Thanks to images captured by the robotic Mars Reconnaissance Orbiter, scientists have further evidence gullies on the Red Planet's surface are carved by the seasonal freezing of carbon dioxide, not water.

The first reports of the formation of gullies on Mars issued in 2000 generated excitement because they suggested the potential presence of on Mars of liquid water, which is necessary for all life as we know it and could have easily explained the surface erosion.

But, comparing dated before-and-after images, researchers determined the timing of the land scarring coincided with seasonal carbon-dioxide frost, and that temperatures on the rocky world would not have allowed for liquid water.

Frozen carbon dioxide, otherwise known as dry ice, does not exist naturally on Earth, but is plentiful on Mars and has been associated to a number of active processes on the neighboring planet, including carbon dioxide gas geysers and lines on sand dunes plowed by blocks of dry ice.

One explanation for the gullies is that gas sublimating from the frost provides enough lubrication for dry material to flow. Otherwise, the weight of seasonal frost buildup may be causing material slides down steep Martian slopes.

Bottom line, the latest imaging sequences sent back to Earth from the orbiter demonstrate the gullies and other surface characteristics on the Martian surface can may be attributed to current, ongoing processes, instead of events that occurred thousands to millions of years ago, as was previously suggested, when the climate might have been more hospitable to liquid water.

This latest report about gullies has been posted online by the journal Icarus.

"As recently as five years ago, I thought the gullies on Mars indicated activity of liquid water," lead author Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center in Flagstaff, Arizona, said in a news release. "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 occurs in winter."

Starting in 2006, Dundas and his colleagues used the High Resolution Imaging Science Experiment camera aboard the MRO to examine the gullies at 356 sites on the Red Planet's surface. Thirty-eight of the sites showed active gully formation, including evidence of new channel segments and increased deposits at the downhill end of some gullies.

"Much of the information we have about gully formation, and other active processes, comes from the longevity of MRO and other orbiters," said Serina Diniega of NASA's Jet Propulsion Laboratory in Pasadena. "This allows us to make repeated observations of sites to examine surface changes over time."

Dundas said he likes how Mars continues to challenge notions about the solar system's evolution.

"Martian gullies are fascinating features," he said, "that allow us to investigate a process we just don't see on Earth."