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There’s been a flutter of excitement at the recent images from NASA’s Perseverance. Not only is the Mars helicopter ingenuity safe and sound on the ground, but in the sky above it, people saw what looks like a rainbow. The only issue is, it is probably not really there.
First of all, what makes a rainbow? You need water droplets in the air and the Sun lowish on the horizon. When those conditions are met, in the direction opposite to the Sun, you’ll see a bright colorful arc. If you’re lucky you might see two, or even rarer combinations due to many other factors coming into play.
But what about Mars? NASA’s Pathfinder mission has observed something akin to a rainbow due to ice crystals in the very tenuous and very dry atmosphere of Mars. Yesterday, some suggested that maybe we were witnessing a dust-bow, where the sunlight was reflected and refracted by particles of dust. The explanation is more mundane: We are seeing a lens flare.
The rear hazard cameras of Perseverance, which snapped the photos, are pointing South looking in the direction of the Sun at the time of snapping. The same rainbow image is also present over multiple days in the same position, indicating it's an artifact from the camera.
Bows on other worlds are possible. Venus has something akin to a rainbow called a "glory" due to the presence of sulfuric acid droplets in the Venusian atmosphere. On Titan, the largest moon of Saturn, it rains methane and it is possible that rainbows (maybe only visible in infrared) can happen there.
