Some of the gullies that cut the
sides of Martian craters were likely formed by meltwater from glaciers that
existed a few million years ago, when Mars was wetter than it is now, a new
study suggests.
The gully features are similar to
ones seen in the Dry
Valleys of Antarctica, say the authors of the study, which is detailed in
the Aug. 25 issue of the journal Proceedings of the National Academy of
Sciences. So this polar region of Earth can act as an analog for Mars'
past.
The gullies, young features
geologically speaking, were discovered
in 2000 by NASA's orbiting Mars Global Surveyor, which is now out of
commission. The discovery came as a surprise because scientists had thought
that Mars was too dry in the past few million years to host liquid water at its
surface, as it is today.
Though water ice was confirmed this
summer by NASA's Phoenix Mars Lander just a few inches below the surface of
Mars' arctic regions, the planet's below-freezing temperatures and low
atmospheric pressure ensure that any ice exposed to the air turns into vapor.
When the gullies were discovered,
some scientists proposed that the features were formed either by dry avalanches
or by groundwater pushing up from below the surface and running down the sides
of craters. But in a study of gullies within a 6.5 mile- (10.5 kilometer-)
diameter crater located in the much larger Newton Crater, James Head of Brown
University and his colleagues found that accumulated ice and snow were more
likely the source of the water that sculpted the gullies.
Ice ages
Head and his team examined
high-resolution images of the crater and its gullies taken by orbiting spacecraft
and found evidence of features that suggested glaciers once covered the crater
floor about 10 million to 20 million years ago.
As Mars' orbit and planetary tilt
(with respect to the sun) changed as a result of the natural wobble of the
planet's axis, glaciation waned, and the bulk of the ice was lost. As the ice
started to sublimate (or vaporize), debris embedded within it began to pile up
on the crater floor. These deposits can be seen on the crater floors now, said
study team member David Marchant of Boston
University.
But debris didn't accumulate as much
on the crater walls, leaving behind hollows where the glaciers once sat.
"Where ice used to stand you have a hole," Marchant told SPACE.com.
As the climate continued to change,
not enough snow fell to maintain the glaciers, and the holes left by the
glaciers could have caught windblown snow and preserved it in drifts. In the
summer, these drifts were likely exposed to warmer summer temperatures, melting
the snow and ice deposits, which then ran down the crater slopes, eroding the
sediment and forming the channels and fans of the
gullies.
Glaciers vs. groundwater
These findings likely rule out the
other mechanisms proposed for creating the gullies — gurgling groundwater and
dry avalanches — in this and other craters, Marchant said.
There's no geological evidence for
dry avalanches, and for groundwater to be the cause, melting would have had to
penetrate to the subsurface. "It's a much more difficult thing to
do," Marchant said. "It's much easier just to melt the surface
snow."
Because this new study, funded by
NASA and the National Science Foundation, shows that glaciers pre-existed the
gullies, it seems likely that melting snow and ice are behind the gully
formations.
"It just seems to fit the
geologic story," Marchant said. "For geology it's all about the
sequence of events."
Antarctic analog
Meltwater in Earth's Antarctic Dry
Valleys — also a very dry region — seems to create some of the same patterns in
gully fans as are in seen on Mars. Head and his colleagues have done field work
in the Dry Valleys to better understand the
processes that sculpt the Martian gullies.
The team suspects that some of the
pockets in these gullies might have held water in various forms in the
recent past, over the last few hundreds of thousands of years, periodically
harboring snow and ice when the conditions were right. The researchers also
think that the Antarctic gullies would be a good area for close-up study
because of the relatively recent interaction between water and the Martian
surface.
"It would be great to have a
rover explore these deposits because they may be one of the few places where
water is periodically present in the recent past. This could have implications
for biology," Head told SPACE.com in an email. Water has long been
considered a requisite feature for any potential
Martian life as it is key to life as we know it on Earth.
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