ESA’s ExoMars Trace Gas Orbiter has discovered a green nightglow in the Martian atmosphere, providing crucial data on atmospheric processes and potential illumination for future Martian missions. This phenomenon, distinct from auroras, marks a significant advancement in our understanding of Mars.
When future astronauts explore Mars’s polar regions, they will see a green glow lighting up the night sky. For the first time, a visible nightglow has been detected in the Martian atmosphere by ESA’s ExoMars Trace Gas Orbiter (TGO) mission.
Under clear skies, the glow could be bright enough for humans to see by and for rovers to navigate in the dark nights. Nightglow is also observed on Earth. On Mars, it was something expected, yet never observed in visible light until now.
The atmospheric nightglow occurs when two oxygen atoms combine to form an oxygen molecule, about 50 km (~30 miles) above the planetary surface.
“These observations are unexpected and interesting for future trips to the Red Planet,” says Jean-Claude Gérard, lead author of the new study and planetary scientist at the University of Liège.
The international scientific team was intrigued by a previous discovery made using Mars Express, which observed the nightglow in infrared wavelengths a decade ago. The Trace Gas Orbiter followed up by detecting glowing green oxygen atoms high above the dayside of Mars in 2020 – the first time that this dayglow emission was seen around a planet other than Earth.
Orbiting the Red Planet at an altitude of 400 km, TGO was able to monitor the night side of Mars with the ultraviolet-visible channel of its NOMAD instrument. The instrument covers a spectral range from near ultraviolet to red light and was oriented towards the edge of the Red Planet to better observe the upper atmosphere.
The nightglow serves as a tracer of atmospheric processes. It can provide a wealth of information about the composition and dynamics of a region of the atmosphere difficult to measure, as well as the oxygen density.
Understanding the properties of Mars’ atmosphere is not only scientifically interesting but it is also key for missions to the Red Planet’s surface. Atmospheric density, for example, directly affects the drag experienced by orbiting satellites and by the parachutes used to deliver probes to the Martian surface.
Nightglow is also observed on Earth, but it is not to be confused with auroras. Auroras are just one way in which planetary atmospheres light up.
“Auroras are produced, on Mars as on Earth, when energetic electrons from the Sun hit the upper atmosphere. They vary across space and time, while nightglow is more homogeneous. Nightglow and auroras can both exhibit a wide range of colors depending on which atmospheric gases are most abundant at different altitudes,” says the article.