Turns out the Red Planet is a little more green than we thought. The ExoMars Trace Gas Orbiter (TGO) has detected a tinge of green in the atmosphere, making it the first time this aurora-like glow has been spotted around a planet other than Earth.
Here at home, green glows in the sky are caused by glowing oxygen, excited by collisions with electrons that stream into the atmosphere from solar wind. While aurora are the most dramatic examples, the sky glows almost constantly. At night it can appear green as molecules previously ripped apart by solar winds begin to recombine.
It was predicted that this same effect should be visible around other planets, but it’s hard to spot since the bright surfaces of planets can wash out the color. Now, astronomers from ESA have managed to detect it around Mars for the first time.
The trick is all in the viewing angle. Some of the most stunning photos of Earthly glows are captured from the unique vantage point of the International Space Station. Rather than looking straight down at a planet’s surface, it’s easier to spot this green airglow looking towards the horizon.
So the ESA team applied the same logic to Mars. They adjusted the angle of the ultraviolet and visible spectrometer (UVIS), an instrument on the TGO that’s often pointed at the ground.
“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,” says Ann Carine Vandaele, co-author of the study.
The team studied the atmosphere from this angle many times for more than seven months, scanning different altitudes between 20 and 400 km (12.4 and 248.5 mi) above the surface twice every four-day orbit. The green emission, produced by oxygen, was present in all cases.
“The emission was strongest at an altitude of around 80 km (49.7 mi) and varied depending on the changing distance between Mars and the Sun,” says Vandaele.
On closer examination, it appears that the green glow in this case mostly comes from oxygen that’s produced when carbon dioxide is split by solar radiation.
“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,” says Håkan Svedhem, a TGO Project Scientist.
The research was published in the journal Nature Astronomy.
Source: ESA