes seen the image.