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High chemical reactions in the Mars atmosphere rip water molecules apart

Mars water is skimming at the top. NASA's MAVEN spacecraft has found water above Mars' upper atmosphere, where its hydrogen and oxygen atoms are ripped off, scientists report on Nov. 13.

“This changes completely how we thought hydrogen, in particular, was being lost in space,” says planetary chemist Shane Stone of the University of Arizona in Tucson.

The surface of Mars was shaped by flowing water, but today the planet is an arid desert (SN: 8/12/14). Previously, scientists thought Mars ’water was lost in a“ slow, steady irrigation, ”as sunlight divided water in the lower atmosphere and hydrogen gradually diffused upward, Stone says.

But MAVEN, which has been orbiting Mars since 2014, has collected water molecules in the ionosphere of Mars, at an altitude of about 150 kilometers. That was surprising: previously the highest water that had been seen was about 80 kilometers (SN: 22/01/18).

That high water varied in concentration as the seasons changed on Mars, with the peak in southern summer, when seasonal dust storms are more frequent (SN: 14/07/20). During a global dust storm in 2018, water levels jumped even higher, suggesting that dust storms will raise the water in a “sudden splash,” Stone says.

The upper atmosphere of Mars is full of charged molecules that are prepared for rapid chemical reactions, especially with water. Thus, the water up there splits rapidly, on average only lasting four hours, leaving hydrogen atoms to float (SN: 27/11/15). That process is 10 times faster than Mars ’previously known ways to lose water, Stone and his colleagues calculated.

This process could explain why Mars has lost the equivalent of a 44-centimeter-deep global ocean in the last billion years, in addition to another 17-centimeter-deep ocean during each global dust storm. That may not explain all of Mars ’water loss, but it’s a start.

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