Satellite spots unique green glow around Mars
Exomars Trace Gas Orbiter, a satellite belonging to the European Space Agency (ESA) has detected glowing green oxygen in Mars’ atmosphere – the first time that this emission has been seen around a planet other than Earth.
On Earth, glowing oxygen is produced during polar auroras when energetic electrons from interplanetary space hit the upper atmosphere. This oxygen-driven emission of light gives polar auroras their beautiful and characteristic green hue.
The aurora, however, is just one way in which planetary atmospheres light up. The atmospheres of planets including Earth and Mars glow constantly during both day and night as sunlight interacts with atoms and molecules within the atmosphere. Day and night glow are caused by slightly different mechanisms: night glow occurs as broken-apart molecules recombine, whereas day glow arises when the Sun’s light directly excites atoms and molecules such as nitrogen and oxygen.
On Earth, green night glow is quite faint, and so is best seen by looking from an ‘edge on’ perspective – as portrayed in many spectacular images taken by astronauts aboard the International Space Station (ISS). This faintness can be an issue when hunting for it around other planets, as their bright surfaces can drown it out.
This green glow has now been detected for the first time at Mars by the Exomars Trace Gas Orbiter (TGO), which has been orbiting Mars since October 2016.
“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,” says Jean-claude Gérard of the Université de Liège, Belgium, and lead author of the new study published in Nature Astronomy.
“Previous observations hadn’t captured any kind of green glow at Mars, so we decided to reorient the UVIS nadir channel to point at the ‘edge’ of Mars, similar to the perspective you see in images of Earth taken from the ISS,” adds co-author Ann Carine Vandaele of the Institut Royal d'aéronomie Spatiale de Belgique, Belgium, and Principal Investigator of NOMAD.
“We modelled this emission and found that it’s mostly produced as carbon dioxide, or CO2, is broken up into its constituent parts: carbon monoxide and oxygen,” says Jean-claude. “We saw the resulting oxygen atoms glowing in both visible and ultraviolet light.”
Simultaneously comparing these two kinds of emission showed that the visible emission was 16.5 times more intense than the ultraviolet.
“This is the first time this important emission has ever been observed around another planet beyond Earth, and marks the first scientific publication based on observations from the UVIS channel of the NOMAD instrument on the Exomars Trace Gas Orbiter,” highlights Håkan Svedhem, ESA’S TGO Project Scientist.
Understanding the properties of Mars’ atmosphere is not only interesting scientifically, but is also key to operate the missions we send to the Red Planet. Atmospheric density, for example, directly affects the drag experienced by orbiting satellites and by the parachutes used to deliver probes to the Martian surface.