The gullies that crisscross the surface of Mars were not carved out by running water, but dry ice, in fact, according to new research.
These channels remain active and tend to form during cold weather, implicating frozen carbon dioxide - known as dry ice - rather than liquid water, which would freeze at such low temperatures.
"As recently as five years ago, I thought the gullies on Mars indicated activity of liquid water," study lead author Colin Dundas of the US Geological Survey's Astrogeology Science Center in Flagstaff, Ariz., said in a NASA statement. "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."
Dundas and his team used the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter (MRO) to examine gullies at 356 sites on Mars, since they were first discovered in 2006. The researchers found that 38 of these sites had active gully formation.
After studying these sites over time, scientists determined that the ongoing surface changes occurred at times just too cold for liquid water to flow. So, dry ice is likely responsible, they said.
On Earth, flowing water creates features that are very similar to the Red Planet's channels. But Earth doesn't have gullies created by dry ice to compare to the Martian landforms.
"The pressure and temperature conditions for dry ice to exist aren't found in nature on Earth," Dundas told Space.com. "On Mars, they occur every winter, forming a seasonal polar cap of dry ice."
The new results may come as a bit of a blow to people who were hoping that water still flows on the Red Planet, as it did billions of years ago. However, Dundas asks people not to give up hope just yet.
For example, features known as "recurring slope lineae" - dark narrow streaks running down Martian slopes - occur during the summer and on warmer, equator-facing slopes.
"The warmer temperatures make water or briny solutions possible," Dundas said. "There are still strong candidates for present-day liquid."
The findings were published in the journal Icarus on July 10.
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