NASA Mars Reconnaissance Orbiter Reveals Dry Ice is Behind Gullies on the Red Planet

NASA's Mars Reconnaissance Orbiter (MRO) has made some new discoveries when it comes to the gullies on Mars' surface. After repeated high-resolution observations, the researchers found that these gullies are primarily formed by the seasonal freezing of carbon dioxide and not liquid water.

In 2000, scientists first reported their findings of formative gullies on Mars. This, in particular, actually suggested the presence of liquid water on the planet.

"As recently as five years ago, I thought the gullies on Mars indicated activity of liquid water," said Colin Dundas, the lead author of the new study, 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."

After first discovering the gullies, the researchers examined them a bit more closely using the High Resolution Imaging Science Experiment (HiRISE) camera on MRO. More specifically, they analyzed gullies at 356 sites on Mars, beginning in 2006. About 38 of these sites showed active gully formation, including increased deposits at the downhill end of some gullies.

The scientists soon noticed that the timing of this gully formation coincided with seasonal carbon-dioxide frost and temperatures that would not have allowed for liquid water. While frozen carbon dioxide, also known as dry ice, doesn't exist naturally on Earth, it can exist on Mars. The researchers believe that carbon-dioxide frost might actually drive gully flows.

While these latest findings reveal that the gullies do not have liquid water, there may be other sites that hold liquid water. A smaller type of surface-flow feature recently found by scientists could still hold the potential for liquid water on Mars.

"I like that Mars can still surprise us," said Dundas. "Martian gullies are fascinating features that allow us to investigate a process we just don't see on Earth."

The findings are published in the journal Icarus.