People around the world have been following NASA’s Perseverance Mars Rover, sifting through the stunning visuals provided by the spacecraft since it landed on the red planet weeks ago. In images associated with the deployment of its helicopter, Ingenuity, a stunning rainbow was seen arcing across the horizon, surprising people about a rain-free environment. NASA has now identified what that rainbow is and how it formed.
Technically, a rainbow is a luminous arc featuring all colors of the visible light spectrum: red, orange, yellow, green, blue, indigo, and violet. It is created by refraction, total reflection, and the dispersion of light passing through water droplets in the air. As light enters the raindrop, it is refracted; this means the path of the light is bent to a different angle. After refraction, some of the light is reflected by the internal, curved, mirror-like surface of the raindrop, and finally is refracted back out the raindrop toward the observer.
The ray illustrated here is significant because it represents the ray that has the smallest angle of deviation of all the rays that can enter the raindrop. This means that much of the sunlight that is refracted and reflected through the raindrop is focused along this path; this is known as the “rainbow ray.” Since the various colors that make up white light all have slightly different wavelengths, each color becomes slightly separated from the others as the light ray is refracted and reflected. Rays that strike the raindrop at an angle of 42 degrees will usually form a concentrated, strong rainbow ray with widely separated colors.
Because raindrops are round, the reflection it creates is circular too. The lower the sun is in the sky, the more of a circular arc while the higher it is, the smaller it’ll be. If the sun is more than 42 degrees above the horizon, no rainbow will be visible.
Anywhere with water droplets and sunlight could see a rainbow, but this isn’t the case on arrid Mars. With people wondering how a “rainbow” formed on Mars, NASA issued a statement that true rainbows aren’t possible on Mars. “Rainbows are created by light reflected off of round water droplets, but there isn’t enough water here to condense, and it’s too cold for liquid water in the atmosphere.”
So if it isn’t a rainbow, what is it? NASA says, “This arc is a lens flare.” Lens flare occurs when non-image forming light hits the image sensor in a camera and contributes to the exposure. Lens flares can come in different forms, from radial streaks from the light source to rainbow circles or arcs. It is one of these rainbow arcs that appears in the images processed by Perseverance.