Mars’s atmosphere has a green glow, much like the Northern and Southern lights on Earth.
Researchers predicted four decades ago that Mars’s atmosphere should emit green light, but it has proved elusive. Jean-Claude Gérard at the University of Liège in Belgium and his colleagues finally spotted the glow by using the European Space Agency’s Trace Gas Orbiter spacecraft to scan the planet’s edge against the dark background of space.
The green glow is given off by oxygen, which forms when the Sun’s radiation breaks apart carbon dioxide in Mars’s atmosphere. The team’s measurements suggest that much of the glow comes from oxygen produced in two regions, 80 and 120 kilometres above the Martian surface.
The spacecraft measured the glow’s intensity in both visible and ultraviolet wavelengths, allowing the researchers to calculate the ratio between the two. The ratio should be the same for other planets. As a result, researchers studying other planets’ glows will be able to confirm that their instruments are working correctly by comparing the intensity ratio in their own measurements with that in the European team’s measurements of Mars.
Correction: An earlier version of this article misspelled Jean-Claude Gérard’s name.