Sunset on Mars

NASA recently release a video of the sun set on Mars as seen by the Curiosity rover.
It’s more a set of time lapsed images as the rover doesn’t take full motion videos. The engineers color corrected the images since the rover’s camera is black and white only. However they know the spectral data of the light, so we know what the color is supposed to be. Thus we see what the actual colors would be of the Martian sunset. Turns out the sunset is blue-ish on Mars. This is caused by the presence of a lot of dust in the Martian atmosphere that scatter the light and makes blue come through instead of red. This is Rayleigh scattering, the same phenomenon that gives up blue skies and red sunsets, kind of the opposite of Mars actually.

Rayleigh scattering is the bouncing of light photons from atoms, molecules, and dust particles. When a photon hits a particle, it can bounce off the particle in a different direction. The wavelength of the deflected photon can change depending on the angle. On Earth, the photons bounce off mostly nitrogen. If the photons are deflected by around 90 degrees, then the light turns blue. If instead the photons are deflected straight forward, then the light turns red. So we have blue days and red sunsets. The Martian atmosphere is mostly CO2, which has different behaviors, thus the blue sunset.

Of all the images that have come back from Mars, these are one of the most inspiring and awe-inducing for me. The desert landscapes of Mars, the geological samples, the chemical data, and all the other data are very interesting scientifically. However they all seem so far removed from our everyday human experiences. But the sunset is something we have all seen many times. Songs, poems, and paintings have been made about the beauty of sunsets. It has romantic connotations. The fact the sun sets on Mars is a given, but it’s probably not a fact we’ve really thought about. It’s a piece, a tie, to our Earthly lives. One day, the first men and women to step foot on Mars will see the sunset and feel a bit more human and closer to home.


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