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Steep Martian gullies thought to have carried liquid water in the past decade were more likely the site of an avalanche of sand and gravel, according to a UA analysis of new images taken from Mars' orbit.

The findings dispute a 2006 study that argued liquid water flowing on Mars created bright streaks visible in pictures taken in 2006 that were not visible in pictures from 1999. The 2006 study, led by Michael Malin of Malin Space Science Systems, used images from the Mars Global Surveyor Mars Orbital Camera.

With their own higher-resolution camera in orbit around Mars, UA scientists re-imaged the same area in an effort to gather more evidence in support of Malin's conclusions.

"We started off expecting this would have a liquid water flow and we were looking to make a stronger case," said Jon D. Pelletier, a University of Arizona associate professor of geosciences and lead author of the study.

"The original imagery looked pretty convincing. It was certainly something that was very fluidized, but if you get sand and gravel down a steep slope, it can have fluid- like behavior," Pelletier said.

The findings are somewhat disappointing for Pelletier because they contradict what he considers the most exciting discovery about Mars in 10 years. Evidence of recent water on Mars would have profound implications for the question of whether life could currently exist on Mars.

"The conclusion of our paper led us to a simpler explanation, that we really do understand water on Mars, and under the current climate conditions, liquid water is probably not present and flowing on Mars today," Pelletier said. "It is a bit disappointing that this really cool news didn't turn out."

The research paper — "Recent Bright Gully Deposits on Mars: Wet or Dry Flow?" — will be published in the March edition of the journal Geology. Pelletier's co-authors are HiRISE camera principal investigator and UA planetary scientist Alfred S. McEwen, UA doctoral student Kelly J. Kolb and Randy L. Kirk of the U.S. Geological Survey in Flagstaff.

The UA team used HiRISE pictures taken at two angles to construct three-dimensional images of the Martian slope in much greater detail than were used in the 2006 study. The HiRISE camera, in orbit aboard the Mars Reconnaissance Orbiter since March 2006, has a resolution five times greater than the MOC.

Pelletier plugged the data from those three-dimensional images — called digital elevation models, or DEMs — directly into computer models of different fluid patterns. The simplest model, liquid water with no sediment, was ruled out quickly.

The modeling continued with different levels of sediment added to the water — basically mudflows — and, at a certain consistency, the liquid-sediment mix would match the observed flow patterns on Mars.

"Either we have this really sediment-rich flow mixed in with some water, or maybe it's just dry," Pelletier said.

A dry, granular flow model fit the Martian images best, but the study did not rule out the possibility that the flows were created by a thick mud, with the consistency of hot lava. The slope began at a 45-degree angle, more than steep enough for sand to build up speed and act like a fluid as it moves downhill, Pelletier said.

Pelletier will continue to study flow patterns on Mars that come within the last decade or so, though not from older features that lack a clear prior image.

"Now we have a collection of these enigmatic flows where they could be dry and they could be wet, and we're doing a systematic test for all of these," he said.

The research is the first to use complex computer modeling techniques to analyze HiRISE images.

"By using computer models, we can do a more precise job at testing the hypothesis of one process or another," Pelletier said.