Terrestrial sunsets are certainly one of the most beautiful sights we can witness on our planet. Dotted with pink, orange and yellow, these events are a riot of colorful shades that delight many photographers and observers. But sunsets are not unique to Earth, and they can be observed from other perspectives elsewhere in the Universe. What color are the sunsets on other planets?
The answer depends on the planet. On Mars, the Sun moves back and forth with a blue glow. On Uranus, the sunset changes from blue to turquoise, according to NASA. And on Titan, one of Saturn's moons, the sky turns from yellow to orange to brown as the sun dips below the horizon. Sunset colors aren't uniform because, in large part, those hues are a product of each planet's atmosphere and how the particles in it scatter sunlight, according to Kurt Ehler, a mathematician at the Truckee Community College.
On Earth, the atmosphere is made up of tiny gas molecules, mostly nitrogen and oxygen, which are more efficient at scattering — that is, absorbing and re-emitting in a different direction — light at short wavelength, such as blue and violet, than for longer wavelengths such as red.
The selective scattering mechanism caused by small molecules is called Rayleigh scattering. It gives us blue skies at noon, but at sunset and sunrise, when sunlight has to travel farther, more of the blue light is dispersed; it is the longer red and yellow wavelengths that reach our field of vision, creating the shades of red we see.
Any planet with a gas-dominated atmosphere will follow a similar pattern of longer wavelength colors, becoming dominant at sunset, Ehler says. On Uranus, for example, the gas particles of hydrogen, helium, and methane in its atmosphere scatter the shorter blue and green wavelengths while absorbing (but largely not re-emitting) wavelengths red longer, according to NASA.
This creates a bright blue sky that turns turquoise at sunset as the blue light is scattered compared to the longer greenish wavelengths. If a planet's atmosphere is dominated by something other than gases, everything about how the sunset appears is going to be different. Take in the Martian blue sunset. “The density of atmospheric gas is only about 1/80 of what it is on Earth. Scattering is dominated by larger dust particles says Ehler.
In a 2014 study that used data from the Mars rover Spirit, Ehler and his colleagues found that Martian dust scatters light very differently from gas molecules. “The reason for the blue Martian sunset is the way the light is scattered by these dust particles .
Related:Why does the sky appear blue?
Gas molecules, like those here on Earth, scatter light in all directions. In contrast, dust scatters it mainly in one direction. Additionally, dust particles scatter red light at much wider angles than blue light. Because blue light is not scattered very widely, it becomes more concentrated, so blue light is about six times more intense than red light on Mars.
When you look at the Martian sunset, you actually see that "the disk of the sun is white, because light does not change color as it passes through the Martian atmosphere. Around the sun there is a bluish glow. And further on, the sky begins to look reddish. There you see scattered red glows at larger angles .
As for the other planets and moons, it is almost impossible to predict what the sunset will look like without having a thorough understanding of their atmospheric composition. If these celestial bodies have a gaseous atmosphere, expect to see longer wavelengths of light at sunset.
NASA video simulating sunsets on different planets:
