Video NASA thinks it now understands why Mars has lost so much of its atmosphere: the Sun stripped it virtually clean.

About four billion years ago, solar winds blasting the Red Planet managed to turn it from a warm, moist world to the cold dry desert we see today, we're told.

"To quote Bob Dylan, the answer is blowin' in the wind," said Bruce Jakosky, Mars Atmosphere and Volatile EvolutioN (MAVEN) principal investigator at the University of Colorado.

The agency's MAVEN spacecraft has been orbiting Mars since September 2014, scanning the upper atmosphere. The probe is studying how the planet interacts with solar winds – charged particles that stream out from the Sun at about one million miles per hour.

These particles either pick up ions from the Martian atmosphere and smash them out of the atmosphere, or push them into the atmosphere where they knock out other particles. Right now, Mars is losing about 100 grams a second – or about 2²⁴ particles per second.

There are two main departure routes for the atmosphere: three quarters of the loss comes from particles streaming away from the night side of the planet, while the remaining quarter shoots up from the polar region.

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The same solar winds tear into Earth, but we are protected by our magnetic field, which prevents much of the atmospheric loss. However, about 4.2 billion years ago, Mars lost its magnetic field; about 500 million years after that point, the bulk of its atmosphere was gone.

Today's rate of atmospheric decay on Mars is nothing compared to the rate at which the planet was robbed of its atmosphere four billion years ago. Back then the Sun was much younger and more energetic. The NASA team thinks that in those days the rate of atmospheric loss could have been a thousand times greater.

The Sun provided a good way to test this hypothesis when it belched out three consecutive solar storms in March this year. This boosted solar wind speeds to more than two million miles per hour and raised the rate of atmospheric loss by a factor of 10 or 20.

Is Earth in danger of something like this happening? Not unless we lose our magnetic field, said Jakosky. Our magnetic poles reverse every few hundred thousand years or so, taking a couple hundred years to switch. During that transition, our magnetic field is weaker than usual, making us more vulnerable, but honestly it's not something to worry about.

The announcement is great news for the MAVEN team, and the probe will continue in an extended mission to gather more data. But it's bad news for those of us who dream of terraforming Mars for eventual human habitation.

"People talk about terraforming Mars and using atmospheric particles that have become trapped in the surface material, as if that's where everything went," Jakosky said. "But with it being stripped away it's not there to use, and it's not possible to bring it back." ®