Gullies on Mars weren’t created by water– What made them?

The exterior of Mars contains many slender, deep channels called gullies, and prevailing opinion held that these channels were formed by running water.

However, new data from NASA’s Mars Reconnaissance Orbiter had indicated these gullies may have been created by carbon dioxide freezing and thawing, according to a study recently published in the journal Geophysical Research Letters.

To reach their conclusion, study researchers analyzed high-resolution data from greater than 100 gully sites across Mars. These data, gathered by the orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), were then associated with images from the orbiter’s High Resolution Imaging Science Experiment (HiRISE) camera and Context Camera (CTX).

No Evidence Found for Liquid Water

The analysis did not reveal mineralogical evidence for liquid water or its by-products are a factor in creating these gullies, the study said.

“The HiRISE team and others had shown there was seasonal activity in gullies — primarily in the southern hemisphere — over the past couple of years, and carbon dioxide frost is the main mechanism they suspected of causing it. However, other researchers favored liquid water as the main mechanism,” Jorge Núñez, the lead author of the paper, said in a news release. “What HiRISE and other imagers were not able to determine on their own was the composition of the material in gullies, because they are optical cameras. To bring another important piece in to help solve the puzzle, we used CRISM, an imaging spectrometer, to look at what kinds of minerals were present in the gullies and see if they could shed light on the main mechanism responsible.”

The study team said new tool to analyze CRISM data called Map-projected Targeted Reduced Data Records helped expedite their work.

“On Earth and on Mars, we know that the presence of phyllosilicates — clays — or other hydrated minerals indicates formation in liquid water,” Núñez said. “In our study, we found no evidence for clays or other hydrated minerals in most of the gullies we studied, and when we did see them, they were erosional debris from ancient rocks, exposed and transported downslope, rather than altered in more recent flowing water. These gullies are carving into the terrain and exposing clays that likely formed billions of years ago when liquid water was more stable on the Martian surface.”

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Image credit: NASA

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