The study authors think Mars's wet period ended before the close of the Noachian epoch, the planet's earliest geologic age. That means Mars was wet sometime between 4.6 billion and 3.8 billion years ago.
Sweet and Sour?
At first blush, the finding appears to contradict a paper published in May in the journal Science.
Harvard University researcher Nicholas Tosca and his colleagues reported finding highly concentrated minerals in the Meridiani Planum region of Mars.
The minerals suggest Mars's surface was once covered in toxic brine—not a place where known life-forms would prosper, the scientists said.
"This goes 180 degrees in the other direction," Mustard said of the current study.
But, he added, the results may still be compatible.
Tosca said in an email, "salty conditions may have followed an era where clay formation dominated, or the two may have overlapped, leading to a diversity of chemical conditions."
Joe Michalski, an astrophysicist at the Université Paris-Sud who was not involved with the new study, called it a "huge contribution" that's "opened the door for ten years of research and debate."
He said astronomers have made good progress interpreting data coming back from orbiting spacecraft.
"It's clear that we need to work together to figure out a way to go to Mars, collect a sample of these rocks, and bring them back to Earth," he said. "That's really going to be the next milestone."
NASA's Mars Science Laboratory, set to launch in the fall of 2009, is the next hope for securing such samples.
Mars scientists will discuss possible landing sites at a workshop in September.
One of Mustard's graduate students, Bethany Ehlmann, will suggest two well-preserved deltas—sediment formations likely left by flowing water—in the Jezero Crater.
Ehlmann and colleagues reported the location last month in Nature Geoscience.
The crater once contained a lake bigger than Nevada's Lake Tahoe, they say.
"If any microorganisms existed on ancient Mars," Ehlmann said in a statement, "the watershed would have been a great place to live."
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