Carried out by Jon D. Pelletier of The University of Arizona in Tucson and his colleagues, the research casts doubt on the 2006 report that the bright spots in some Martian gullies indicate that liquid water flowed down those gullies sometime since 1999. For disapproving the earlier finding, Pelletier and his colleagues used topographic data derived from images of Mars from the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
As part of the research, Pelletier used the stereoscopic computer-generated topographic maps known as digital elevation models (DEMs) to figure out how particular landscape features form. DEMs are made using images of the landscape taken from two different angles.
"The Mars Reconnaissance Orbiter spacecraft is designed to regularly point at targets, enabling high-resolution stereo images," McEwen said.
The research team made a DEM of the crater in the Centauri Montes region where the Malin team found a new bright streak in a gully.
Once the DEM was constructed, Pelletier used the topographic information along with a commercially available numerical computer model to predict how deposits in that particular gully would appear if left by a pure water flood versus how the deposits would appear if left by a dry avalanche.
"The dry granular case was the winner," said Pelletier. "I was surprised. I started off thinking we were going to prove it's liquid water," he added.
"What we'd hoped to do was rule out the dry flow model - but that didn't happen," said McEwen, the HiRISE principal investigator and director of UA's Planetary Image Research Laboratory.
According to Pelletier and McEwen, an avalanche of dry debris is a much better match for their calculations and also what their computer model predicts. "Right now the balance of evidence suggests that the dry granular case is the most probable," said Pelletier.
"There are other ways of getting deposits that look just like this one that do not require water," he added.
One of the team's next steps is using HiRISE images to examine similar bright deposits on less-steep slopes to sort out what processes might have formed those deposits.