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updated 10/10/2010 12:24:59 PM ET 2010-10-10T16:24:59

NASA has approved a new unmanned mission to Mars, one aimed at explaining exactly how the Red Planet lost most of its atmosphere.

A spacecraft is scheduled to launch in late 2013 and begin orbiting around Mars about 10 months later for a yearlong study. Scientists suspect that the sun has been stealing off Martian air for eons, and they expect the new probe to put that theory to the test.

NASA has given the $438 million project, which was first proposed in 2008, a heady name: the Mars Atmosphere and Volatile Evolution mission. Scientists are calling it "Maven" for short. [ Video NASA's Maven Mars Probe . ]

"Maven will examine all known ways the sun is currently swiping the Martian atmosphere, and may discover new ones as well," said Joseph Grebowsky, the mission's project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., in a statement.

Lost Martian atmosphere

Astronomers have found clues to Mars' lost atmosphere by studying images of the planet from orbit and from the surface. Mars is dotted with formations resembling dry riverbeds and minerals, which suggests Mars once had a thicker atmosphere and was warm enough for water to exist as a liquid on the surface.

But somewhere along the road, something changed.

Mars has been cold and dry for billions of years. Its atmosphere is so thin that any liquid water on the surface would quickly boil away, and the sun's ultraviolet radiation scours the ground. Such harsh conditions may have meant the end for any Martian life, if it ever existed.

The lead suspect in the loss of the Martian atmosphere is the sun, chiefly through its solar wind a million-miles-per-hour stream of charged particles. A global magnetic field shields Earth's atmosphere by diverting most of the solar wind around it. But Mars lost its magnetic field billions of years ago, researchers said. So its atmosphere was exposed to the solar wind and probably stripped away.

The sun's emissions could have stolen the Martian atmosphere in other ways, too, scientists said. For example, solar heating could give some atmospheric atoms enough speed to escape Mars' gravity and zip off into space.

Other factors could have contributed to the loss of Mars' atmosphere. Mars has more than 20 ancient craters larger than 600 miles (968 km) across. So it could have been massive impacts that blasted large amounts of the Martian atmosphere into space.

Huge Martian volcanoes that erupted after the impacts, like Olympus Mons, could have replenished the atmosphere by venting massive amounts of gas from the planet's interior. But without the protection of a magnetic shield, any replacement atmosphere also would have been stripped away by the solar wind, scientists said.

Maven's job

Other Mars-watching spacecraft have caught hints of how Mars lost its air.

For example, flows of ions from Mars' upper atmosphere have been seen by both NASA's Mars Global Surveyor and the European Space Agency's Mars Express spacecraft.

"Previous observations gave us 'proof of the crime' but only provided tantalizing hints at how the sun pulls it off the various ways Mars can lose its atmosphere to solar activity," said Grebowsky, adding that Maven will also track how atmospheric losses change over time. "Linking different loss rates to changes in solar activity will let us go back in time to estimate how quickly solar activity eroded the Martian atmosphere as the sun evolved."

The Maven spacecraft can also examine how much Mars water has been lost to space by measuring the planet's hydrogen isotope ratios, scientists said.

Isotopes are versions of an element that differ in atomic weight. For example, deuterium is a heavy version of hydrogen. Normally, two atoms of hydrogen join to an oxygen atom to make a water molecule, but sometimes the heavy and rare deuterium takes a "normal" hydrogen atom's place.

On Mars, "normal" hydrogen would escape faster because it is lighter than deuterium. Since the lighter version escapes more often, over time the remaining Martian atmosphere would have less and less hydrogen compared to deuterium.

The Maven team will measure the amount of hydrogen compared to the amount of deuterium in Mars' upper atmosphere. They will compare that ratio to the ratio Mars probably had when it was young, an estimate based on observations of comets and asteroids, which are believed to be pristine "fossil" remnants of our solar system's formation.

Maven is also expected to determine how much Martian atmosphere has been lost over time by measuring the isotope ratios of other elements in the air, such as nitrogen, oxygen and carbon.

"This project is a vital complement to past, present, and future Mars missions," said Michael Meyer, lead Mars scientist for NASA's Mars Exploration Program in Washington. "Maven will take us a step closer in learning about the evolution of our intriguing celestial neighbor."

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