Sedimentary rock on Mars suggests large, ancient lake beds
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Examples of layered outcrop landscapes photographed by Mars Global Surveyor (Images copyright Science Magazine)
(CNN) -- Scientists looking at satellite images of Mars have detected
evidence of sedimentary rock dating back billions of years, suggesting
that the planet once teemed with large lakes.
If Mars harbored life in its early history, fossils might be found
within such sedimentary rock layers, according to planetary
NASA plans to send numerous rovers and satellites to Mars this decade
to search for signs of water or life. But the agency might have to wait
for the next generation of spacecraft before it can search the newly discovered sites.
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The outcrops, some several kilometers thick, are situated inside steep
gullies, inside craters and between craters, locations too dangerous
for the current slate of NASA probes to visit.
"Such locations are inaccessible to presently conceived lander/rover
missions, which are dictated by engineering constraints rather than
science objectives," said Michael Malin and Kenneth Edgett in a recent
correspondence to the American Geophysical Union.
'Hundreds of layers'
Malin and Edgett, authors of the new report, are conducting an
extensive study of high-resolution images taken by the Mars Global
Surveyor, which has orbited the red planet since 1997.
The two created a scientific stir in June when they announced the
discovery of visual evidence of recent water flow near the surface of Mars.
According to their study, to be published in the December 8 issue
of Science, Mars has numerous layered geologic outcrops that date back
at least 3.5 billion years, early in the planet's geologist history.
The prevalence of such outcrops in basins and craters suggest that
water carried the sediments into the depressions and formed lakes
inside them, they said.
"Some of the images of these outcrops show hundreds and hundreds of
identically thick layers, which is almost impossible to have without
water," Malin said in a statement.
Malin and Edgett acknowledge that many questions remain. Mars has no
traces of gullies or streams through which water might have transported
the sediments. They speculate that erosion might have wiped out signs
of such channels.
Malin likened the geologic history of Mars to a jigsaw puzzle.
"In the center of the puzzle, we have these layered rocks,
which are good evidence of an extremely dynamic environment," Malin said.
"On either side of this well-developed puzzle piece, we have
mysteries. In any case, Mars sedimentary rocks suggest a very active early history for the planet."
They also allow that other processes might be
responsible for the sedimentary layering. Periods of high atmospheric
pressure, caused by fluctuations in carbon dioxide levels, could have
increased the ability of the air to carry surface dust.
Mars scientists James Head III of Brown University greeted the new report with excitement.
"I think they've made a compelling case that sedimentation took place," he said.
"One of the interesting things about this new (Mars Global Surveyor)
data -- it's kind of like looking at Mars under the microscope. You can
see things you couldn't possibly see before."
"Seeing layers is really important. It means we can get to a new level
of discussion about the origins of these things," said Brown, who last December said Surveyor images landforms that resembled ancient coastlines.
"If conditions might have been appropriate for life, these are
important candidate sites to look for fossils," Head added. On Earth,
sedimentary rock layers are prime locations to find the fossil remains
of ancient life forms.
Other red planet researchers were not so enthusiastic.
"Maybe there are more details about what has been shown before, but
there is nothing strikingly new to the Mars science community," said
Kenneth Tanaka of the U.S. Geologic Survey.
"The question is, what is the source of that layering. There are
different ways you can get sedimentary layers. What they seem to prefer
is to say that it was done by water. But they also say it might be dry
sources," said Tanaka, who has proposed that carbon dioxide, not water,
could have shaped geologic features on Mars.
"Was it water, carbonated water or something even more exotic?" he
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