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 Opportunity, along with its now-defunct twin, Spirit, landed 10
years ago for concurrent 90-day missions to look for clues of the
past existence of water.
Both rovers did so, confirming evidence collected by orbiting
spacecraft that Mars, the planet believed to be most like Earth in
the solar system, was not always the cold, dry desert that appears
today.
In August 2012, Curiosity, equipped with an onboard chemistry lab,
arrived for follow-up investigations to determine if Mars had other
ingredients essential for supporting life.
The answer, returned very early in the ongoing mission, was a
definite 'yes'.
On the other side of the planet, meanwhile, Opportunity has been
analyzing water-bearing rocks at the rim of an ancient impact crater
called Endeavour.
Rather than the chemical fingerprints of acidic, salty water found
at previous sites, Opportunity discovered telltale clays called
smectites that form in Ph-neutral water.
"It's like drinking water," planetary scientist Ray Arvidson, with
Washington University in St. Louis, said in an interview.
"This would have been a niche for whatever life at the time
existed," he said.
The finding adds to an emerging picture of a planet that spent its
first billion years or so warmer than it is today, with pools of
fresh water on its surface, scientists say.
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Gradually, water activity declined and what did exist became acidic,
scientific findings reveal, and then, beginning about 3 billion
years ago, Mars dried up.
"Most of the activity on Mars in terms of habitability and water
activity was concentrated in the first billion or so years," said
Opportunity lead scientist Steve Squyres, with Cornell University in
New York.
Opportunity is expected eventually to head south toward a ridge on
the rim of Endeavour Crater that appears to contain a much richer
cache of clay-bearing rocks.
Curiosity, which is exploring an area known as Gale Crater, is
driving toward a three-mile high mountain of layered deposits.
By studying rocks at various levels, scientists expect to not only
get a better idea of how long the planet was able to sustain life,
but where conditions might be favorable to perverse key evidence,
such as organic carbon.
The research appears this week in the journal Science.
(Editing by Kevin Gray and Gunna Dickson)
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