- A salty sea once washed over the plains of Mars at the
Opportunity rover's landing site, creating a life-friendly environment
more earthlike than any known on another world, NASA scientists announced
today.
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- The rover found evidence for the shores of a large body
of surface water that contained currents, which left their marks in rocks
that developed at the bottom of the sea. Opportunity found a distinct chemical
makeup in the rocks and unique layering patterns that must have been generated
by slow-moving water in an evaporating sea, researchers said.
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- The discovery casts fresh light on the possibility that
critters could have gained a toehold on the red planet when it was younger,
warmer and wetter. Geologists inside and outside NASA are elated over the
discovery, saying it could resolve a decades-long debate over whether Mars
ever was in fact warmer.
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- Scientists don't yet know how deep the ocean was, exactly
when it existed or for how long.
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- The finding builds on the March 2 announcement that Meridiani
Planum, where Opportunity landed, had long ago been soaked with water.
Geologists could not tell from those initial results whether the water
was above the surface or only underground.
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- "We think Opportunity is now parked on what was
once the shoreline of a salty sea on Mars," Cornell University's Steve
Squyres, principal science investigator for the Mars rover mission, said
in a statement provided to SPACE.com prior to a press conference today.
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- The rocks would be excellent preservers of biological
signs, if life ever existed on Mars, Squyres said. That makes Meridiani
Planum a prime target for future missions that would search for evidence
of past biology.
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- An ancient sea also implies that early Mars was warmer
than today, said University of Colorado geologist Bruce Jakosky, who was
not directly involved in the finding. And he said it suggests that any
possible microbes on Mars would not have had to rely only on relatively
inefficient subsurface, geochemical energy, but might have used direct
sunlight as an energy source.
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- "If it's surface water, that would allow the possibility
of photosynthetic organisms," Jakosky told SPACE.com. "Once you
can tap into sunlight, it leaves open the possibility for a much greater
abundance of life if it was ever there."
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- Scientists so far have no firm evidence that Mars was
ever inhabited, however.
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- Sedimentary signs
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- The crucial clues that came together in recent days included
the detection of chlorine and bromine, indicating a salty sea had evaporated
over time, scientists said. Also significant were crisscrossing layers
of sediment in the rock that revealed they formed beneath currents of moving
water.
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- Early interpretations of the same lines of research led
to the previous discovery that the rocks were once soaked in water, but
it wasn't clear if the water was present when the rocks formed, or if the
water came later. Increased confidence in the bromine finding strengthened
the case that the particles which formed the rocks had precipitated out
of surface water, with salt concentrations that increased as the water
evaporated.
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- Some layers within the rock are at telltale angles to
the main layers. Scientists call the patterns "crossbedding."
Other features, called festooned layers, involve smile-shaped curves produced
by shifting, rippled shapes of loose sediments under a current of water.
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- The patterns indicate sediment the size of sand grains
had bonded together into ripples in water that was at least 2 inches (5
centimeters) deep -- and possibly much deeper, said John Grotzinger, a
rover science team member from the Massachusetts Institute of Technology.
The water flowed at a rate somewhere between 4 and 20 inches every second
(10 to 50 centimeters per second). That's up to about 1 mile an hour.
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- "Ripples that formed in wind look different than
ripples formed in water," Grotzinger said.
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- The rocks may have been cast in a salt flat that was
alternately wet and dry, Grotzinger added. Similar environments on Earth,
at the edge of oceans or in desert basins, sometimes have currents of water
that produce the type of ripples seen in the Mars rocks.
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- The researchers cautioned that their results so far do
not allow them to determine if the sea was deep and long-lasting, or if
it was very shallow and receded rapidly. Squyres said, however, that they'd
found something "you could swim in" as opposed to the sort of
water that could only be drawn from a well.
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- The findings were vetted by six scientists not on the
rover team, prior to being released today. Included in the data were several
dozen close-up images of the smiling structures.
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- "I was astonished," said one of the outside
experts, Dave Rubin, a sedimentologist with the U.S. Geological Survey.
"There on Mars were sedimentary structures that we see on Earth."
Rubin said he knew of no processes that could better explain what the images
show. "I think the explanation the team put together is the best explanation
for those rocks."
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- Brighter prospects
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- The discovery re-ignited enthusiasm over Mars as a potential
well for biology, at least in the past. (Researchers are unsure whether
any life that ever developed on Mars -- if it did -- could have endured
into the present era, with Mars being cold and dry.)
