Scientists had lost hope of ever seeing a phenomenon on the red planet.
Ice crystals in the atmosphere can make the appearance of a bright spot on either side of the sun. Scientists think that a similar optical trick could happen on other planets. Scientists hadn't seen a sun halo despite decades of robotic explorers on the surface of Mars.
That is until December 15, 2021.
Mark Lemmon is a planetary scientist at the Space Science Institute, a nonprofit research institute in Boulder.
"I've been involved with this for a long time, and we've looked for halos in a lot of images," he said. I was going to have a hard time finding an explanation for that. I thought it would be a lot of work to figure out what was going on because everything has been a false alarm.
There are amazing photos from the Perseverance rover's first year on Mars.
It takes certain conditions to make a halo around the sun.
It takes a lot of sunlight to see pencil-like ice crystals in the atmosphere. The size of the halo is determined by the hexagonal structure of water-ice crystals, which is 22 degrees away from the sun.
The sun is on its own if the clouds are too thick or if the crystals are too small. Crystals of other chemicals in the atmosphere can have the same effect, but they have different crystal structures. Lemmon said it's like a fingerprints that tells you what the element was, what the shape was, and then a little bit about the size.
The team looked at how big a halo of dry ice would be, but it didn't match what Perseverance had seen.
Lemmon and his colleagues did not check any other things.
The bright ring may have been created by the camera, but the feature didn't match other images. The sun and halo appeared in three different spots in the frame of the photo, which was part of a sequence of five pictures.
It wasn't dust, that's for sure. Lemmon, now the lead author on a paper describing the work, said that they have a lot of pictures that show you what kind of features you get from dust in the sky. There was only one candidate.
Both as a way of understanding Mars and also as a way of understanding atmospheres more generally has been done by scientists.
Lemmon said that every planet has an atmosphere that is a laboratory. We can't understand something completely if we look at one example and then try to figure out what it would be like if things changed.
Scientists need a better view from the surface of the Red Planet because the fleet of orbiters has excelled at gathering atmospheric data. Lemmon said "everything that we can learn about the atmosphere anywhere on Mars, looking at the clouds, looking at the dust storms, looking at dust devils, we have to validate how it plays out there on the surface."
Scientists don't keep the rover's cameras pointed at the ground because Perseverance has a small weather station. Sky cam, a camera that only looks up and snaps images each sol, is one of the things Perseverance has. Lemmon identified the halo by snapping a picture of it.
Ice crystals can grow larger than any that scientists have measured directly, and the halo around the sun teaches scientists about the atmosphere of Mars. Lemmon is hesitant to draw any conclusions because he only has one example.
He said that the most important thing for them is that they have learned that it is possible. We have to look harder for it at Perseverance.
Lemmon is looking forward to the next time Mars enters the same season as the December 2021 observations, which will be called aphelion cloud belt season. Lemmon said they would spend the entire season looking for this and be ready to investigate if they see anything.
There is a paper about the research in the journal.
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