Mars may be known as the red planet, but a European spacecraft found oxygen that is causing a green glow in the planet's atmosphere, according to a study.
On Earth, this occurs in the form of auroras at the planet's poles, but this is the first time this kind of glow has been seen around another planet.
The study about the European Space Agency's ExoMars Trace Gas Orbiter's findings was published in the journal Nature Astronomy.
Earth's polar green glow happens when electrons from space collide with our upper atmosphere, but the atmospheres of Earth and Mars both glow during the day and at night thanks to sunlight. At night, a glow is created when molecules in the atmosphere that broke apart come back together. During the day, the glow occurs when sunlight agitates atmospheric molecules and atoms, like nitrogen and oxygen.
Astronauts on the International Space Station have the best view of Earth's faint green night glow because their perspective is "edge on," meaning they can see it at an angle that makes the glow more visible.
That's because our planet has a bright surface, which can overwhelm this faint glow. The same is true of other planets, which is why this detection of the green glow around Mars is so exciting.
"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," said Jean-Claude Gérard, lead study author and astronomer and professor at the Université de Liège in Belgium, in a statement.
"However, this emission has been predicted to exist at Mars for around 40 years — and, thanks to (Trace Gas Orbiter), we've found it."
The ExoMars Trace Gas Orbiter established an orbit around Mars in October 2016. Some of the orbiter's instruments, collectively referred to as NOMAD (Nadir and Occultation for Mars Discovery), were pointed down at the Martian surface during orbit. These instruments include the ultraviolet and visible spectrometer, or UVIS.
Hoping to find a green glow, they oriented the instruments to get an "edge on" view of Mars and scan at different altitudes twice per orbit between April 24 and Dec. 1, 2019. The altitudes ranged from 12.4 to 249 miles from the surface of the planet.
The telltale green glow was found in all of the data gathered during this time — and an impressive feat for the orbiter, considering that Mars' atmosphere during the daytime glows much brighter than the nightside and makes the green glow harder to detect, the researchers said.
"The emission was strongest at an altitude of around 80 kilometers and varied depending on the changing distance between Mars and the Sun," said Ann Carine Vandaele, study co-author and principal investigator of NOMAD at the Institut Royal d'Aéronomie Spatiale de Belgique in Belgium, in a statement.
What's in a green glow?
When comparing the two planets, the researchers found that Mars' green glow is different than that of Earth's.
"We modeled this emission and found that it's mostly produced as carbon dioxide, or CO2, (and) is broken up into its constituent parts: carbon monoxide and oxygen," Gérard said. "We saw the resulting oxygen atoms glowing in both visible and ultraviolet light."
While this agrees with the theoretical models that suggested Mars would have this glow, it's much stronger than the visible emission created by Earth.
"This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics," Gérard said.
Observing the glowing planetary atmospheres can reveal their composition and energy they gain from both sunlight and the sun's solar wind, or stream of charged particles that move across the solar system.
This is also critical for understanding auroras. By studying Mars' green glow, the researchers can understand the structure of this layer in the planet's atmosphere, better understand its altitude range and even observe any changes in reaction to the sun.
And understanding the Martian atmosphere in greater detail means that space agencies can better prepare as they send orbiters or deliver spacecraft to the surface. A continuation of the ESA's ExoMars mission includes landing the Rosalind Franklin rover on the Martian surface in 2022. It will be Europe's first planetary rover.
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