After circling the Red Planet for more than a full Martian year, NASA's MAVEN spacecraft has discovered the culprit behind much of Mars' missing air: a young, rambunctious sun.
The findings, published in Friday's edition of the journal Science, point to solar wind and radiation as one of the main drivers behind atmospheric loss — and may help scientists better understand how a once-habitable planet became such an arid world.
But over the intervening eons, Mars lost the thick atmosphere that allowed water to remain stable in liquid form. The planet transformed into the rusty, dusty rocky body we see today.
"If it was warm and had liquid water, it could have had the conditions necessary for life to arise," said study coauthor Marek Slipski, a graduate student in planetary science at the University of Colorado, Boulder. "One way to learn about what this early atmosphere could have been like is to understand how it's been stripped away, and how much has been removed."
Scientists are particularly interested in what happened to the carbon dioxide that used to be in the Martian atmosphere — as a greenhouse gas, it would have helped keep the planet relatively warm. But carbon can be lost from the atmosphere in lots of ways — by being stored in rock, for example — which makes its loss to space difficult to track.
Argon, on the other hand, is a noble gas that hardly reacts with other atoms. That means it can provide a better measure of the escape route the carbon dioxide may have taken.
And that's where MAVEN comes in. Launched in 2013, the Mars Atmosphere and Volatile Evolution Mission has been orbiting our rusty, dusty neighbor since 2014, collecting data on the planet's thin upper atmosphere — the layer of air where molecules can escape to space.
Slipski and his coauthors examined the abundance of two argon isotopes, argon-36 and its heavier sibling argon-38. Because of that slight difference in mass, the isotopes tend to separate out over time, with more of the heavier one staying in the lower layers while the lighter one ends up far more abundant in the higher layers. This makes argon-36 far more susceptible to getting knocked out of the atmosphere by the solar wind and solar radiation.
The scientists also had measurements of the abundance of argon isotopes in the lower atmosphere thanks to landers like the Mars Curiosity rover. By comparing the ratios of the two isotopes in different layers of the atmosphere, the researchers found that about 66% of the planet's argon-36 was lost to space through a process known as sputtering. That's where high-speed ions carried by the solar wind slam into Mars and knock atoms out of the atmosphere, like a cue ball on a pool table.
It's not clear how much the loss of argon does or doesn't correlate with overall atmospheric loss, Slipski said. But it does show that sputtering may be responsible for the disappearance of much of the Red Planet's air, and at least 10% to 20% of its atmospheric carbon dioxide, if not more.
The story makes sense: In the solar system's early days, the sun was a much more active star, with more intense ultraviolet radiation and stronger solar winds that would more easily strip a planet's atmosphere if weren't protected by a strong magnetic field, as Earth is. (Mars once had a magnetic field but lost it fairly early in its history.)
The findings could shed light on the potential habitability of distant planets. They could also help scientists determine whether life may have ever made a home on Mars.
"Once we understand better what the atmosphere was like and how long it was there and what the conditions were at the surface, we can try to understand whether life could have existed there in the past," Slipski said.
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8:15 p.m.: This article has been updated with additional details and comments.