This article was originally published at The Conversation. The publication contributed the article to Space.com’s Expert Voices: Op-Ed & Insights.
Every once in a while, you may look up towards the sun and see strange bright lights on either side of it. Or perhaps you’ll be sitting in an aircraft, looking out the window at its shadow and see a circle of light, like a halo below (known as glories). Or, if you’re really adventurous, maybe you’ll even be out on a midnight walk with a full moon lighting your way, and see what appears to be a rainbow encircling the moon.
These are all beautiful examples of atmospheric optical phenomena. And a new paper has suggested they may appear in alien skies too.
These celestial wonders can tell us a lot about the state of the atmosphere at home on Earth as well as on other planets. Rainbows, for instance, the most well-known of these phenomena, can only form when light passes through spherical liquid droplets, like our normal rain on Earth. Therefore, there must be spherical liquid droplets in the atmosphere where the rainbows are observed.
Most planet atmospheres have some kind of crystalline aerosols (clouds of tiny particles) in them, from sodium chloride in Io (one of Jupiter’s moons), to carbon dioxide crystals in Mars. On Earth, these are generally ice crystals, often found in clouds as snowflakes. The orientation of these crystals, and how they change the light, dictates the type of optical phenomena you can see.
Sun dogs are another of these phenomena, where bright lights appear on either side of the Sun, sometimes even splitting white light into the colors of the rainbow. They form because of the light being bent by horizontally oriented hexagonal ice crystals high up in the atmosphere. If you want the best chance of seeing these, you should try to be at the same latitudes as Europe or Argentina during wintertime. Look for high altitude wispy clouds that are in front of the Sun, and you might get lucky.
Horizontal ice crystals can also create light pillars in extremely cold conditions, which look like colored beams of light trailing to clouds over head. Vertical crystals form parhelic circles – a circle of light at the same height as the Sun. And crystals aligned with the electric fields above thunderstorms create crown flashes.
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