A simulated view of what Mars’ night sky might look like at its poles.
Credit: NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.– E. W. Knutsen
The night sky at Mars’ poles looks nothing like our own. Instead of an inky black spread, those who eventually step foot on these Martian regions will likely find themselves under a vast expanse of glowing green. For the first time, researchers in Europe have witnessed Mars’ green night sky in the visible light spectrum, further confirming a decades-old suspicion about the Red Planet’s eerie atmosphere.
The bizarre shade is made possible by airglow, an effect that occurs both on and off Earth. Airglow is produced via two routes: Atoms and molecules in the upper atmosphere shed their excess energy or capture free electrons after sunlight ionizes them. Both phenomena involve the ejection of photons, the tiny packs of electromagnetic energy we call light. The resulting effect is similar to that behind auroras, which are driven by solar wind instead of everyday solar radiation; the visible difference is that airglow occurs constantly, while auroras are more sporadic.
Airglow and an aurora occurring over Earth.
Scientists have wondered about Mars’ capacity for airglow since the early 1980s, when researchers in the United States and Belgium observed airglow spectra within the ultraviolet spectrometer data from multiple Mariner spacecraft. In 2012, the OMEGA spectro-imaging instrument aboard the European Space Agency’s Mars Express detected similar tell-tale patterns within the infrared spectrum. Finally, in 2020, researchers used the ESA’s ExoMars Trace Gas Orbiter (TGO) to see via visible light that airglow occurs during the daytime at Mars’ poles.
Now, researchers have confirmed the eerie phenomenon’s presence at night, too. Experts from Belgium, Spain, and the United Kingdom used the ExoMars TGO again to spot airglow—which they call “nightglow”—at Mars’ poles. The effect occurs between 40 and 60 kilometers (roughly 25 to 37 miles) in the winter during the three-body recombination of oxygen atoms.
“This O2 nightglow should be observable from a Martian orbiter as well as from the Martian surface with the naked eye under clear sky conditions,” the team writes in a paper for Nature Astronomy.
This means future astronauts who arrive on Mars might work under a glowing green sky—quite the adjustment, especially for those who try to make a new home on the Red Planet.