ESA ExoMars Trace Gas Orbiter detects green glow around Mars

The ESA has a spacecraft in orbit around Mars called the ExoMars Trace Gas Orbiter. Around the Red Planet, a phenomenon previously only seen around the Earth has been discovered by ExoMars. The green glow is a phenomenon caused by oxygen in the Martian atmosphere. On Earth, glowing oxygen is produced during polar auroras when energetic electrons from interplanetary space collide with the upper atmosphere.

The oxygen-driven emission of light gives polar auroras on Earth the characteristic green hue. Scientists say that the atmospheres of Earth and Mars will continuously glow during both day and night due to sunlight interacting with atoms and molecules in the atmosphere. Day and night glow are caused by different mechanisms.

At night, glow occurs as broken-part molecules recombined. In the day, glow arises when the sunlight directly excites atoms and molecules, like nitrogen and oxygen, in the atmosphere. It can be challenging to view the green night glow as it's very faint on most planets. That faintness makes the green glow challenging to find around other planets, even when explicitly searching it out.

ExoMars has detected the green glow around Mars for the first time since it began orbiting the Red Planet in 2016. Scientists predicted that the green glow existed around Mars for the last 40 years, and it has only now been discovered. Researchers say the previous observations hadn't captured any kind of green glow around Mars, so they decided to reorient the UVIS nadir channel to point at the "edge" of Mars, similar to a perspective used in images of Earth taken from the ISS showing the green glow.

The scientists say between April 24 and December 1, 2019, researchers used the NOMAD-UVIS to scan altitudes from 20 to 400 km above the Martian surface twice per orbit. When the images were analyzed, the green oxygen glow was found in all of them. The glow was strongest at an altitude of around 80 km and varied depending on the distance between Mars and the sun.