Gullies on Mars Probably Not Carved by Water

The gullies, three- to 33 feet in width, on a scarp in Mars' Hellas impact basin.

The hunt for what carved channels into polar-facing slopes on Mars -- a process that is ongoing today -- took a new twist on Friday with the release a study showing no chemical fingerprints of flowing water at more than 100 gully sites.

Similar features on Earth are carved by flowing liquid water. Mars' cold temperatures and a low atmospheric pressure means that liquid water would be transient, if it exists at all.

Nevertheless, telltale signs of water, in the form of hydrated salts, has been tied to another seasonal feature on Mars known as recurring slope lineae, or RSL.

Scientists are beginning to take test images of potential RSL sites in Gale Crater with NASA's Mars rover Curiosity, project scientist Ashwin Vasavada told DNews.

"These things really aren't understood yet," Vasavada said.

Scientists consider gullies to be features that have an alcove on top, a channel and an apron of material at the bottom. RSLs are characterized by seasonal darkening and fading, not how the ground is shaped.

Most RSLs are found on equator-facing slopes, as opposed to the pole-facing locations of gullies, leading scientists to theorize that the features stem from different processes.

Gullies are found all over Mars, with most located between 30 degrees and 50 degrees latitude in the northern and southern hemispheres.

In the new study, planetary scientist Jorge Nunez with Johns Hopkins University Applied Physics Laboratory, and colleagues correlated high-resolution images of more than 100 gullies taken by Mars Reconnaissance Orbiter with chemical data obtained by the spacecraft's Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, instrument.

Several mechanisms have been proposed over the years to explain gully formation, including the melting of ground ice or the melting of a relict, regolith-covered snow pack. Another option is carbon dioxide frost activity that doesn't involve liquid water at all, the study said.

To help narrow the options, scientists looked for minerals, such as clays, silica, zeolites, sulfates, carbonates, or chlorides in the gullies which could indicate past water activity.

"We might have expected to see spectral evidence for liquid water such as hydrated salts as observed at RSL sites if present or recent liquid water activity had played a role in gully formation and evolution," the study said.

"We find no such evidence for brines in any of the gullies we have investigated," Nunez wrote.

The scientists conclude that their observations indicate "a limited role for long-lived liquid water in the formation and modification of Martian gullies, and support a stronger role for carbon dioxide frost-related processes."

The research is published in this week's Geophysical Research Letters.

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