Scientists may be closer to answering the question of how enigmatic Martian gullies form. According to research presented by François Costard, François Forget (University of Paris), and others last week at a meeting of planetary scientists, changes in the planet's obliquity (polar tilt) can lead to melting of shallow subsurface ice.

Forget notes that in Jameson Land, Greenland, the tops of permafrost layers between 80 and 220 meters thick can thaw episodically during the summer. Near the poles, as the ground soaks in more sunlight, the mean soil temperature can rise above freezing, and gullies form as a result. "These gullies don't involve water runoff, but are instead debris flows — the melt of subsurface water mixed with rock," says Forget.

Permafrost regions on Mars may be analogous. Mars's obliquity is known to cycle radically between 0° and 60°. Currently the planet is tilted at 25.2° to its orbit. But during periods of higher obliquity (above 30°) the poles can experience enough time in the Sun to warm the topmost ground layers above freezing. The model also explains why the Martian gullies tend not to congregate near the equator. During periods of high obliquity, the poles get more Sun than the planet's midsection.

Costard and Forget's theory argues strongly against several existing scenarios used by scientists to explain the gullies. One largely debated idea involves the existence of subsurface aquifers that are capped by ice plugs. When a plug cracks, the water comes out, forming a gully. But no one could explain how the aquifers could stay liquid within a permafrost layer given Mars's limited geothermal activity. Outflows of thawing carbon dioxide ice have also been proposed.

The model isn't flawless though. A major hurdle to overcome involves keeping water liquid on the surface. Given the current atmosphere of Mars, any liquid water should rapidly evaporate away. A possible solution is that during periods of high obliquity, Mars's atmosphere is thicker, perhaps due to the sublimation of polar carbon dioxide ices (see story below). A thicker atmosphere could maintain liquid water long enough to form the gullies.