More Wet Gullies Options

[Tom Tamlyn]

A new paper argues that HiRISE photos show evidence for recent short-lived surface water flows. Neat pictures on Phil Plait's site (my source for the news).

It would be interesting to learn if there was any MGS coverage of the same area.

TTT

[Dysgraphyk]

A new paper showing recurring dark streaks.
Nature comments : "Water seems to flow freely on Mars"

McEwen, A. S. et al. Nature Geosci. http://dx.doi.org/10.1038/ngeo2014 (2013).

River-like valleys attest to the flow of water on ancient Mars, but today the planet is dry and has an atmosphere that is too thin to support liquid water on the surface for long. However, intriguing clues suggest that water may still run across the surface from time to time.

In 2011, for example, researchers who analysed images from NASA's Mars Reconnaissance Orbiter (MRO) spacecraft observed dark streaks a few metres wide that appeared and lengthened at the warmest time of the year, then faded in cooler seasons, reappearing in subsequent years2. "This behaviour is easy to understand if these are seeps of water," says planetary scientist Alfred McEwen of the University of Arizona in Tucson, who led that study. "Water will darken most soils."

The streaks, known as recurring slope lineae, initially were found at seven sites in Mars's southern mid-latitudes. The water may have come from ice trapped about a metre below the surface; indeed, the MRO has spotted such ice in fresh impact craters at those latitudes.

McEwen and his colleagues have now found the reappearing streaks near the equator, including in the gargantuan Valles Marineris canyon that lies just south of it. The MRO has turned up 12 new sites — each of which has hundreds or thousands of streaks — within 25 degrees of the equator. The temperatures there are relatively warm throughout the year, says McEwen, and without a mechanism for replenishment, any subsurface ice would probably already have sublimated.

He says that this suggests that water may come from groundwater deep in the crust (...)

But even though McEwen says that water is the most likely explanation for the streaks, he is not sure of the sources. Some of the streaks seem to begin at the tops of ridges, too close to the surface to easily be explained by subsurface aquifers. So the water may come instead from atmospheric water vapour that is pulled into salts in the soil and later released.

"It is quite difficult to understand how [the streaks] can occur with the current understanding of Mars," says Gerhard Kminek, vice-chair of COSPAR's planetary protection panel and planetary protection officer at the European Space Agency. "And that makes it more interesting of course."

source : Nature news