Blue Sunset on the Red Planet

On July 4, 2023, NASA’s Perseverance Rover shot this sunset photo on planet Mars. The unusual bluish color is caused by the composition of the thin atmosphere. On Earth, the sunlight interacts with mainly nitrogen and oxygen. On Mars, the sunlight interacts with iron-rich dust that hangs in the atmosphere. This ultimately scatters lower-frequency red light around the sky during the day. At sunset though, the red light is scattered out and mostly blue light remains — not unlike a hazy day on Earth with smoke or blowing dust.

It’s pretty much the opposite case on Earth, with blue skies during the day and red sunsets. Follow those links to learn more about why that happens. And check out the Earth–Mars sunset comparison below:

Atmospheric scientist Mark Lemmon of Texas A&M University explains it this way: “The colors [on Mars] come from the fact that the very fine dust is the right size so that blue light penetrates the atmosphere slightly more efficiently. When the blue light scatters off the dust, it stays closer to the direction of the Sun than light of other colors does. The rest of the sky is yellow to orange [often described as “salmon”], as yellow and red light scatters all over the sky instead of being absorbed or staying close to the Sun.”

Martian twilight also lasts longer because of that fine dust. Dust can be suspended very high up in the atmosphere, especially after a dust storm, so the Sun’s rays will continue to illuminate it and scatter blue light long after sunset. Same thing at sunrise.

One final difference to note: The comparison photos are both at the same scale, so the difference in the apparent size of the Sun can be easily seen. From Earth, the Sun has an angular diameter of 0.5°. On Mars that diameter is only 0.35°. Thus, the Sun itself sets more quickly, as measured from the time it first touches the horizon, till the time the last sliver of it disappears. Of course, that assumes Earth and Mars are rotating at the same rate, and remarkably, they are to within about 3%. One Mars day lasts 24 hours 37 minutes.

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