Miércoles 13 de Diciembre de 2006, Ip nº 183

Water flows on mars before our very eyes
Por David Shiga

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.

Ice shell
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.

Active today
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° and 60° from the equator – are too high for the gas to get frozen in the first place.

Just dust?
Allan Treiman of the Lunar and Planetary Institute in Houston, Texas, US, agrees that something flowed recently to make the observed changes.

But he is not convinced that water was involved. "There is no direct evidence of water in the images – only that something flowed downhill. My money is on sand and dust, because there's lots and lots of sand and dust on Mars."

Streaks on slopes have been observed before and interpreted as the result of dust avalanches. But these appear to be a separate phenomenon from the new light-coloured gully deposits, the researchers say.

Newly formed dust streaks have been observed, but are always dark. The dust streaks are also usually observed in areas where the surface clearly has a thick coating that could be dust, unlike the two craters in question. And dust streaks have never been observed on the same slopes where gullies carve into the surface.

The formation of new gullies has been observed before also, but these were on the sides of sand dunes, and were more clearly related to avalanching sand (see Landslips, impacts and eroding ice revealed on Mars).

Melting snow
If the deposits are the result of liquid water flow, the source of the water is not clear. Malin's team suggests it comes from underground aquifers, perhaps kept liquid at low temperatures with the help of high salt concentrations.

Christensen says it could result from the removal of dust from a hypothetical layer of snow, which would then melt when exposed to sunlight.

The SHARAD radar on MRO is potentially capable of detecting any underground pockets of water that the flows might have come from, Malin says. "We're hopeful that as SHARAD flies of over these locations it may be able to detect these subsurface aquifers," he says.

The new evidence that liquid water may flow on Mars today boosts the chances that life could be present, Christensen says. "I believe that we have found places on Mars where you could take terrestrial life forms that live on snow or in aquifers and put them there and they would survive," he says.

Malin's team also reports in the same study the formation on Mars of 20 new craters between 2 and 150 metres across since 1999, confirming the previously estimated rate of crater formation and reinforcing the view that crater-free areas of Mars must truly be young or recently modified.

The discovery may be one of the last from MGS, which went silent shortly before its 10th launch anniversary in early November, and has not been heard from since (see Europe joins hunt for missing Mars probe).

  06/12/2006. New Scientist Magazine.