How The Sun Is Stripping Away Mars’s Atmosphere In Sudden Bursts

Solar Wind Waves Are Ripping Chunks of Mars’s Atmosphere Into Space

Mars is losing its atmosphere, and scientists have just caught one of the more dramatic ways it happens. Concentrated clouds of charged gas, born from a wave pattern familiar to anyone who has watched wind ripple across a lake, are hurling chunks of the Martian atmosphere into space in short, intense bursts. Researchers used two spacecraft watching Mars at once to catch these bursts in the act and pin down, for the first time, roughly how big they are.

Solar wind, the constant stream of charged particles blowing off the Sun, strips gas from Mars over time through several processes. Scientists already understood two steady pathways: a dayside plume where ions get flung outward from the top of the atmosphere, and a nightside channel where ions get swept into Mars’s magnetic tail before drifting off into space.

This new study, published in the journal Science Advances, adds a third: short bursts called plasma clouds, generated by something called the Kelvin-Helmholtz instability, that pack 10 to 100 times more escaping gas than those two steadier channels. Nobody yet knows how much that adds up to across the whole planet over time, but catching the bursts in the act is a first step toward finding out.

Two Spacecraft Confirm Plasma Clouds Form at Mars

Earlier studies had only one spacecraft to work with, which created a real problem. Scientists could see plasma clouds forming, but they had no way to check what the solar wind was doing further out at the same moment. Without that context, nobody could tell whether the clouds were triggered by disturbances arriving from the Sun or whether Mars was generating them on its own.

That changed with Tianwen-1, China’s Mars orbiter that arrived in 2021. Paired with NASA’s long-running MAVEN spacecraft, the two probes gave researchers a complete picture for the first time. While Tianwen-1 sat upstream watching the solar wind, MAVEN watched the boundary where that wind meets the Martian atmosphere. During the events described in the paper, Tianwen-1 recorded a generally steady solar wind with no major disturbances, while MAVEN simultaneously detected plasma clouds forming and escaping nearby. That let the team rule out solar wind gusts as the trigger, pointing instead to the Kelvin-Helmholtz instability, much like wind stirring ripples across a lake until they curl into rolling vortices at the water’s edge.

Solar Wind Waves Trigger Mars’s Plasma Clouds

Mars lacks a strong global magnetic field, unlike Earth, so nothing deflects the solar wind before it reaches the upper atmosphere. Where the fast-moving solar wind meets the slower gas ions native to Mars, the sharp difference in speed whips up the same kind of vortex, trapping pockets of Martian ions and flinging them outward as a plasma cloud. Most carried somewhere between 1 million and 100 million ions escaping through every square centimeter each second, a patch of atmosphere roughly the size of a fingernail.

Plasma Clouds Are Smaller Than Scientists Assumed

Earlier single-spacecraft estimates suggested these clouds stretched anywhere from about 2.5 to 6 times the radius of Mars, a range too wide to calculate how much gas any single cloud carried away. With the two probes positioned about 1,900 kilometers apart during one isolated event, MAVEN detected a plasma cloud that Tianwen-1 completely missed, meaning that cloud had to be smaller than the distance separating them. That single case shows at least some plasma clouds are far more compact than earlier estimates assumed.

They also don’t happen evenly around the planet. “Instead, it is mainly observed on one side of the planet, depending on the direction of the solar wind electric field,” said lead author Chi Zhang, a research scientist at Boston University’s Center for Space Physics. That lopsidedness gives researchers another clue for predicting where and when these events are likely to strike.

Plasma Clouds May Have Shaped Mars’s Ancient Climate

Billions of years ago, Mars likely had a thicker atmosphere and liquid water on its surface. “Mars is thought to have once been potentially habitable, with a thicker atmosphere and surface liquid water,” Zhang said. “Understanding how it became the cold, dry planet we see today is important for understanding how planetary environments evolve over time.” Sunlight breaking apart atmospheric molecules currently drives most of Mars’s ongoing gas loss, but the solar wind is believed to have mattered far more in the past, when a younger, more energetic Sun blew a much stronger wind. That could have made wave-driven escape more common on early Mars, though this study did not calculate how often it happened.

Whatever the final tally turns out to be, it won’t stop at Mars. Boston University co-author Chuanfei Dong noted the same wind-driven process could play out anywhere lacking a strong magnetic field, “including some exoplanets.” NASA’s newly arrived ESCAPADE mission is expected to pick up that thread as MAVEN winds down after more than a decade at Mars.

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