Maybe it was a nice idea while it lasted. In 2006 a team of scientists published two images from NASA’s Mars Global Surveyor probe — the first, from orbit in 1999, showing the slope of a crater, the second, from 2005, showing that something had created a gully in the crater’s side. What could it be? The researchers at the time said they thought it could be water, spurting from underground and freezing as it slid down the crater wall. But today a second team says…maybe not. To review: Here’s that image from 1999: And here’s the second, of exactly the same spot, six years later: Tantalizing, no? “We are talking about liquid water that is present on Mars right now," said Ken Edgett, the scientist who led the analysis, at the time. "It could be acidic water, it could be briny water, it could be water carrying all types of sediment, it could be slushy–but H2O is involved." But now, Jon D. Pelletier and colleagues at the University of Arizona in Tucson have reexamined that crater wall, using higher-resolution imagery from a newer probe, Mars Reconnaissance Orbiter. They’ve also done some computer modeling, and they conclude that a small landslide — sand or gravel — is a better explanation for what the orbiting cameras saw. The deposit could have some liquid in it, but water would spread slightly differently as it came down the slope, especially at the bottom. Take a look at the newer picture, and the Arizona team’s interpretation of it: I traded e-mails with Pelletier and with Alfred McEwen, one of his co-authors. "If it was bright due to water ice it should have disappeared in the summer or at least changed shape if being resupplied somehow, but instead it had the identical outlines after a full summer," McEwen wrote. "I was surprised," says Pelletier. "I started off thinking we were going to prove it’s liquid water." The team is publishing its conclusions in the journal Geology; the abstract is HERE. To a lot of people this finding may feel disappointing; many scientists studying Mars will openly admit they want to find water there. But as MeEwen wrote, "we should always be skeptical, especially about a brand-new observation and preliminary interpretations."