Seasonal carbon dioxide frost, not liquid water, is the main driver in forming gullies on Mars today, according to a recent U.S. Geological Survey study that relied on NASA's Mars Reconnaissance Orbiter's (MRO) repeated high-resolution observations.

Martian gullies are landforms typically consisting of steep channels, usually having a recessed head, that feed into a fan of material deposited at the bottom. The discovery of active gullies was first reported in 2000, which generated excitement due to consideration that they might result from action of liquid water.

Mars has water vapor and plenty of water ice, but liquid water, a necessity for all known life, has not been confirmed on modern Mars. The new report, published in the journal Icarus, is available online.

"As recently as five years ago, I thought the gullies on Mars indicated activity of liquid water," said USGS scientist Colin Dundas, lead author of the new report. "We were able to get many more observations, and as we started to see more activity and pin down the timing of gully formation and change, we saw that the activity is in winter."

A smaller type of seasonal flow seen on some slopes on Mars may involve liquid water, but is yet to be determined. These flows are called recurring slope lineae (RSL), and are sometimes found within small channels but not systematically associated with larger gullies.

Dundas and collaborators used the High Resolution Imaging Science Experiment (HiRISE) camera on MRO to examine each of 356 Martian sites with gullies at least twice, beginning in 2006. Thirty-eight of the sites showed activity, such as cutting a new channel segment or adding material to the apron-shaped deposit at the downhill end of a gully.

Wherever the timing of before-and-after observations enabled determining the season of gully activity, it was a time too cold for the possibility of melting water-ice, but consistent with seasonal carbon dioxide frost.

"RSL and mass movements in Martian gullies are two distinct types of slope activity," said Dundas "It's not hard to tell them apart in HiRISE images. The classic Martian gullies are much larger than RSL. Many of them are more the size that you'd call ravines on Earth."

Frozen carbon dioxide, commonly called dry ice, does not exist naturally on Earth, but it is plentiful on Mars. It has been linked to active processes on Mars such as geysers of carbon dioxide gas from springtime sublimation of dry ice, and blocks of dry ice that plow lines on sand dunes by sledding down dunes on cushions of sublimated gas.

One mechanism for how carbon dioxide frost might drive gully flows is by gas that is sublimating from the frost, providing lubrication for dry material to flow. Another might be slides due to accumulating weight of seasonal frost buildup on steep slopes.

Work by Dundas and others has previously pointed to winter timing of gully formation on dune and non-dune slopes, with suggested involvement of seasonal changes in frozen carbon dioxide. The new report adds evidence for the changes. The findings also make a new point that the pace of gully formation that has now been documented is swift enough that all of the fresh-appearing gullies seen on Mars can be attributed to current processes.

Some earlier hypotheses attributing the gullies to action of liquid water have suggested they formed thousands to millions of years ago when climate conditions were possibly more conducive to Mars having liquid water due to variations in the planet's tilt and orbit.

Dundas' co-authors on the new report are Serina Diniega of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, and Alfred McEwen of the University of Arizona, Tucson.

"Much of the information we have about gully formation and other active processes on Mars comes from the longevity of MRO and other orbiters," said Diniega. "This enables repeated observation of sites to examine changes over time."

Data will appear in an upcoming special issue of Icarus with multiple reports about active processes on Mars, including RSL.

"I like that Mars can still surprise us," Dundas said. "Martian gullies are fascinating features where we can investigate a process that we just don't see on Earth."