Artist's impression of jets of carbon dioxide and sand roaring upwards into the sky as spring begins at the Martian south pole. © Arizona State University/Ron Miller The NASA probe Mars Odyssey has spotted jets of carbon dioxide erupting from the red planet’s south pole. The gas geysers are quite unlike anything seen on Earth and carry debris hundreds of metres above the surface.

Mars Odyssey has been in orbit around Mars since 2001 and is partly designed to search for subsurface water. Astronomers used the probe’s Thermal Emission Imaging System (THEMIS) to study a small region of the south polar cap, which like on Earth is composed of water ice mixed with layers of dust and sand. In the southern the pole is in shadow for several months. During this time the temperature drops to -130°C, cold enough for carbon dioxide to freeze out of the atmosphere and form a thin coating over the ice. More dust and ice covers this material in a slab about a metre thick.

Dark spots (left) and fans (right) mark the sites of carbon dioxide geysers in this Mars Global Surveyor picture. © NASA/JPL/Malin Space Science Systems When spring begins and the sunlight falls on the pole again, the carbon dioxide begins to turn to gas (a process called sublimation). Eventually pressure builds up and the gas bursts through weak spots in the overlying slab, jetting out at about 150 km an hour. Images from THEMIS show dark spots and fans and spider-shaped markings where the jet erodes the surrounding terrain.

The gas geysers last for several months until the overlying slab sublimates away completely. Phil Christensen from Arizona State University believes a nearby explorer would see a dramatic and exotic sight:

If you were there… you’d be standing on a slab of carbon-dioxide ice. All around you, roaring jets of CO₂ gas are throwing sand and gas a couple of hundred feet into the air… This is unlike anything that occurs on Earth.