NASA spacecraft finds solar ‘cannonballs’ may have stripped Mars of its water — proving decades-old theory

The Ancient Martian Environment: A Watery Past

Mars today is a barren world, with a thin atmosphere and surface temperatures that rarely allow liquid water to exist. Yet, the planet’s landscape tells a different story. Features such as dried river valleys, lake beds, and minerals that form only in the presence of water indicate that Mars once hosted long-lasting bodies of liquid water, including lakes and possibly shallow seas. These conditions imply that Mars had a much denser atmosphere capable of trapping heat and maintaining surface pressure sufficient for liquid water to persist.

Understanding when and how Mars lost this atmosphere is crucial to piecing together its climate history and assessing how long the planet might have been hospitable to life. Scientists have long suspected that solar wind—a continuous flow of charged particles from the sun—played a major role in stripping away Mars’ atmosphere.

Sputtering: The ‘Cannonball’ Effect Eroding Mars’ Atmosphere

One of the most significant processes suspected of atmospheric loss is sputtering. This occurs when high-energy ions from the solar wind collide with atoms and molecules in the upper atmosphere, transferring enough energy to knock these particles free from Mars’ gravitational hold and send them into space. The process has been likened to a cannonball splashing water out of a pool, with the solar wind ions acting as the cannonballs and the atmospheric particles as the splashed water.

Until recently, sputtering had only been theorized based on indirect evidence. NASA’s MAVEN mission, which has been orbiting Mars for nearly ten years, has now captured direct observations of sputtering in action. By analyzing data from multiple instruments aboard MAVEN, researchers have mapped the distribution of argon—a heavy, chemically inert noble gas—in Mars’ upper atmosphere. Because argon does not easily react or become charged, it serves as an excellent tracer for sputtering-induced atmospheric escape.

Direct Evidence from MAVEN: Mapping Argon Escape

The MAVEN spacecraft detected unexpectedly high concentrations of argon at altitudes where solar wind particles interact with the Martian atmosphere. These concentrations were far greater than what would be expected if argon were simply drifting upward under gravity, providing clear evidence that sputtering is actively lifting argon atoms and removing them from Mars.

This observation confirms that sputtering is not just a theoretical mechanism but an ongoing process contributing to atmospheric loss. Since argon is a heavy gas, its escape indicates that lighter gases like water vapor and carbon dioxide could have been lost even more readily, accelerating the thinning of Mars’ atmosphere over time.

Implications for Mars’ Climate Evolution and Habitability

The MAVEN data revealed that sputtering occurs at a rate approximately four times higher than previous models had predicted. Moreover, the process intensifies during solar storms, suggesting that in Mars’ early history—when the sun was more active and the planet lacked a protective magnetic field—sputtering could have been even more efficient at stripping away the atmosphere.

Without a magnetic shield, Mars’ atmosphere was exposed directly to the full force of the solar wind. This exposure likely accelerated atmospheric erosion, pushing Mars past a critical threshold where liquid water could no longer remain stable on the surface. The loss of a thick atmosphere and the resulting drop in temperature and pressure transformed Mars into the frozen desert we observe today.

These findings establish sputtering as a key driver in Mars’ atmospheric loss and, by extension, its climate and habitability history.

Next Steps: Peering Into Mars’ Ancient Past

While MAVEN’s observations provide compelling evidence of sputtering’s role in atmospheric loss, scientists still need to reconstruct Mars’ atmospheric evolution over billions of years to fully understand the process’s impact. This involves combining MAVEN’s data with isotopic analyses, ancient climate models, and geological clues from the Martian surface.

Determining whether sputtering was the dominant mechanism or one of several contributing factors in Mars’ atmospheric erosion will help clarify how and when Mars transitioned from a habitable environment to its current state. This knowledge is essential for future exploration and for understanding the potential for life on Mars and other planets.

What this means

NASA’s MAVEN mission has provided a breakthrough in understanding how Mars lost its atmosphere and, with it, the liquid water necessary for a potentially habitable environment. By directly observing sputtering—the solar wind-driven ejection of atmospheric particles—scientists have confirmed a decades-old theory about Mars’ atmospheric erosion. These findings not only illuminate the planet’s climatic transformation but also guide future research into Mars’ ancient environment and the broader processes that shape planetary atmospheres. As scientists continue to analyze MAVEN’s data alongside geological and isotopic evidence, we move closer to unraveling the full story of Mars’ transition from a watery world to the arid desert we see today.