New research suggests that liquid water has not been flowing on the surface of Mars within the past decade.
An investigation by the University of Arizona casts doubt on a 2006 report speculating that bright spots in some Martian gullies indicated the former presence of liquid water.
"It rules out pure liquid water," said lead author Jon D Pelletier, associate professor of geosciences at the University of Arizona.
HiRISE has been providing the most detailed images of Mars ever taken from orbit since 2006.
The researchers applied basic fluid physics under Martian conditions to determine how a flow of pure liquid water would look on the HiRISE images compared with an avalanche of dry granular debris such as sand and gravel.
"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."
Finding liquid water on the surface of Mars would indicate the best places to look for current life, according to co-author Alfred S McEwen, professor of planetary sciences at the University of Arizona.
"What we had hoped to do was rule out the dry flow model, but that did not happen," he said.
An avalanche of dry debris is a much better match for the scientists' calculations and computer model predictions, according to Pelletier and McEwen.
"Right now the balance of evidence suggests that the dry granular case is the most probable," said Pelletier.
The research does not rule out the possibility that the images show flows of very thick mud containing 50 per cent to 60 per cent sediment.
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