Blue sunset over Mars by NASA Curiosity rover

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.

Read the full essay at Sky Lights by Dan Heim.