Mars Sunset Is Blue

On Earth, a setting sun paints the sky in fiery shades of red and orange. Witnessing the same event from the surface of Mars, however, would reveal a startlingly different and ethereal scene: a distinct blue glow concentrated around the distant, setting sun. This otherworldly phenomenon isn't an artist's rendering but a reality first captured in stunning color by NASA's Spirit rover in 2005 and observed since by rovers like Curiosity. The reason for this cosmic color swap lies entirely in the planet's thin, dusty atmosphere.

The science behind the spectacle is a fascinating reversal of what happens on our own world. Earth’s thick atmosphere is dominated by tiny gas molecules that scatter short-wavelength blue light, which is why our daytime sky is blue. At sunset, the sun's light travels through more atmosphere, scattering away the blues and leaving the longer-wavelength reds and oranges to reach our eyes. On Mars, the atmosphere is thin but full of fine, reddish dust particles. These particles excel at scattering red light forward, giving the Martian (Review) daytime sky its familiar pinkish or butterscotch hue. But as the sun sets, this same dust scatters the red light away from the direct line of sight, allowing the blue light to travel a more direct path to the observer, creating that mesmerizing blue halo.