In 2004,
NASA's Opportunity rover found evidence in Martian soils that water had once
flowed across the surface there, buoying hopes that the red planet may once
have supported primitive life.
But a new
study throws some cold water, and a big pinch of salt, on those hopes.
"Liquid
water is required by all species on Earth and we've assumed that water is the very
least that would be necessary for life on Mars,"
said study team member Nicholas J. Tosca, a Harvard University postdoctoral
researcher. "However, to really assess Mars' habitability we need to
consider the properties of its water. Not all of Earth's waters are able to
support life, and the limits of terrestrial life are sharply defined by water's
temperature, acidity and salinity."
Tosca and
his team analyzed salt deposits in the 4-billion-year-old Martian rock
investigated by Opportunity (and by spacecraft orbiting the planet). The new
analysis shows that the water that would have flowed across these ancient
Martian rocks may have been exceedingly
briny.
"Our
sense has been that while Mars is a lousy environment for supporting life
today, long ago it might have more closely resembled Earth," said Andrew
H. Knoll, also of Harvard and on the study team. "But this result suggests
quite strongly that even as long as four billion years ago, the surface of Mars
would have been challenging for life. No matter how far back we peer into Mars'
history, we may never see a point at which the planet really looked like
Earth."
The
research was presented
in February at the annual meeting of the American Association for the
Advancement of Science in Boston.
Halophiles,
or organisms that can tolerate high-salinity waters, are known to exist in
places on Earth, but they likely evolved from organisms that lived in purer
waters, scientists think, making it unlikely that life would actually arise
initially in extremely briny waters.
The high
salinity, however, "doesn't rule out life forms of a type we've never
encountered," Knoll added, "but life that could originate and persist
in such a salty setting would require biochemistry distinct from any known
among even the most robust halophiles on Earth."
Knoll and
Tosca also say the finding doesn't rule out the possibility that less salty
waters once flowed on the planet, though Meridiani Planum, where the Opportunity
rocks were found, is believed to have been one of the wetter, more hospitable
places on the planet.
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