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Two years ago, Mars went undercover. Martian dust storms are common, but every decade or so, for reasons unknown, a monstrous one goes global, veiling the planet. The storms can be a mortal threat to exploration: The one in 2018 [url=https://www.sciencemag.org/news/2019/01/winds-fail-revive-nasa-s-opportunity-rover]killed off NASA’s Opportunity rover[/url] by coating its solar panels in dust. But now, researchers say the storms may also be one of the culprits in the ultimate martian cold case: how the once-wet planet lost its water.
[color=#333333][size=3][font=Roboto, "Helvetica Neue", Helvetica, Arial, sans-serif][img]https://www.sciencemag.org/sites/default/files/styles/inline__450w__no_aspect/public/ca_1113_Mars_dust_storm.jpg?itok=kMfIUZsv[/img][/font][/size][/color]
[color=#666666][size=1][font=Roboto, "Helvetica Neue", Helvetica, Arial, sans-serif][font=Roboto, "Helvetica Neue", Helvetica, Arial, sans-serif]In 2018, Mars was enveloped by dust storms (seen here in a Mars Express image) that helped water escape the planet.[/font]
[right][color=#808080][size=1]ESA/DLR/FU BERLIN/CC BY-SA 3.0 IGO[/size][/color][/right]
[/font][/size][/color]Fossilized rivers and deltas etched across Mars [url=https://www.sciencemag.org/news/2020/06/nasa-s-new-rover-will-collect-martian-rocks-and-clues-planet-s-ancient-climate]suggest water flowed there billions of years ago[/url]. Most of it must have somehow escaped to space—yet researchers thought water vapor could not travel high in the frigid, thin atmosphere without condensing into snow and falling back to the surface. New data from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, [url=https://science.sciencemag.org/cgi/doi/10.1126/science.aba5229]published today[/url] in Science, show how churning dust storms may in effect pump water into space. “These escape processes are an effective way to make Mars dry,” says Anna Fedorova, a planetary scientist at the Space Research Institute of the Russian Academy of Sciences.
One known escape process comes from the Sun’s ultraviolet (UV) light, which can split small amounts of water near the surface of Mars, sending hydrogen and oxygen—both lighter than the planet’s mostly carbon dioxide air—percolating to the top of the atmosphere, where they are lost into space. But scientists assume water loss by this mechanism is a trickle.
During the 2018 storm, however, Shane Stone, a graduate student at the University of Arizona, was looking at data from MAVEN, which has studied the planet’s upper atmosphere since 2014. One MAVEN instrument directly samples the gossamer atmosphere as the probe dips to its lowest orbital altitude of 150 kilometers, and Stone and his colleagues couldn’t believe what it was reporting: While the dust swirled at lower altitudes, a deluge of water was reaching the edge of space. “This was really a smack in the face,” Stone says. “The global dust storm stands out in the data like nothing else.”
Earlier hints that dust storms might somehow be lofting water came in 2014, when two teams reported on UV observations made in 2007, after the last global dust storm, [url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL061803]by the Hubble Space Telescope[/url] and the [url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013GL058578]Mars Express orbiter[/url]. The teams noticed a fluorescent fog of hydrogen in the upper atmosphere, which faded as the southern hemisphere’s summer ended and the storm subsided. The only plausible source for that hydrogen was water. “That was the first hint of something weird going on,” says Michael Chaffin, a planetary scientist at the University of Colorado, Boulder, who led the Mars Express work.
Since then researchers using instruments on MAVEN and the European Space Agency’s Trace Gas Orbiter (TGO) [url=https://www.nature.com/articles/s41586-019-1097-3]have found evidence for water[/url] high in the atmosphere during the southern hemisphere’s summer, when solar heating stirs up dust. That was true even when there wasn’t a full-fledged dust storm, says Fedorova, who led the TGO work, [url=https://science.sciencemag.org/content/367/6475/297]published in January[/url] in [color=#333333][font=Roboto, Helvetica Neue, Helvetica, Arial, sans-serif][size=3]Science[/size][/font][/color].
This article first appeared on www.sciencemag.org
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