for National Geographic News
As the recently landed Phoenix probe begins to unstow its robotic arm today, scientists are eagerly waiting for its analysis of Mars's arctic soil, which could offer new clues to the red planet's habitability.
But data from the rover Opportunity is already suggesting that water on early Mars billions of years ago may have been fit for pickling—not supporting—life.
That's because the water was thick with salt and other minerals, making it far too briny for life as we know it, according to a new study.
Nicholas Tosca of Harvard University and colleagues studied mineral clues from the surface of Mars sent back by the rover and used computers to turn back the clock.
"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 Knoll, a study co-author also from Harvard.
But instead the team found that the soil's mineral content would have made that liquid a salty, toxic stew.
"No matter how far back we peer into Mars's history, we may never see a point at which the planet really looked like Earth," Knoll said.
For their study, the researchers looked at the same mineral formations that have been used as proof that the planet once had abundant water.
"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," lead author Tosca said.
"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."
The team's findings are based on a property called water activity.
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