Gullies on Mars are likely not being formed by flowing liquid water, scientists using data from NASA's Mars Reconnaissance Orbiter have found.
The finding will allow researchers to further narrow theories about how martian gullies form, and unveil more details about Mars' recent geologic processes.
The term "gully" is used for features on Mars that share three characteristics in their shape: an alcove at the top, a channel, and an apron of deposited material at the bottom.
Gullies are distinct from another type of feature on martian slopes, streaks called "recurring slope lineae," or RSL, which are distinguished by seasonal darkening and fading, rather than characteristics of how the ground is shaped.
Water in the form of hydrated salt has been identified at RSL sites. The new study focuses on gullies and their formation process by adding composition information to previously acquired imaging.
Researchers from the Johns Hopkins University Applied Physics Laboratory (APL) in the US examined high-resolution compositional data from more than 100 gully sites on Mars.
The data, collected by the orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), was then correlated with images from the High Resolution Imaging Science Experiment (HiRISE) camera and Context Camera (CTX).
The findings showed no mineralogical evidence for abundant liquid water or its by-products, thus pointing to mechanisms other than the flow of water - such as the freeze and thaw of carbon dioxide frost - as being the major drivers of recent gully evolution.
Gullies are a widespread and common feature on the martian surface, mostly occurring between 30 and 50 degrees latitude in both the northern and southern hemispheres, generally on slopes that face toward the poles.
On Earth, similar gullies are formed by flowing liquid water; however, under current conditions, liquid water is transient on the surface of Mars, and may occur only as small amounts of brine even at RSL streaks.
The lack of sufficient water to carve gullies has resulted in a variety of theories for the gullies' creation, including different mechanisms involving evaporation of water and carbon dioxide frost.
"On Earth and on Mars, we know that the presence of phyllosilicates - clays - or other hydrated minerals indicates formation in liquid water," said Jorge Nunez of APL.
"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," he said.
"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," Nunez said.
The findings were published in the journal Geophysical Research Letters.
