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.
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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.
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"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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