The NASA probe MAVEN has discovered a new type of aurora on Mars. It's happening on the day side of the planet, making observations extremely challenging. The phenomenon is created when the protons from the solar wind hit the thin Martian atmosphere, exciting the gas and making it glow. This is the first time a proton aurora has been seen on the Red Planet. 

As discussed in Nature Astronomy, the discovery has important implications for the space environment around Mars. The Red Planet hasn’t got a magnetic field like Earth, but it can still move the solar wind. There is a bow shock region, where plasma from the solar wind and from the atmosphere interact, that turns away charged particles hitting it.  

“The Martian proton auroras are more than a light show,” co-author Jasper Halekas of the University of Iowa said in a statement. “They reveal that the solar wind is not completely diverted around Mars, by showing how solar wind protons can sneak past the bow shock and impact the atmosphere, depositing energy and even enhancing the hydrogen content there.”

MAVEN, which stands for Mars Atmosphere and Volatile Evolution mission, studies both the atmosphere and the space environment of the Red Planet. And this discovery was possible because two instruments witnessed something unusual at the same time. Whenever the Solar Wind Ion Analyzer picked up an influx of protons from the Sun, the hydrogen in the upper atmosphere of Mars was seen lighting up by the Imaging Ultraviolet Spectrograph.

The events were obviously connected and gave researchers the key to understanding the phenomenon. How’s the light being emitted and how can the solar protons penetrate the bow shock region? Well, it turns out that these protons are committing a bit of theft.

“As they approach Mars, the protons coming in with the solar wind transform into neutral atoms by stealing electrons from the outer edge of the huge cloud of hydrogen surrounding the planet. The bow shock can only divert charged particles, so these neutral atoms continue right on through,” explained lead author Justin Deighan from the University of Colorado, Boulder.

Proton aurorae happen on Earth but due to our planet’s strong magnetic field, their effect is limited to small regions above the poles. More intriguing is the possibility that such phenomena take place around Venus or Titan. Both objects have lots of hydrogen in their atmospheres, and protons might be ready to steal from them too.