Rather amazing time-lapse photos in the article if you access it. Large
areas of Mars
show water streaks growing in size based on seasonal variation.
BR
Nature
Water seems to flow freely on Mars
Any areas of water could be off-limits to all but the cleanest spacecraft.
* _Maggie McKee_
(http://www.nature.com/news/water-seems-to-flow-freely-on-mars-1.14343#auth-1)
10 December 2013
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Dark streaks that hint at seasonally flowing water have been spotted near
the equator of Mars_1_
(http://www.nature.com/news/water-seems-to-flow-freely-on-mars-1.14343#b1) .
The potentially habitable oases are enticing
targets for research. But spacecraft will probably have to steer clear of them
unless the craft are carefully sterilized — a costly safeguard against
interplanetary contamination that may rule out the sites for exploration.
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 years_2_
(http://www.nature.com/news/water-seems-to-flow-freely-on-mars-1.14343#b2) .
"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.
(http://www.nature.com/news/dummy-jpg-7.14209?article=1.14343)
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, which could have implications for Martian life: "The subsurface is
probably the best place to find present-day life if it exists at all
because it is protected from the radiation and temperature extremes," he says.
"Maybe some of that water occasionally leaks out onto the surface, where we
could see evidence for that subsurface life."
Off-limits
Yet the possibility of running water could put the sites off-limits for
future spacecraft unless they are carefully sterilized. The international
guidelines of the Committee on Space Research (COSPAR) of the Paris-based
International Council for Science say that sites that may host life, called
'special regions', should only be visited by probes that have been thoroughly
treated to prevent microbes from hitching a ride from Earth. "You wouldn't
want to send a dirty spacecraft to these places because you'd have the
potential to not discover what you're looking for, but what you took with
you,"
says John Rummel, chair of COSPAR's panel on planetary protection.
Spacecraft or their components that come into contact with special regions
have to be sterilized meticulously with heat, hydrogen peroxide vapour or
ionizing radiation to kill off as much Earth life as possible, rather than
simply swabbed with alcohol. This raises the price of a mission, which could
effectively end an intended visit to one of these areas, says McEwen.
Indeed, heating the two Viking spacecraft in the 1970s is estimated to have
represented 10% of the mission's cost. But Rummel says that trustworthy
results justify added costs.
Rummel and McEwen are part of an advisory group to NASA that is studying
what should be classified as a special region — currently defined as an area
that gets warmer than –25°C for a few hours a year and has available water.
The group is expected to present a report to COSPAR in August. Rummel says
that regions with reappearing streaks could be added to the special
regions under the current definition because of their potential connection
with
water.
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."
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