ESA’s Trace Gas Orbiter Detects Glowing Oxygen
Using data from the NOMAD (Nadir and Occultation for Mars Discovery) ultraviolet and visible spectrometer instrument on board ESA’s ExoMars Trace Gas Orbiter (TGO), a team of researchers has detected green line dayglow emission in the atmosphere of Mars.
An artist’s impression of ESA’s ExoMars Trace Gas Orbiter detecting the green glow of oxygen in the Martian atmosphere. This emission, spotted on the dayside of Mars, is similar to the night glow seen around Earth’s atmosphere from space. Image credit: ESA.
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).
“One of the brightest emissions seen on Earth stems from night glow,” said Dr. Jean-Claude Gérard, a scientist at the Université de Liège.
“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.”
“Previous observations hadn’t captured any kind of green glow at Mars, so we decided to reorient the ultraviolet and visible spectrometer (UVIS) nadir channel to point at the ‘edge’ of Mars, similar to the perspective you see in images of Earth taken from the ISS,” said NOMAD principal investigator Dr. Ann Carine Vandaele, a researcher at the Institut Royal d’Aéronomie Spatiale de Belgique.
Between April 24 and December 1, 2019, the scientists used NOMAD-UVIS to scan altitudes ranging from 20 to 400 km from the Martian surface twice per orbit.
When they analyzed these datasets, they found the green oxygen emission in all of them.
“The emission was strongest at an altitude of around 80 km and varied depending on the changing distance between Mars and the Sun,” Dr. Vandaele said.
To better understand this green glow at Mars, and compare it to what we see around our own planet, the team dug further into how it was formed.
“We modeled this emission and found that it’s mostly produced as carbon dioxide is broken up into its constituent parts: carbon monoxide and oxygen. We saw the resulting oxygen atoms glowing in both visible and ultraviolet light,” Dr. Gérard.
Simultaneously comparing these two kinds of emission showed that the visible emission was 16.5 times more intense than the ultraviolet.
“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,” Dr. Gérard said.
“This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics.”
“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 TGO,” said TGO project scientist Dr. Håkan Svedhem, of ESA.
“It demonstrates the remarkably high sensitivity and optical quality of the NOMAD instrument. This is especially true given that this study explored the dayside of Mars, which is much brighter than the nightside, thus making it even more difficult to spot this faint emission.”
A paper on the findings was published in the journal Nature Astronomy.
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J. Gérard et al. Detection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations. Nat Astron, published online June 15, 2020; doi: 10.1038/s41550-020-1123-2
