Green glow seen in Mars atmosphere: What is it and how did we spot it?

It is caused by sunlight releasing oxygen atoms from carbon dioxide. The detection was made by ExoMars Trace Gas Orbiter, which has been orbiting Mars since October 2016

                            Green glow seen in Mars atmosphere: What is it and how did we spot it?

Scientists have detected a green glow in the atmosphere of Mars, making it the first time that such a phenomenon has been observed around a planet other than Earth. Such an emission – which comes from oxygen atoms when they are excited by sunlight – has been predicted to exist at Mars for around 40 years, but this is the first time that it has been spotted in Mars atmosphere by the ExoMars Trace Gas Orbiter (TGO). The TGO has been orbiting Mars since October 2016. 

“The oxygen emission at 557.7 nm is a ubiquitous component of the spectrum of the terrestrial polar aurora and the reason for its usual green color. It is also observed as a thin layer of glow surrounding the Earth near 90 km altitude in the dayside atmosphere, but it has so far eluded detection in other planets. Here we report dayglow observations of the green line outside the Earth,” says the study published in Nature Astronomy.

“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,” says lead author of the study Jean-Claude Gérard, Université de Liège, Belgium, in the analysis.

Scientists can gather a lot of information about the composition and dynamics of an atmosphere by studying the glow of planetary atmospheres. It can reveal how energy is deposited by both the Sun’s light and the solar wind – the stream of charged particles emanating from our star. The researchers further explain that understanding the properties of Mars’ atmosphere is not only interesting scientifically, but critical in operating the missions sent 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. “This type of remote-sensing observation, coupled with in situ measurements at higher altitudes, helps us to predict how the martian atmosphere will respond to seasonal changes and variations in solar activity. Predicting changes in atmospheric density is especially important for forthcoming missions, including the ExoMars 2022 mission that will send a rover and surface science platform to explore the surface of the Red Planet,” says Håkan Svedhem, TGO project scientist from the European Space Agency (ESA).

Such an emission,  which comes from oxygen atoms when they are excited by sunlight, has been predicted to exist at Mars for around 40 years. (Getty Images)

The 2016 ExoMars Trace Gas Orbiter is the first in a series of Mars missions to be undertaken jointly by two space agencies, ESA and Russian space agency Roscomos. A key goal of this mission is to gain a better understanding of methane and other atmospheric gases that are present in small concentrations (less than 1% of the atmosphere) but could be evidence for possible biological or geological activity.

The glow detected in the study is different from classic aurora like the Northern and Southern Lights. 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 characteristic green hue. The aurora, however, is just one way in which planetary atmospheres light up. The scientists explain that 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,” they say.

On Earth, the green nightglow is quite faint. This faintness can also be an issue when hunting for it around other planets, as their bright surfaces can drown it out. The current research explored the dayside of Mars, which is much brighter than the nightside, thus making it even more difficult to spot the faint emission.

Airglow in the Earth's atmosphere observed from the International Space Station. (ESA)

The research team was able to detect the emission using a special observing mode of the TGO. An instrument on the orbiter called the Nadir and Occultation for Mars Discovery (NOMAD) as well as 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. Between April 24 and December 1, 2019, the NOMAD-UVIS was used to scan altitudes ranging from 20 to 400 kilometers from the Martian surface twice per orbit. When experts analyzed these datasets, they found the green oxygen emission in all of them.

“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 kilometers and varied depending on the changing distance between Mars and the Sun,” says co-author Ann Carine Vandaele from the Institut Royal d'Aéronomie Spatiale de Belgique, Belgium, and principal investigator of NOMAD. 

To better understand the green glow on Mars, the team also modeled this emission and found that it is mostly produced as carbon dioxide, and is broken up into its constituent parts: carbon monoxide and oxygen. They saw the resulting oxygen atoms glowing in both visible and ultraviolet light. According to the analysis, 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 have 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,” says Gérard.

Such an understanding is important in characterizing planetary atmospheres and related phenomena – such as auroras. By deciphering the structure and behavior 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.

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