We all know Mars as the “Red Planet.” But the latest photo from ESA’s ExoMars Trace Gas Orbiter show that its atmosphere glows – green. The orbiter has detected glowing green oxygen in Mars’ atmosphere, which is exciting on its own. But there’s more – this is the first time that this emission has been seen around a planet other than Earth.
The green glow (aurora) on Earth
Let’s start by explaining what this green glow is on Earth. After all, we’ve seen it here. It’s usually found in the far north and south as auroras. But thanks to recent solar storms, we’ve seen auroras in places closer to the equator.
However, auroras aren’t the only source of the greenish glow in the sky. The atmospheres of planets, both Mars and Earth, glow all the time, day and night. It happens as sunlight interacts with atmospheric atoms and molecules. Dayglow and night glow arise from slightly different mechanisms: night glow comes from recombining broken-apart molecules. In contrast, day glow is made when the sunlight directly excites atoms and molecules like nitrogen and oxygen.
Earth’s green night glow is quite faint. The best perspective to see it is “edge-on,” as the ISS astronauts have captured it in many breathtaking images. This faintness poses a challenge in detecting night glow around other planets, as their brighter surfaces can obscure it.
The green glow on Mars
As I mentioned, this is the first time this green glow has been detected on Mars. The ExoMars Trace Gas Orbiter (TGO) caught it, and the spacecraft has been orbiting Mars since October 2016.
“One of the brightest emissions seen on Earth stems from night glow,” says Jean-Claude Gérard of the Université de Liège, Belgium, and lead author of the new study published in Nature Astronomy. “More specifically, from oxygen atoms emitting a particular wavelength of light that has never been seen around another planet.”
Gérard notes that this emission has been predicted to exist on Mars for around 40 years. “Thanks to TGO, we’ve found it.”
Gérard and colleagues used a special observing mode of the TGO to spot this emission.
One of the orbiter’s cutting-edge instrument suites, NOMAD (Nadir and Occultation for Mars Discovery), encompasses the ultraviolet and visible spectrometer (UVIS). NOMAD is capable of operating in various configurations, including one that directs its instruments straight down at the Martian surface, a configuration known as the “nadir” channel.
Ann Carine Vandaele of the Institut Royal d’Aéronomie Spatiale de Belgique, Belgium is a co-author with Gérard, and Principal Investigator of NOMAD. She adds:
“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.”
Between April 24 and December 1, 2019, the team used NOMAD-UVIS to scan altitudes ranging from 20 to 400 kilometers 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 kilometers and varied depending on the changing distance between Mars and the Sun,” Vandaele adds.
The importance of these findings
Examining the glow of planetary atmospheres unveils plenty of information about their composition and dynamics. We also learn how energy is imparted by both the Sun’s light and the solar wind, a stream of charged particles emanating from our star.
The team started a thorough investigation to delve deeper into the processes behind Mars’ green glow and compare it to our own planet. “We modeled 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 Gérard. “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.
“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. This suggests we have more to learn about how oxygen atoms behave, which is hugely important for our understanding of atomic and quantum physics.”
Understanding the green glow of Mars’ atmosphere is crucial for characterizing planetary atmospheres and related phenomena like auroras. By unraveling the structure and behavior of this glowing layer, scientists can gain valuable insights into an altitude range that has remained largely unexplored. They can also monitor how this layer changes in response to variations in the Sun’s activity and Mars’ orbital position.
Håkan Svedhem, ESA’s TGO Project Scientist, explains:
“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. 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.”
Understanding the properties of Mars’ atmosphere is, above all, fascinating and relevant to science. But it’s also essential for successfully operating missions to the Red Planet. Atmospheric density, for instance, directly impacts the drag experienced by orbiting satellites and the parachutes used to deliver probes to the Martian surface.
[via Space.com; Image credits: NASA]
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