A little over a year ago, NASA announced it had found strong evidence of liquid water flowing, at least temporarily, on the surface of Mars. Pictures taken a few years apart showed flow-like gullies in the sides of craters, and there were a few different pieces of evidence that these were due to sudden flooding of liquid water downhill. Here are the original shots:
However, a new study just released says that these images fit better with being dry grains flowing downhill.
The researchers, led by Jon D. Pelletier of The University of Arizona, used HiRISE, the very high-resolution camera on board the Mars Reconnaissance Orbiter, to look at the same regions as observed before. By taking images at different angles, they could establish a digital elevation model, a topographical map of the same crater shown in the earlier announcement. They then modeled the way liquid water would flow under Martian conditions compared to how dry grains would flow. To their surprise, they found that dry grains were a better match. From their press release:
"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 on Mars, said co-author Alfred S. McEwen, a UA professor of planetary sciences.
"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.
An avalanche of dry debris is a much better match for their calculations and also what their computer model predicts, said Pelletier and McEwen.
While this isn't conclusive, it does seem compelling (they can't rule out very thick mud, incidentally). The press release doesn't have any statements from the scientists who made the previous announcement about water, and I'll be very curious indeed to hear what they have to say.
If it holds up, it's too bad. I'd love to see better evidence of ubiquitous water on (or immediately under) the surface of Mars. But facing reality is what we have to do. Of course, as our tools get better, we'll get better at figuring this stuff out, too. It helps that so many people involved are so very clever.