An orbiting spacecraft has detected a strange green glow on Mars that had previously only been seen on one planet – Earth.
The ExoMars Trace Gas Orbiter (TGO) spotted glowing green oxygen in Mars’s atmosphere similar to the northern lights seen on Earth.
It’s something scientists had been hunting for four decades until the European Space Agency (ESA) satellite spotted it, according to a study published in Nature Astronomy.
On Earth, glowing oxygen is produced during polar auroras when energetic electrons from interplanetary space hit the upper atmosphere, creating a distinctive green glow.
But the atmospheres of Earth and Mars also glow constantly during both day and night as sunlight interacts with atoms and molecules within the atmosphere.
On Earth, green night glow is quite faint, but is visible in some of the spectacular images taken by astronauts aboard the International Space Station (ISS).
This green glow has now been detected for the first time on Mars by the ExoMars TGO, which has been orbiting Mars since October 2016.
Jean-Claude Gerard, of the Universite de Liege in Belgium, said: “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.
“However, this emission has been predicted to exist at Mars for around 40 years – and, thanks to TGO, we’ve found it.”
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Gerard and colleagues were able to spot this emission using a special observing mode.
One of the orbiter’s advanced suite of instruments, known as NOMAD (Nadir and Occultation for Mars Discovery) and including the ultraviolet and visible spectrometer (UVIS), can observe in various configurations, one of which positions its instruments to point directly down at the martian surface – also referred to as the ‘nadir’ channel.
Co-author Ann Carine Vandaele, of the Institut Royal d'Aeronomie Spatiale de Belgique, said: “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.
“The emission was strongest at an altitude of around 80 kilometres and varied depending on the changing distance between Mars and the Sun.”
This understanding is key to characterising planetary atmospheres and related phenomena – such as auroras.
By deciphering the structure and behaviour of this green glowing layer of Mars’ atmosphere, scientists can gain insight into an altitude range that has remained largely unexplored, and monitor how it changes as the Sun’s activity varies and Mars travels along its orbit around our star.