Watch the sun set against a deep-blue Martian sky

The Red Planet has blue sunsets.

At least, that’s how it looked to the Curiosity rover as its Mast Camera was pointed toward the Martian horizon one evening last month.

One of the Mastcam’s two imaging systems experiences colors very much like a human would if she were standing on the surface of Mars — except that the Earthling would be even more sensitive to blue wavelengths of light.

Those blue wavelengths tend to dominate in the part of the Martian sky that is close to the sun. That’s because the relatively shorter wavelengths are able to sneak through the dusty Mars atmosphere more efficiently than the longer wavelengths toward the red end of the spectrum of visible light. The effect is magnified at sunset, when light must travel through a thicker slice of atmosphere than it does in the middle of the day, when the sun is overhead. (If you’re having trouble visualizing this, check out this diagram from EarthSky.org.) The same geometry causes sunsets on Earth to look red.

Curiosity has sent back thousands of pictures from the surface of Mars. Usually, both the ground and the sky have reddish-brown hues. The pictures taken April 15 represent Curiosity’s first attempt to capture color images of a Martian sunset, according to NASA.

The rover witnessed this particular sunset at the end of its 956th Martian day. The pictures were taken from Gale Crater over a period of 6 minutes and 51 seconds. The images were color-calibrated before being released to the public, but NASA says the blue sky is the real deal. 

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