Air stripper: Solar wind (from the left) blowing against Mars tears atmosphere-filled plasmoids (yellow) from the tops of magnetic umbrellas.
Credit: Steve Bartlett/NASA
SYDNEY: A new study reveals that the solar wind strips away the atmosphere of Mars by ripping chunks of air away, sending them tumbling into deep space.
The surprisingly violent mechanism could also help solve a longstanding mystery, say researchers.
"It helps explain why Mars has so little air," said David Brain an astronomer at the University of California, Berkeley, who presented the discovery at the 2008 Plasma Workshop, held in Huntsville, Alabama, last month.
Relentless chipping away
There have been several theories about what happened to the Red Planet's atmosphere.
An asteroid hitting Mars long ago might have detached a large portion of the atmosphere in a single violent upheaval – or the loss might have been gradual, the result of billions of years of relentless chipping away by the solar wind.
Now, using data from NASA's retired Mars Global Surveyor (MGS) spacecraft, Brain's study entertains a third possibility; a daily ripping process intermediate between the other two mechanisms.
In 1998, the MGS discovered that Mars has a peculiar magnetic field. Instead of a global bubble, like the field generated by the Earth, the Martian field is in the form of 'magnetic umbrellas' that sprout out of the ground and reach beyond the top of the atmosphere.
These umbrellas number in the dozens and cover perhaps 40 per cent of the planet's surface, mainly in the southern hemisphere. For years, researchers thought the umbrellas protected the Martian atmosphere, shielding pockets of air beneath them from erosion by the solar wind.
Magnetic umbrellas
Surprisingly, though, the new study shows that the opposite can be true too: "The umbrellas are where coherent chunks of air are torn away," said Brain.
To make the discovery he studied data from the 25,000 orbits made of Mars by the MGS. During one of those orbits, MGS passed through the top of a magnetic umbrella. Brain noticed that the umbrella's magnetic field had linked up with the magnetic field in the solar wind.
The researchers are not 100 per cent sure what happened next, but Brain has a hypothesis that fits the data.
"The joined fields wrapped themselves around a packet of gas at the top of the Martian atmosphere, forming a magnetic capsule a thousand kilometres wide with ionised air trapped inside," he said. "Solar wind pressure caused the capsule to 'pinch off' and it blew away, taking its cargo of air with it."