Rainbows usually don't usually stir up controversy. But earlier this week the Mars Perseverance rover took a photo of one that did. It looked like the real thing: a big bow with colors in the correct order from red outside to blue inside. So what was it? And are rainbows even possible on Mars?
No and no. Mars has an extremely thin atmosphere with only minute amounts of water vapor. Sometimes that vapor condenses to form cirrus-like clouds, but they're made of ice crystals, not liquid water. Rainbows form when sunlight is refracted and reflected inside raindrops. Light enters the drop, reflects off the back side and then spreads into a spectrum of color as it exits. Each drop contributes to the whole of the rainbow.
Raindrops can't exist in the Martian atmosphere because the temperature and pressure are too low for liquid water to be stable. The air is more than 100 thinner than at Earth and daily temperatures range from around 103° below (–75° C) to 32° (0° C) near the surface in some locales. Even if you could create artificial rain the drops would freeze or boil away almost instantaneously. Mars today is as inhospitable to rainbows as it is to life.
Instead, the colorful arc is something much more prosaic — a lens flare. NASA posted the photo and explanation in the rover's official Twitter account earlier this week. Perseverance's left rear-hazard-avoidance camera (hazcam) took the photo on April 4. Unlike the front hazcams, the rear ones aren't equipped with sunshades to block the sun's glare, so they occasionally record artefacts and spurious reflections. You'll see similar effects if you point your camera in the direction of the sun without a lens shade.
That said, colorful and exotic halos created by clouds composed of carbon dioxide and water ice crystals are possible on Mars. During late autumn and winter at either Martian pole the air gets cold enough for carbon dioxide, which makes up 95 percent of the atmosphere, to condense as dry-ice crystals and form a thick hood of clouds. CO2 snow falls from the sky and accumulates to a depth of several feet, temporarily enlarging the polar cap. When the clouds part in spring, the cap is one of the brightest and most obvious features visible with a telescope.
Les Cowley, who maintains Atmospheric Optics, an excellent website devoted to how light interacts with ice, water droplets and dust in the atmosphere, suggests that under the right conditions astronauts on Mars could see some amazing halos like the simulated one above.
Dry ice is transparent enough and refracts light even more strongly than water. Ice crystals in earthly clouds typically have six faces and are shaped like flat hexagonal plates or hexagonal columns that look like tiny pencils. Carbon dioxide crystals come in a variety of shapes that include cubes, octahedrons (eight sides) and cuboctahedrons with 8 triangular faces and six square faces. Teeming in wispy clouds blown by chill winds, there's good reason to believe they create a rich variety of halo phenomena, some with delicate hints of color.
And it doesn't end there. Jupiter and Saturn, bedecked with thick clouds of ammonia and ammonium hydrosulfide crystals, likely create their own unique atmospheric displays. So too the cloud-crystal-rich planets Uranus and Neptune and even dwarf planet Pluto. Close or far, ice creates ephemeral masterpieces in the sky.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.