Astronomers have discovered a faint green glow in the atmosphere of Mars due to the presence of oxygen. This phenomenon is well known on Earth where the glow can be see in the aurorae and at the edge of the atmosphere, but it has never been seen around another world before.
Spotted by the European Space Agency and Roscosmos’ ExoMars Trace Gas Orbiter (TGO), the observations are reported in Nature Astronomy. The spacecraft has been circling Mars since October 2016, its instruments monitoring the composition of the atmosphere and how it changes with the seasons.
“One of the brightest emissions seen on Earth stems from night glow. More specifically, from oxygen atoms emitting a particular wavelength of light that has never been seen around another planet,” lead author Dr Jean-Claude Gérard of the Université de Liège, said in a statement. “However, this emission has been predicted to exist at Mars for around 40 years – and, thanks to TGO, we’ve found it.”
Mars' atmosphere is just 1 percent of Earth’s own in terms of pressure and is overwhelmingly made of carbon dioxide. According to the study, the oxygen detected here is a direct product of that carbon dioxide. A small fraction of those molecules is broken apart by the ultraviolet light from the Sun, releasing carbon monoxide and oxygen.
The oxygen is seen glowing in visible light but also in the ultraviolet. The visible emission is 16.5 times stronger than the UV one, something quite different from our planet’s own.
“The observations at Mars agree with previous theoretical models but not with the actual glowing we’ve spotted around Earth, where the visible emission is far weaker,” added Dr Gérard. “This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics.”
The observations were conducted last year, between April 24 and December 1, 2019, using the NOMAD (Nadir and Occultation for Mars Discovery) suite of instruments to scan the atmosphere of Mars from 20-400 kilometers (12.4-249 miles) in altitude. The instruments were aimed at the edge of the planet, and the observations were conducted twice every orbit or about 24 times per day. The oxygen emission was present in every single observation.
“The emission was strongest at an altitude of around 80 kilometres and varied depending on the changing distance between Mars and the Sun,” said co-author Dr Ann Carine Vandaele of the Institut Royal d'Aéronomie Spatiale de Belgique, Belgium, Principal Investigator of NOMAD.
There are still many unknowns when it comes to the Martian atmosphere but these kinds of observations provide new ways to probe the evolution of the planet’s atmosphere over time.