Today, Earth is the only clearly habitable world in the Solar System. But more than 3.5 billion years ago, it may have been joined by at least one other: Mars. We believe it used to have a thick atmosphere, liquid water, and maybe even life on the surface, until something happened to that atmosphere and turned it into a cold, barren world.

Now, thanks to NASA’s MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, we have our best idea yet as to what killed the Red Planet – and all fingers are firmly pointed at the Sun. Three different processes have been identified that stripped our neighbor of its atmosphere and ultimately robbed it of its habitable environment: solar wind, extreme ultraviolet photons from the Sun, and most importantly, solar storm events.

This research is one of 44 new papers being published in Geophysical Research Letters(GRL) today and featured in Science tomorrow. Since it arrived in September 2014, MAVEN has been studying the upper atmosphere of Mars, thousands of kilometers above the surface, in addition to its ionosphere and its magnetosphere – the remnants of a much stronger magnetic field it once had – to determine how Mars became almost uninhabitable.

“On early Mars, with liquid water present and abundant at the surface, one could imagine life existing there,” Bruce Jakosky, principal investigator on the MAVEN mission and lead author on one of the papers in GRL, told IFLScience. “Then the atmosphere disappeared, and any life that was there would have had to migrate under the surface. So we’re learning what processes can affect habitability.”

Instruments on MAVEN were used to make direct measurements of the rate of atmospheric loss from Mars, analyzing the oxygen and carbon dioxide ions being emitted by the planet. There is still an atmosphere around Mars, but based on this new data it is losing what remains at a rate of 100 grams (0.22 pounds) per second – a small but significant amount over time.

Crucially, solar storm events – bursts of energy and particles such as interplanetary coronal mass ejections (ICMEs) – seem to greatly accelerate the rate of loss. MAVEN was able to observe this while monitoring Mars’ remaining magnetic field and found that rope-like tendrils of magnetism extended 5,000 kilometers (3,100 miles) into space during these events.

This resulted in more ions such as oxygen and carbon dioxide being carried from the Red Planet into space, reducing the size of the atmosphere. This finding is illustrated in a data visualization on the cover of tomorrow's issue of Science, seen right.

It means that large chunks of the Martian atmosphere were likely wiped off when Mars lost most of its magnetic field and the Sun was more active over 3.5 billion years ago, resulting in rapid atmospheric loss.

“We saw the effect of a solar storm hit, and when it did, it increased the loss rate by a factor of 10 or 20,” said Jakosky. “So we think early in Mars’ history, when solar storms were more abundant and more intense, when the solar wind was more intense, the loss rate would have been greater than it is today. That’s the key result we’re focused on.”

How Mars lost its magnetic field and allowed the Sun to wipe away most of its atmosphere remains a mystery. But the research can explain how Mars lost most of the carbon dioxide it once had, suggesting it was simply blown into space.

“The gas that used to be in the atmosphere has been removed entirely from the planet. It’s not locked up in the crust, it’s not in the polar caps – it’s gone,” said Jakosky. “One of the problems has always been focused on looking within the crust for a reservoir of carbon-bearing minerals, and they haven’t found it. So I think this really resolves one of the fundamental issues about Mars, where did the volatiles go, all the carbon dioxide.”