Gullies on Mars may be sculpted by dry ice processes, rather than flowing liquid water, according to a study published online this week in Nature Geoscience. The study suggests that, although the Martian gullies may look like water-sculpted gullies on Earth, the underlying processes may be very different.

Gullies-small valleys cut into slopes-found at the Martian mid-latitudes appear similar in shape to gullies formed by flowing streams on Earth, suggesting a common origin. However, the modern surface environment of Mars is too cold at the gully locations to support large volumes of liquid water. The gullies are less than a few million years old-with some actively forming today-and seasonally active on slopes during times that coincide with defrosting carbon dioxide (CO2) ice on the Martian surface, indicating that dry ice may be involved in their formation.

Cedric Pilorget and Francois Forget propose that, as a layer of CO2 ice that coats a sloped surface defrosts, trapped CO2 gas builds up beneath the ice layer, eventually destabilizing the underlying soils and triggering a flow of gas and debris. The authors use a numerical model to simulate this process and show that the dry-ice-driven process can explain many of the gullies’ observed features, including their distribution across the Red Planet.

In an accompanying News & Views article, Colin Dundas notes that: “When dealing with other worlds, we must take care to remember that unfamiliar processes are possible and even likely in alien environments.”