Dec. 21--Gullies on Mars that appeared to have possibly been carved by water were probably dug out by great chunks of dry ice, a pair of French researchers say.
Even as certain lines of evidence mount for the very occasional occurrence of water on present-day Mars, the findings published by the journal Nature Geoscience serve as a reminder that not every familiar-looking geological feature on Mars has an analogue on Earth.
"When dealing with other worlds, we must take care to remember that unfamiliar processes are possible and even likely in alien environments," Colin Dundas of the U.S. Geological Survey's Astrogeology Science Center, who was not involved in the research, wrote in a commentary on the work.
As NASA orbiters and rovers probe the Red Planet from above and on the ground, there's been growing support for the idea that water was once plentiful on our dry, dusty neighbor. The Mars Science Laboratory rover, known affectionately as Curiosity, has found signs of a long-lasting series of lakes that rose and fell in Gale Crater over many millions of years. Analyses of pebbles in the crater have revealed hints of a sustained river as well. Other papers have posited the idea that Mars was once had a large ocean that slowly disappeared over the eons.
All of this was a Mars in the distant past. But even today, researchers are beginning to think there could be some liquid water -- albeit very harsh, salty water -- that on very rare occasions briefly exists on the surface today. The recent discovery that dark streaks known as 'recurring slope lineae' could have been left by contemporary liquid water compelled NASA officials to wax enthusiastic about the planet's current habitability.
"It suggests that it would be possible for there to be life today on Mars," John Grunsfeld, associate administrator of NASA's Science Mission Directorate in Washington, said at a briefing announcing those results (also published in Nature Geoscience).
Not all streaks on slopes are potential signs of a wet present; far from it, in fact. A pair of French researchers has analyzed different features -- Martian gullies in the planet's mid-latitudes, deep channels that on Earth would have looked like they were carved by streams or sliding wet debris. But the proposed explanations would require groundwater discharge, which would be unlikely in the spots where many of these gullies are found, on lone peaks and sand dunes.
