Is water flowing on Mars right now? Ask any planetary physicist two years ago, and the response would have been that the planet is probably dry, bar deposits of non-frozen water hidden a few kilometres beneath the surface. But towards the end of 2006, images from NASA’s Global Surveyor spacecraft published in Science revealed what seemed to be sediment left by rivulets that had flowed down the sides of craters in recent years. It strengthened the possibility, however remote, that Mars is supporting life.

A new study by Jon Pelletier and colleagues of the University of Arizona in the US, together with Randy Kirk from the US Geological Survey, also based in Arizona, suggests the Global Surveyor team may have jumped the gun. Analysing stereo images taken from space by the High Resolution Imaging Science Experiment (HiRISE) on board NASA’s Mars Reconnaissance Orbiter, the researchers have mapped the topography of the Centauri Montes region on Mars where the sediment had been spotted in 2006. They then performed computer simulations to see how this 3D land profile would shape the flow of either water or granular material.

The Arizona researchers found that a flow of water, in which drag would be due to surface roughness, would produce wide channels that end at a rounded point. On the other hand, the flow of granular material, in which drag would be due to viscous and yield stresses, would produce fairly narrow channels that peter out into finger-like extremities. These fingers can clearly be seen in the HiRISE images, leading Pelletier and colleagues to conclude that the tracks at Centauri Montes are not sediment, but are probably, in fact, the remnants of a mini-landslide (Geology 36 211).

“Our findings show that the previous evidence for water was far from definitive,” Pelletier told physicsworld.com. “Of course we cannot prove that liquid water has not flowed on Mars, because it’s always possible that some very minor trickle of liquid water has occurred that is simply too small to see.”

Michael Malin of Malin Space Science Systems in San Diego, US, which operated the camera on Global Surveyor and which published the original analysis in Science, was not aware of the Arizona group’s work so does not yet want to pass judgement. He admits, though, that he “would not be surprised that some of the light-toned features seen in gullies are not formed by water deposition.” He added that definitive observations may not be possible with orbiting satellites.

Existing doubts

There have already been doubts about the conclusions of the 2006 report. Malin’s team attributed the whitish colour of the tracks to ice, frost or salt — deposits that would favour the theory of water flow. If they are indeed ice or frost, they ought to sublime — that is, turn into vapour — over a period of 1–2 Martian years (1.9–3.8 Earth years).

However, other images taken by HiRISE since 2005 have showed no change in the brightness of the tracks that would accompany sublimation. Spectra taken in 2007 by the Compact Reconnaissance Imaging Spectrometer (CRISM), also on board the Mars Reconnaissance Orbiter, have showed no evidence for salts, again implying that there has been no liquid water.

But these results alone have not been enough to rule-out a water origin. Now, although planetary physicists will agree that water has flowed on the surface within the last million years or so — HiRISE scientists also recently discovered the bed of an ancient Martian lake — and that ice is prevalent to some extent, it could be some time before a consensus on present-day water flow is reached. “We have only been closely monitoring Mars for ten years, so it is wrong to say that if we don't have consensus now we will never achieve it,” says Pelletier. “It's early days yet.”