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Which distant Mars-alikes could we live on? Ask these Red Planet data-sifters

MAVEN probe observations shed light on atmosphere loss

Scientists are applying knowledge gained from studying Mars to calculate how the atmospheres of theoretical exoplanets would behave.

David Brain, professor at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder and co-investigator for NASA's MAVEN (Mars Atmosphere and Volatile EvolutioN) mission, told a meeting of the American Geophysical Union yesterday how his team used data gathered from MAVEN to estimate how a Mars-like planet might behave when in orbit around a different star.

Mars' atmosphere has been scraped away over four billion years since it lost its protective magnetic field (magnetosphere), which was likely caused by meteorite strikes. The MAVEN probe has been in Martian orbit since 2014, measuring the rate of atmospheric loss due to various causes and sending data back to Earth for the team in Colorado to analyse.

"The MAVEN mission tells us that Mars lost substantial amounts of its atmosphere over time, changing the planet's habitability," said Brain. "We can use Mars, a planet that we know a lot about, as a laboratory for studying rocky planets outside our solar system, which we don't know much about yet."

The calculations were based on a Mars-like planet orbiting a small M-class star. This class is not as bright as our Sun so the theoretical planet would have to be much closer to its star than Mars is in order to develop under the same conditions. The distance between the theoretical star and planet would be less than that between the Sun and Mercury.

At this distance, however, it is believed there would be five to 10 times more ultraviolet radiation than Mars receives. As UV rays are a significant source of atmospheric damage, both causing it itself and exacerbating other causes, the calculations showed that an "exo-Mars" orbiting an M-class star would be habitable for five to 20 times less time than ur-Mars would have been. A particularly active star prone to solar storms would decrease this period by a factor of 1,000.

This is only the case for Martian conditions, however. A different planet could have its own chemical processes capable of replenishing the atmosphere, or have a greater mass or magnetic field that would fend off the stellar wind, which would give the atmosphere greater protection.

Bruce Jakosky, MAVEN's principal investigator, said: "Habitability is one of the biggest topics in astronomy, and these estimates demonstrate one way to leverage what we know about Mars and the Sun to help determine the factors that control whether planets in other systems might be suitable for life." ®

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