When spring begins in the southern hemisphere of Mars, the sun’s heat creates changes in the frozen southern ice cap. Phil Christensen of Arizona State University led a team that monitored Mars’ southern ice cap using the THEMIS camera on NASA’s Mars Odyssey spacecraft.

The team discovered that dark spots, fan-shaped features, and spiders (radiating channels formed by the outflow) began forming when the sun reached just a half a degree high. Over several days, the new features became more numerous.

Christensen believes the dark spots mark vents where high pressure carbon dioxide gas bursts through a three-foot layer of dry ice. (Dry ice is frozen carbon dioxide.) The carbon dioxide as frost, mixed with surface dust and sand, move downward through the dry ice all winter long. In springtime, the sun heats the dark dust and sand which heats the dry ice. The ice turns to a gas and builds pressure beneath the icy surface. The trapped carbon dioxide gas breaks free, taking dark dust and sand with it. The dust-laden gas jets or vents carve out spidery ravines and the dust makes the dark spots.