Glowing green oxygen has been detected in the atmosphere of Mars, the first time such a phenomenon has been observed on a planet outside of the Earth.
This emission of green light around Mars was first predicted around four decades ago, and now it has finally been identified using the ExoMars Trace Gas Orbiter (TGO,) according to a study published in the journal Nature Astronomy.
The TGO spacecraft has been orbiting the Red Planet since October 2016 as part of a mission operated by the European Space Agency (ESA) and Roscosmos, the Russian space agency.
Constantly glowing green oxygen is also present in Earth's atmosphere. On the dayside, the emission is the result of the sun's radiation exciting oxygen molecules, causing them to emit light in a particular frequency. A similar glow can also be seen on Earth's nightside, although it is caused by a slightly different mechanism—molecules that have been broken apart by solar radiation recombining again.
The Earth's green glow is quite faint and so is best seen when looking at the atmosphere "side-on" from space—a view that is often captured in images by astronauts aboard the International Space Station.
Scientists observed the green oxygen on Mars using instruments on the TGO known as NOMAD (Nadir and Occultation for Mars Discovery) and UVIS (the ultraviolet and visible spectrometer) to look at the planet's horizon, scanning between altitudes 12 and 249 miles twice per orbit for more than seven months in 2019.
Analysis of the data collected revealed green oxygen emissions on Mars' dayside, which had never been detected before around the planet.
"The emission was strongest at an altitude of around 80 kilometers and varied depending on the changing distance between Mars and the sun," Ann Carine Vandaele, NOMAD's principal investigator from the Institut Royal d'Aéronomie Spatiale de Belgique, Belgium, said in a statement.
According to the researchers, the green emission is probably produced when carbon dioxide in the Martian atmosphere is broken up into its constituent parts: carbon monoxide and oxygen. This process is similar to the one that produces the green glow on Earth's nightside
The researchers detected the resulting oxygen atoms glowing in the visible spectrum, as well as in ultraviolet light, although the latter emission was more than 16 times weaker.
"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," Jean-Claude Gérard, lead author of the study from the Université de Liège, Belgium, said in a statement. "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 scientists say the latest findings provide us with a better understanding of the chemistry and dynamics of Mars's upper atmosphere, which could have significant implications.
Håkan Svedhem, ESA's TGO Project Scientist, told Newsweek: "This is the first step in trying to understand the evolution of Mars' atmosphere—from the early atmosphere, until today and into the future. It is also a part of the puzzle of how the terrestrial planets have evolved all together, with their individual similarities and differences.
"A good understanding of the atmosphere in this region, and its variation with latitude, time of day, season and dependence on the Solar cycle, is important for the planning of future missions to the surface of Mars. Obviously, all landers have to pass through this region, and here, friction between air molecules and the spacecraft body starts becoming high and generates a lot of heat. The design has to be made to properly fit the environment."