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- "The particular type of rock Opportunity is finding,
with evaporite sediments from standing water, offers excellent capability
for preserving evidence of any biochemical or biological material that
may have been in the water," said Squyres, the rover mission's chief
scientist.
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- The discovery quite literally brightens the prospects
for past life in another important way.
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- Had the water been only subsurface, life would have had
to rely on geochemical energy, such as the decay of rocks into methane.
On Earth, dependence on geochemical energy is a limiting factor for underground
microbes, said Jakosky, the University of Colorado geologist who is also
director of the university's NASA-sponsored Center for Astrobiology. He
helped pick the rover landing sites but has not been directly involved
in the science explorations.
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- Organisms that depend on geochemical energy are less
diverse and of more limited scope than life that flourishes in sunlight,
Jakosky explained.
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- "This dramatic confirmation of standing water in
Mars' history builds on a progression of discoveries about that most earthlike
of alien planets," said Ed Weiler, NASA associate administrator for
space science. "This result gives us impetus to expand our ambitious
program of exploring Mars to learn whether microbes have ever lived there
and, ultimately, whether we can."
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- Weiler said the results are "beyond our wildest
expectations" for the rover mission. "If you have an interest
in searching for fossils on Mars, this is the first place you'd want to
go."
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- Any such fossils would almost surely be microbial and
not detectable by the current rovers.
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- Most scientists agree that finding signs of past or present
life will likely require sending human geologists or, in the near term,
sending a robot to bring back samples for study in laboratories on Earth.
Meridiani Planum is, for now, the best destination for such a mission,
which NASA has slated for launch sometime in the next decade.
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- "Someday we must collect these rocks and bring them
back to terrestrial laboratories to read their records for clues to the
biological potential of Mars," said James Garvin, lead scientist for
Mars and lunar exploration at NASA Headquarters.
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- Hints at 'warm and wet' Mars
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- Finding a sea on Mars also suggest the planet was once
warmer. Scientists have been arguing for decades over whether Mars was
once warm and wet or just wetter and cold.
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- "I think that this seems to point toward Mars being
warm enough and wet enough to allow standing water" in the distant
past, Jakosky said.
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- Squyres cautioned against making climate interpretations
from the new finding just yet. He said it's possible the sea might have
existed under a cover of ice in a cold climate.
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- But geologist Bob Craddock, science advisor to the under
secretary for science at the Smithsonian Institution, says he can't figure
any way to develop the sedimentary deposits seen by Opportunity other than
in a salty sea that evaporates in a climate warmer than what exists on
Mars today.
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- "You need a sediment supply for those ripples,"
Craddock told SPACE.com. "If the lake is frozen with any appreciable
depth, then the surrounding rivers and streams are frozen solid and are
not providing any sediments."
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- Craddock, who is not involved in the rover mission, concluded:
"These deposits and their mineralogy clearly indicate that the climate
of Mars was different in the past. For the first time we're looking evidence
that an alien planet that was earthlike at one point in its history."
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- Shorelines everywhere?
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- Opportunity had spent its entire time on Mars, since
landing in late January, inside a shallow crater studying soil and the
exposed shelf of bedrock. The most recent and telling observations came
by taking 152 microscopic pictures of a rock named Last Chance.
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- The findings add to previous rover discoveries of hematite,
a mineral typically formed in water, and the layered rocks being laden
with salts, which led scientists to conclude the region was at least soaked
with groundwater.
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- Other investigations from orbiting spacecraft have revealed
possible shorelines elsewhere on Mars, but no ground measurements have
confirmed any of those findings, and some scientists had remained skeptical
that Mars ever harbored any significant surface water.
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- Scientists have said they see no visible shorelines surrounding
the Meridiani Planum, another reason they can't yet deduce the size of
the ancient body of water. And Squyres said the apparent shoreline that
Opportunity has detected may have been part of a moving phenomenon -- meaning
that many shorelines may have developed through time as the body of water
evaporated.
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- The rover left the crater this week and will now try
to examine what scientists think is a larger rock outcropping nearly a
half-mile (700 meters) away at the Endeavor crater. To get there, the rover
will roam across a vast, flat plain that is, overall, the size of Oklahoma.
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- At the larger Endeavor outcropping, researchers hope
to find more extensive layers and read them like pages of a history book,
to learn more about the depth, breadth and timing of the ocean that long
ago graced the red planet.
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