The Red Planet can also be green, according to scientists.
For the first time, a visible nightglow has been detected in the Martian atmosphere by ESA’s ExoMars Trace Gas Orbiter (TGO) mission.
The illumination from the nightglow could be enough to light the way for future astronauts as the glow can be as bright as moonlit clouds on Earth.
ESA explains: “When future astronauts explore Mars’s polar regions, they will see a green glow lighting up the night sky.
“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 Earth, it is not to be confused with auroras which are produced when energetic electrons from the Sun hit the upper atmosphere.
Atmospheric nightglow occurs when two oxygen atoms combine to form an oxygen molecule, about 50 km (164042 feet) above the planetary surface.
ESA explains that the oxygen atoms have been on a journey: they form on Mars’s dayside when sunlight gives energy to carbon dioxide molecules, making them split apart.
When the oxygen atoms migrate to the night side and stop being excited by the Sun, they regroup and emit light at lower altitudes.
“This emission is due to the recombination of oxygen atoms created in the summer atmosphere and transported by winds to high winter latitudes, at altitudes of 40 to 60 km (196850.4 feet) in the Martian atmosphere,” explains Lauriane Soret, a researcher from the Laboratory of Atmospheric and Planetary Physics of the University of Liège, in Belgium, and part of the team that published the discovery in Nature Astronomy.
“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.
These atoms also travel to the nightside and then recombine at lower altitudes, resulting in the visible nightglow detected in the newly published research.
Orbiting the Red Planet at an altitude of 400 km (1312336 feet) , 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 NOMAD experiment is led by the Royal Belgian Institute for Space Aeronomy, working with teams from Spain (IAA‐CSIC), Italy (INAF‐IAPS), and the United Kingdom (Open University), among others.
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 colors 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,” adds ESA.
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