Water flows on Mars, before our very eyes
18:00 06 December 2006
NewScientist.com news service
Something recently flowed through this Martian gully, indicated by arrows, depositing a light-coloured material between December 2001 and September 2005
This 24-metre-wide crater was first spotted in December 2003. It was not present in an earlier image from April 2001 (Image: NASA/JPL/MSSS) Liquid water has flowed on the surface of Mars within the past five years, suggest images by the now lost Mars Global Surveyor (MGS). The results appear to boost the chances that Mars could harbour life.
In 1999, MGS spotted gullies carved on the sides of Martian slopes. Thousands of gullies have been imaged since then, most recently by the Mars Reconnaissance Orbiter (MRO) (see Stunning snaps from the best camera ever sent to Mars).
Many scientists believe the gullies were carved by liquid water, although others have argued they are due to avalanches of carbon dioxide gas or rivers of dust.
The gullies appear to have formed sometime in the past several hundred thousand years, since impact craters have not accumulated on top of them. But exactly how long ago material flowed through them has not been clear.
Now, new flows have appeared in two of the gullies monitored by MGS, showing that they have been active within the past several years. The research was led by Michael Malin of Malin Space Science Systems in San Diego, California, US. That company operates the Mars Orbiter Camera on MGS, which acquired the images.
One gully on a crater wall that was imaged in 2001 was found to have filled with light-coloured material when it was re-imaged in 2005. A similar new light-coloured deposit appears in a 2004 image of crater gullies previously imaged in 1999.
The researchers suggest the deposits were made by liquid water flowing out from beneath the surface. The researchers estimate that each flow would have involved 5 to 10 swimming pools' worth of water.
It would have been similar to a flash flood in the desert, says team member Ken Edgett of Malin Space Science Systems. "If you were there and this thing was coming down the slope, you'd probably want to get out of the way," he says.
Any liquid water exposed to Mars's atmosphere would quickly freeze, but Malin's team says even if the exterior of the flow rapidly freezes, water could continue flowing much farther inside this ice shell, developing into a thick mixture of ice and sediment that would eventually freeze completely.
In Mars's thin atmosphere, ice left on the surface would quickly sublimate, changing from a solid to a gas, and disappear. But water vapour diffusing out from deeper in the mixture of ice and sediment could repeatedly coat the surface with frost, maintaining its light colour long enough for MGS to spot it, the researchers say.
Alternatively, salt deposited from salty water or sediment placed there by water flow may be responsible for the light colour.
MGS team member Phil Christensen of Arizona State University in Tempe, US, who was not involved in this study, says he is convinced that the gullies were formed by the action of liquid water.
"It says something is actively going on today in at least some of these gullies and one intriguing possibility is that water was released," he told New Scientist.
"I think they make a pretty good case that these aren't simply dust avalanches or some wind-related process," he says. He adds that the sublimating carbon dioxide scenario is even less likely, because temperatures in the regions where the gullies are found – between 30