Undoubtedly one of the biggest science stories of 2015 was the discovery of water on Mars, with running liquids believed to be forming observable channels on the surface. But another piece of evidence for liquid water – namely gullies on the surface – should be called into question, according to new research.
A study published in Nature Geoscience suggests that these gullies, dark streaks formed at mid to high latitutes, were likely caused by dry ice composed of carbon dioxide, a theory that has been proposed before. Authors Cedric Pilorget and Francois Forget from the University of Paris explained how much of Mars is covered in a carbon dioxide frost, or dry ice, which is much thinner nearer the equator and thaws due to sublimation – with the ice turning directly into a gas.
During this defrosting process, pores beneath the ice can be filled with ice and shifted. The movement of gases beneath the surface can then destabilize the soil above, causing a debris flow as a result of the gas – and not water. The authors say this can create features such as the gullies we can see on Mars today. Many of these gullies were formed less than a million years ago, and even in the present day, which means there must be an ongoing process.
“We conclude that Martian gullies can result from geologic dry ice processes that have no terrestrial [Earth-like] analogues and do not require liquid water,” they write, adding: “The role of liquid water in gully formation should, therefore, be reconsidered, raising the question of the importance of its occurrence in Mars’ recent past.”
If their research is correct, it would have significant implications for our understanding of Mars. We are fairly certain the Red Planet had large bodies of liquid water a few hundred million years ago, suggesting it was once habitable, but the recent suggestion that liquid water is still present on the surface – despite the seemingly unfavorable surface conditions, namely the low temperatures and low pressure – suggested it may still be habitable, raising the possibility that microbial life lives there.
This research does not call into question the salty recurring slope lineae (RSL) that caused such a commotion earlier this year. But it does suggest that liquid water may not be so abundant on the surface of Mars.
“The RSL occur in times and places far too warm for CO2 frost, so this is not a viable formation model for RSL,” Dr Alfred McEwen, professor of planetary geology at the University of Arizona, told IFLScience. “I think this study will help to convince the science community that the CO2-driven gully model is viable and should be considered the main gully-forming mechanism today.”
We're fairly certain Mars once had water – but how much does it have today? NASA/GSFC
In an accompanying News and Views article, Colin Dundas of the Astrogeology Science Center at the U.S. Geological Survey (USGS) in Arizona suggests that processes on Mars may not as closely resemble those on Earth as we thought.
“When dealing with other worlds, we must take care to remember that unfamiliar processes are possible and even likely in alien environments,” he writes. “The Martian gullies may look like terrestrial landforms shaped by liquid water, but it may be CO2, not water, that is the culprit.”
The debate will likely rage on about how prevalent water is on Mars today. But we may have to temper some expectations regarding how much we are expecting to find, particularly in these somewhat mysterious gullies.