With bombs: This is how the Sun stripped Mars of its atmosphere and water.

In the complex puzzle of Mars's past, there is one piece that planetary geologists have been trying to fit into place for decades. At the dawn of its existence, the red planet, now arid and dry, may have been a bustling, watery world, with mighty rivers feeding lakes as large as seas. Vast expanses of water that perhaps harbored the promise of life.

Today, however, the neighboring world is nothing more than a desolate, parched, frozen desert, where water, in the form of ice, is scarce and nothing more than the ghost of a once wet past. Where did all the water on Mars go, and why? This is the question, the missing piece of the puzzle that has long tormented scientists.

Now, after more than a decade of meticulous observation and analysis, NASA's MAVEN (Mars Atmosphere and Volatile Evolution) mission, orbiting Mars since November 2013, has just revealed an elusive mechanism that could be the missing piece. An 'atmospheric escape' process called ' sputtering ' that could be the key to understanding how Mars came to lose the vast majority of its water. The discovery has just been published in ' Science Advances '.

The Martian surface tells no story: it's crisscrossed with unmistakable scars from a much wetter past. Valleys that were once rivers, dry lakebeds, and minerals that only form in the presence of water are direct testimony to a time when liquid water flowed freely across the planet.

And we know that, to preserve all that liquid water, Mars must have had, in the past, a much denser atmosphere than it has today, one like Earth's, capable of trapping heat and maintaining high surface pressure. Understanding when and how that atmosphere waned is essential to reconstructing the Red Planet's climatic evolution and determining, in turn, how long it may have remained habitable.

Over the past few years, scientists have accumulated increasing evidence that the solar wind, the constant stream of charged particles emitted by the Sun, could be responsible for the loss of much of the Martian atmosphere. In April 2022, the international astronomical community witnessed firsthand how a powerful solar flare 'passed over' Mercury, stripping off part of its atmosphere in a single blow . It was a violent and punctual episode, different from and much more powerful than the solar wind, which is a continuous flow that never ceases, but the event made very clear our star's ability to 'strip' its planets.

The sputtering detected by the MAVEN mission is an escape process in which atoms are blasted out of the atmosphere by energetic charged particles. "It's like taking a bomb into a pool," explains Shannon Curry, MAVEN principal investigator at the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder and lead author of the study. "But the bomb, in this case, is the heavy ions slamming into the atmosphere very quickly and splattering neutral atoms and molecules." It is, therefore, a constant "bombardment" of tiny cannon shots that, like the water in a pool, "splatters" atoms and molecules from Mars' upper atmosphere, freeing them from the planet's gravitational pull and launching them into space.

Although scientists had previously found indirect clues to this 'sputtering,' they had never observed it directly. Previous evidence, in fact, came from the analysis of argon isotopes in the upper atmosphere of Mars. Lighter isotopes settle higher in the atmosphere than their heavier counterparts. It was discovered that there were far fewer light isotopes than heavy isotopes of argon in the Martian atmosphere, and it turns out that these lighter isotopes can only be removed by sputtering. "It was as if we had found the ashes of a campfire," explains Shannon Curry. "But we wanted to see the actual fire—in this case, sputtering—directly."

To witness sputtering in action, the MAVEN team needed simultaneous measurements at the right place and time from three instruments onboard the spacecraft: the Solar Wind Ion Analyzer, the Magnetometer, and the Neutral Gas and Ion Mass Spectrometer. Researchers also needed measurements on both the dayside and nightside of the planet at low altitudes, which took years of observation.

Finally, combining data from these instruments allowed the study's authors to create a new type of map of argon sputtering in relation to the solar wind. This map revealed the presence of argon at high altitudes at the exact locations where energetic particles hit the atmosphere and "sputter" argon, showing sputtering in real time. The researchers also discovered that the process is four times more abundant than previously predicted, and that this rate increases during solar storms.

Direct observation of sputtering in the Martian atmosphere confirms that this process, as suspected, was a major source of atmospheric loss in Mars's early history, when the Sun's activity was much more intense. It is important to remember that in its early days, the young Sun was more active and emitted stronger and more frequent solar wind outflows than today, which would have amplified the effect of sputtering.

"These results," Curry says, "establish the role of sputtering in the loss of Mars' atmosphere and in determining the history of water on Mars." The discovery will help scientists finally understand the conditions that allowed liquid water to exist on the Martian surface and the implications this had for the planet's habitability billions of years ago. If Mars ever supported life, it likely developed when the planet was warmer and wetter. Sputtering offers us a window into Mars' past and allows us to better understand how long the Red Planet may have been a favorable environment for life.

Furthermore, this study may also have implications for determining the habitability of other exoplanets. If an exoplanet has an atmosphere and is exposed to a strong stellar wind, sputtering could be an important mechanism of atmospheric loss, affecting its potential to host life.