Many photographs taken from low-Earth orbit, especially from the Space Station while on the night side of Earth, show a thin line of green or yellowish light just above the limb of our planet. This is called airglow, and it’s caused by excited oxygen atoms high in the atmosphere releasing energy in the form of both visible and ultraviolet light. Up until recently this has only been seen around Earth. But between April and December of 2019, ESA’s ExoMars Trace Gas Orbiter detected glowing green oxygen in the daytime atmosphere of Mars, making it the first time this phenomenon has been observed around a planet other than our own.“One of the brightest emissions seen on Earth stems from night glow [airglow]. More specifically, from oxygen atoms emitting a particular wavelength of light that has never been seen around another planet,” says Jean-Claude Gérard of the Université de Liège, Belgium, and lead author of the new study published in Nature Astronomy.

[Editor’s note: ultraviolet airglow was first detected at Mars by Mariners 6 and 7 in 1969.)

In order to see the green airglow at Mars at all, researchers thought to aim TGO’s suite of instruments to look at the limb—that is, the “edge”—of the planet, in the same way that airglow is visible in images of Earth.

“Previous observations hadn’t captured any kind of green glow at Mars, so we decided to reorient the UVIS nadir channel to point at the ‘edge’ of Mars, similar to the perspective you see in images of Earth taken from the ISS,” said co-author Ann Carine Vandaele of the Institut Royal d’Aéronomie Spatiale de Belgique, Belgium.

This observation doesn’t suddenly mean that Mars has secret source of breathable oxygen hidden on (or under) its surface, though.

“We modelled this emission and found that it’s mostly produced as carbon dioxide, or CO₂, is broken up into its constituent parts: carbon monoxide and oxygen,” says Jean-Claude. “We saw the resulting oxygen atoms glowing in both visible and ultraviolet light.”

Comparing these two kinds of emission showed that the visible emission of airglow at Mars was 16.5 times more intense than the ultraviolet — unlike what’s seen here on Earth.

“The observations at Mars agree with previous theoretical models but not with the actual glowing we’ve spotted around Earth, where the visible emission is far weaker,” adds Jean-Claude. “This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics.”

Read the full news here: ESA – ExoMars spots unique green glow at the Red Planet

The ExoMars program is a joint endeavour between the Roscosmos State Corporation and ESA. Apart from the Rosalind Franklin rover mission, now slated to launch in 2022, it includes the Trace Gas Orbiter launched in 2016. The TGO is already both delivering important scientific results obtained by its own Russian and European science instruments and relaying data from NASA’s Curiosity Mars rover and InSight lander. The primary goal of the mission is to determine if there has ever been life on Mars, and to better understand the history of water on the planet.