After circling the Red Planet for a whole Martian year, NASA’s MAVEN spacecraft has discovered the culprit behind much of Mars’ missing air: a young, rambunctious sun.

The findings, published in 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 today.

Billions of years ago, scientists think, the Red Planet may have looked a little more like a Pale Blue Dot – with puffy clouds in a thick, Earth-like atmosphere over rivers, lakes, and perhaps even seas of liquid water. Depending on how long they lasted, those conditions were potentially ripe for life to form. But over the intervening eons, Mars lost the thick atmosphere — particularly the world-warming greenhouse gas, carbon dioxide — that allowed liquid water to remain stable and 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 co-author 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.”

So where exactly did all that air go? Launched in 2013, MAVEN (short for 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 — for more than an entire year on Mars (a Martian year is equivalent to nearly two Earth years).

For this paper, the scientists looked at the abundance of two argon isotopes, Ar-36 and its heavier sibling Ar-38. Because of that slight difference in mass, the isotopes tend to separate out over time, with more of the heavier isotope staying in the lower layers while the lighter isotope ends up far more abundant in the higher layers. This leaves the lightweight Ar-36 far more susceptible to getting knocked out of the atmosphere by the solar wind and solar radiation over time.

The scientists are particularly interested in what happened to the atmospheric carbon dioxide, which as a greenhouse gas would have helped to keep Mars 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 more difficult to track. Argon, as a noble gas, does not really react with other atoms and so provides a better measure of that particular escape route.

The scientists also had measurements of the argon isotope abundances in the lower atmosphere thanks to landers like the Mars Curiosity rover. By looking at the differences in the abundance of the two isotopes in different atmospheric layers, the researchers found that the planet has lost about 65% of the argon that was once in its atmosphere to space. That’s thanks to a process known as sputtering, 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 that 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 some of its carbon dioxide. It 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, and would more easily strip a planet’s atmosphere if it did not have a strong magnetic field protecting it, as Earth does.

The findings could shed light on the potential habitability of distant planets, and could help scientists in their quest to understand 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.