An eerie green glow has been discovered on the night side of the Red Planet – but sadly not from Martian streetlights.
However, the illumination could be enough to light the way for future astronauts as the glow can be as bright as moonlit clouds on Earth.
The mysterious aura was spotted by the European Space Agency’s ExoMars Trace Gas Orbiter (TGO) mission.
‘When future astronauts explore Mars’s polar regions, they will see a green glow lighting up the night sky,’ said the ESA.
‘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.’
While nightglow is seen on our planet, it is not to be confused with auroras which are produced when energetic electrons from the Sun hit the upper atmosphere.
But the Martian nightglow is actually the result of a dizzying journey by the planet’s rare oxygen supply.
During the Martian day, sunlight energises carbon dioxide molecules, ripping them apart into their individual atoms – one carbon and two oxygen.
When the loose oxygen atoms roam across to the night side, pairs join back together to form an oxygen molecule, giving off a green glow as they do about 30 miles above the surface.
‘These observations are unexpected and interesting for future trips to the Red Planet,’ said Professor 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.
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. It can also reveal how energy is deposited by both the Sun’s light and the solar wind – the stream of charged particles emanating from our star.
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 and auroras can both exhibit a wide range of colours depending on which atmospheric gases are most abundant at different altitudes.
‘The green nightglow on our planet is quite faint, and so is best seen by looking from an ‘edge on’ perspective – as portrayed in many spectacular images taken by astronauts from the International Space Station,’ added ESA.
The study is published in the journal Nature Astronomy.
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