for National Geographic News
Mars likely had liquid water early in its past—but it was probably too acidic and oxidizing for life, scientists say.
That's the latest news from the longer-than-expected visits to the red planet by NASA's rovers Spirit and Opportunity, said Andrew Knoll, a Harvard University researcher and member of NASA's Mars program.
"That's not a very good place to live, and it's a worse place for the kind of chemistry that we think gave rise to life on Earth," he said.
Knoll and other Mars scientists presented their latest results in Boston Friday at the annual meeting of the American Association for the Advancement of Science.
Related findings will be published in an upcoming issue of the Journal of Geophysical Research—Planets.
Spirit and Opportunity have been traversing the Martian surface for nearly 1,400 Martian days—well beyond their expected life spans of 90 days apiece.
The machines' most celebrated findings have come in geology, including evidence of water in the planet's past. But they've also shown that the water was high acidic and briny with dissolved minerals.
"At first, we focused on acidity, because the environment would have been very acidic. Now, we also appreciate the high salinity," Knoll said. "This tightens the noose on the possibility of life."
Even if life formed in such an unlikely scenario—there are salt-tolerant microbes on Earth, after all—they probably got a killing blow from meteorites.
About 3.9 million years ago Mars was pummeled by a heavy bombardment similar to the one that has pockmarked Earth's moon.
"We know that large meteorites can have a devastating effect on life," Knoll said. "There would have been a very high probability that the planet would have been hit by sterilizing meteorites."