‘Dry ice, not water, responsible for many gullies on Mars’

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 US Geological Survey’s Astrogeology Science Centre, 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 has been growing support for the idea that water was once plentiful on our dry, dusty neighbour. 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 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 analysed 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.

“Martian gullies have attracted considerable attention because they resemble terrestrial debris flows formed by the action of liquid water,” lead author Cedric Pilorget, a planetary scientist at the Institut d’Astrophysique Spatiale in France, wrote in an e-mail. “This has been puzzling because such features are geologically recent [less than a few million years old] and have thus formed during the current cold and dry Martian era. Was water involved, to what extent and what could it mean in terms of habitability?”

Instead, the researchers pointed to another possible cause: frozen carbon dioxide. Carbon dioxide is the by far the most abundant gas in the Red Planet’s atmosphere, and much of it sticks to the surface in winter, stretching from metre-thick ice caps near the poles to surface frost in the mid-latitudes.

And, as it just so happens, “the distribution of frost correlates with the regions where gullies are most prominent”, Dundas wrote. “Moreover, gully activity appears to be seasonal, with a marked preference for the winter and spring — when CO₂ frost is observed on mid-latitude slopes.”

Scientists have had an idea that frozen carbon dioxide (also known as dry ice) could have been the cause of such gullies for a while now, but they weren’t sure how it worked, said Alfred McEwen, a planetary scientist at the University of Arizona and leader of the HiRISE camera on NASA’s Mars Reconnaissance Orbiter, which has taken images tracking the development of current gullies on Martian slopes.

“This activity did not involve liquid water; we’ve known that for some time,” McEwen said. “But how do you explain the gullies exactly, especially since they look so much like water-carved gullies?”

In the new paper, the scientists studied the frost on slopes at different locations and in different orientations with respect to the sun, developed models to analyse how carbon dioxide would act at different temperatures, and presented a mechanism that could explain the gullies’ creation.

Since dry ice can freeze into a thick translucent slab, sunlight can travel all the way to the ground and cause the dry ice to start to sublimate — go from a solid directly to a gas — at the slab’s base.

“This causes pressure to rise beneath the ice slab, and eventually causes the slab to lift and break apart, rapidly expelling both gas and entrained regolith material,” Dundas said.