NASA

NASA announced today that its Mars Atmosphere and Volatile Evolution (MAVEN) mission has helped the agency figure out why the Martian atmosphere is deteriorating.

The discovery of water on Mars suggests that, at some point, the Martian atmosphere was rather lush. “Mars appears to have had a thick atmosphere warm enough to support liquid water which is a key ingredient and medium for life as we currently know it," NASA scientist John Grunsfeld said in a statement."

The MAVEN researchers' findings are published across several articles in Science and Geophysical Research Letters.

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The question, then, is how did the Martian atmosphere become so thin and cold? Thanks to the MAVEN mission, NASA has an answer: solar wind, made up of the particles streaming from the sun's atmosphere. NASA explains:

MAVEN measurements indicate that the solar wind strips away gas at a rate of about 100 grams (equivalent to roughly 1/4 pound) every second… a series of dramatic solar storms hit Mars’ atmosphere in March 2015, and MAVEN found that the loss was accelerated. The combination of greater loss rates and increased solar storms in the past suggests that loss of atmosphere to space was likely a major process in changing the Martian climate.

MAVEN's principal investigator Bruce Jakosky said the stripping process was a slow one. "Like the theft of a few coins from a cash register every day, the loss becomes significant over time," he said, adding, "We've seen that the atmospheric erosion increases significantly during solar storms, so we think the loss rate was much higher billions of years ago when the sun was young and more active.”

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The New York Times offers details on how the solar wind affects Mars' atmosphere:

The air disappears in mainly two ways. Sometimes an electron is knocked off an atom in the upper atmosphere, and then the charged atom is accelerated away by the electric and magnetic fields of the solar wind. Particles of air can also be knocked into space through collisions with incoming solar wind particles, like billiard balls.

Luckily, the NASA scientists explain during a press conference, Earth's atmosphere is protected from major atmospheric loss by our magnetic field. Phew.

Danielle Wiener-Bronner is a news reporter.