Once again, the ‘water on Mars’ subject made it to the headlines: researchers claim that recent gully activity that took place in the last few years (as documented by photographs taken in 1999 and 2005) suggests that watery sediment flows (debris flows) are shaping the planet’s surface as we speak.
The problem is, of course, that it is difficult to keep water liquid in an environment where the temperature is usually way below 0 degrees Celsius and the atmospheric water vapor pressure is also very low. And, as far as I am concerned, the morphology of the gullies and of the associated deposits does not rule out deposition from dry granular flows at all. Of course, several papers have been written on the subject; here is, for example, an opinion from Allan Treiman (2003):
The salient features of the Martian gullies [Malin and Edgett, 2000, 2001] are consistent with their origin as dry flows of eolian sediment: gully deposits are fine granular material (erodable by wind); eolian sediment are available where gullies form; the distribution of gullies are consistent with deposition of sediment from wind; and the orientations of gullies are similarly consistent with wind patterns. Further, it is clear that granular materials can flow as if they were Bingham liquids, and granular flows can produce landforms with all of the geomorphic features of Martian gullies. No known data concerning the gullies (chronological, geomorphic, or geologic) falsify this hypothesis, so it is worth further investigation.
I just find it interesting that, by the time the story reaches the media, all the uncertainties disappear, and the story is unequivocal: watery flows must occur on Mars today, period.