Earth is not the only planet with an atmosphere that glows green: Astronomers have observed the same ethereal phenomenon on Mars, according to a study published Monday in the journal Nature Astronomy.
The emerald sheen high in the Martian atmosphere was observed by the European Space Agency’s Trace Gas Orbiter, which has been circling the Red Planet since 2016. The glow, which astronomers say is triggered by interactions between the sun’s light and oxygen molecules in Mars’ atmosphere, could help researchers better understand the composition of the planet’s atmosphere and how it behaves.
It’s also the first time that the distinct green lights have been seen on a planet beyond Earth, according to Jean-Claude Gérard, an astronomer at the Université de Liège in Belgium and lead author of the study. The lights are similar to auroras on Earth, but unlike auroras, Mars' green glow appears as a thin band around the planet.
“This emission has been predicted to exist at Mars for around 40 years – and, thanks to TGO [the Trace Gas Orbiter], we’ve found it,” Gérard said in a statement.
Auroras on Earth — the colorful light displays that can be seen at high latitudes — occur when charged particles from the sun collide with Earth’s magnetic field and mix with molecules in the atmosphere. Auroras fluctuate with the sun’s activity, but Earth’s green glow is different because the light is continuous — albeit faint.
On Mars, this so-called night glow can be tricky to spot, but Gérard and his colleagues were able to observe the lights by pointing one of the instruments aboard the Trace Gas Orbiter directly at the surface of Mars from an edge-on perspective.
From April 24 to Dec. 1, 2019, the astronomers scanned altitudes from about 12 miles to 250 miles above the Martian surface twice each time the spacecraft circled the planet. The researchers were able to detect the green glow at all altitudes, with the strongest emission found at around 50 miles above the surface.
The scientists used these observations to examine what causes the green glow and found that the light comes from oxygen atoms that were stripped from carbon dioxide.
On Earth, the green glow is driven by oxygen atoms in the upper atmosphere interacting with electrons from interplanetary space. These stunning displays are sometimes known as polar auroras.
Like on Mars, the emerald lights in Earth’s atmosphere can be faint unless seen edge-on, which is why many of the most dramatic views of the phenomenon have come from photos taken by astronauts aboard the International Space Station.
Though on both planets the green hue is characteristic of oxygen in the atmosphere, the astronomers noted some differences in the resulting emissions of light.
“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,” 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.”
The findings also have important implications for planetary science missions to Mars. Understanding the composition of the Red Planet’s atmosphere is crucial for operating orbiters around Mars or landing rovers on the surface, because these spacecraft are all affected by the density of the Martian atmosphere.
“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,” Håkan Svedhem, a Trace Gas Orbiter project scientist at the European Space Agency who was not involved with the new study, said in a statement.