常速英语:Study finds Mars water too salty to sustain life

As the newly deployed Phoenix spacecraft begins its mission in search of life near Mars' northern pole, scientists say an analysis of Martian deposits suggest there was once water on the planet, but it was too salty to sustain life. VOA's Jessica Berman reports.

Scientists at Harvard University in Massachusetts and Stony Brook University in New York analyzed salt deposits in a four-billion-year-old rock that was explored by the U.S. space agency's Mars exploration rover, Opportunity, and by using information from spacecraft orbiting the Red Planet.

The data confirm that water once existed on Mars. Scientists have eagerly sought evidence of water because it is necessary for life.

But in a new study published in the latest issue of the journal Science, researchers conclude that the water that existed on Mars billions of years ago was too salty for any life form to tolerate, at least in Meridiani Planum, a vast region where the rock was found.(Www.hxen.com)

"Meridiani Planum, this locality where the Mars rover landed, may have actually been one of the best places for life to have existed on early Mars," said Nicholas Tosca, the study's lead author. "But the interesting thing is it itself was a pretty harsh place in terms of salinity because when we looked elsewhere [in] other localities where we had information about salt minerals, we found the salinity was much, much worse."

Tosca says the salt content of the water in Meridiani - which scientists believe was the consistency of a thick brine - does not completely rule out that life once existed there.

"If there was any opportunity for life on Mars, it happened when Mars was a very young planet - so it happened very long ago - and the window of opportunity was short," he said. "The conditions were very harsh in terms of salinity and so life needed to get its act together and grab a foothold before the climate turned quite cold and dry like it is today."

Tosca adds it is always possible Mars harbored ancient life forms unfamiliar to scientists on Earth.

Tosca says the Phoenix mission to scoop up and analyze ice beneath the Martian surface near the planet's northern pole will provide more evidence of the planet's chemistry, helping to rule in or out the existence of life on Mars.