Auroras on Mars don't look quite like they do on Earth — but they still produce a weird and fascinating light show.
NASA's Perseverance rover has snapped pictures of a visible-light aurora from the Martian surface.
It's the first time an aurora has been observed from the surface of a planet other than Earth, and the first time visible auroras have been seen on Mars.
The weak green light was photographed in March last year, with the pictures just released in a new study published in Science Advances.
What causes auroras on Mars?
Auroras on Earth are formed by particles from the Sun interacting with the planet's magnetic field.
But auroras have also been observed on nearly every other planet in the Solar System.
The light show was first detected around Mars 20 years ago from space.
Unlike Earth, Mars doesn't have a global magnetic field — instead, its magnetism is patchy and varies across the planet's surface.
This means its auroras are caused by slightly different phenomena.
One type of aurora Mars sees, called a solar energetic particle (SEP) aurora, comes from super-energised particles from the Sun hitting the Martian atmosphere directly.
These have been observed in ultraviolet light before, from NASA's MAVEN instrument that orbits around Mars.
But when the Sun emitted a solar flare and a coronal mass ejection (CME) in the direction of Mars last year, a team of researchers spotted an opportunity to see the auroras from the planet's surface.
They scrambled to ready the Perseverance rover, which is trundling through Mars' Jezero crater.
How did they spot the aurora?
The team used computer models to predict when and where the aurora would be visible in the Martian sky, and what colour and intensity of light they could expect the rover to spot.
They expected particles from the CME would react with oxygen atoms in the Martian atmosphere, causing a faint glow in a very precise shade of green.
Elise Knutsen, a physicist at the University of Oslo and lead author of the study, said the team needed to pick a strong CME to test their models.
“When we saw the strength of this one, we estimated it could trigger an aurora bright enough for our instruments to detect,” Dr Knutsen said.
Days after the solar flare, two instruments on Perseverance recorded the exact shade of green in Mars' sky the researchers had predicted.
Hannah Schunker, a physicist at the University of Newcastle who wasn't involved in the research, called the study a "neat observation".
"It's a nice test of the Martian atmosphere models — and also a little more indirectly, it can be used to test our models of the coronal mass ejections," Dr Schunker said.
So how does it compare to what we see on Earth?
Disappointingly for future space travellers, it's unlikely the aurora would be as dazzling as those seen on Earth.
"Even on Earth, when we observe auroras, they often look quite different to the beautiful images we see," Dr Schunker said.
"The photographic images that are taken of the auroras on Earth are often exposed for some minutes, so you get these really bright, vivid colours. Whereas if you see it with your eyes, it doesn't look quite so bright and quite so vivid, although still impressive."
Mars, meanwhile, has fainter auroras to start with: lacking Earth's magnetism, the solar particles produce less light when they crash into the planet's atmosphere.
"There might be a detectable change in the atmosphere, but it would not be strong and it would not be very obvious to the eye," Dr Schunker said.
While it might not be the best light show in the Solar System, the research will be useful for better understanding solar weather from different angles, according to Dr Schunker.
"Mars is not aligned with Earth at the moment. So we would not have experienced this particular CME," she said.
We might be able to expect more observations like this in coming months, she said, thanks to "solar maximum": the point in its roughly 11-year cycle when the Sun produces the highest number of flares and sunspots.
This is why people on Earth have seen so many auroras in the past year, often much further away from the poles than they can normally be spotted.
"We're approaching, or we’re in the middle of, solar maximum at the moment. We never know if we've reached solar maximum until we're on the other side of it," Dr Schunker said.
