Beijing time on August 5, according tomedia reports, sunset, the sky between the red and the light red is a unique beauty on earth. But what color is the sky when other planets in the solar system sunset? The answer depends on the specific planet. On Mars, for example, both sunrise and sunset glow in blue. On Uranus, the sky changes from blue to blue-green at sunset. On Titan, as the sun gradually sinks below the horizon, the sky changes from yellow to orange and then brown.
Pictured is a sunset image of Mars taken by NASA’s Courage Mars rover in 2005.
The colors of the sky vary at sunset because they are largely the result of particles scattering light in the atmospheres of each planet. Many people will think that the color of sunset on Mars is formed in the same way as on Earth. But that is not the case.
The Earth’s atmosphere is made up of tiny gas molecules, mostly nitrogen and oxygen. These gas molecules scatter short-wave light such as blue light and purple light more efficiently than long-wave rays that are red. (Scattering refers to the process by which a molecule absorbs light and retransmits it in all directions.) This phenomenon of selective scattering of light by small molecules is called Riley scattering. Because of this phenomenon, the sky at noon on Earth is blue, but at sunrise and sunset, the sun moves in the atmosphere for longer, so more blue light is scattered along the way, eventually passing on to the red and yellow light of longer wavelengths in our eyes, creating a bright color of the glow at sunset.
Any planet whose atmosphere is made up mainly of gas follows a similar pattern, with sunset-time waves dominating. For example, hydrogen, helium, and methane molecules in Uranus’s atmosphere scatter blue and green short-wave light, and most of the red-ish long-wave light is absorbed and not re-emitted, so the sky on Uranus is generally blue, but it turns blue-green at sunset, because, like Earth, a large amount of blue light is scattered during propagation, with the rest of the wavelengths mostly shorter green light.
When light at certain wavelengths is scattered, they are re-emitted in all directions.
But if the main component of a planet’s atmosphere is not gas, the sunset on that planet will look very different. Take Mars, for example. The density of gas in the Martian atmosphere is only one-eight-tenths of that of Earth, so light on Mars is mainly scattered by larger dust molecules.
A study in 2014 using data collected by the Courage Mars rover found that dust on Mars scatters light in very different ways from gas molecules. The sunset on Mars is blue, caused by the pattern of dust particles scattering light.
Gas molecules scatter light in all directions, while dust particles, in contrast, scatter light forward. In addition, dust particles scatter red light at a much wider angle range than blue light, so blue light is more concentrated, and blue light on Mars is about five times stronger than red light.
If you look at the sunset on Mars, you’ll see that the sun is also white, because the light doesn’t change color as it passes through Mars. But the sun is surrounded by a blue glow, and the sky is redder outwards because the angle of red light scattering is wider.
As for other planets and moons, we must fully understand their atmospheric composition, otherwise it is almost impossible to predict their sunset. But if it is known that the atmosphere of an object is made up mainly of gas, we can predict that the color of the object at sunset must be dominated by long waves of light, Heller said.
But if the atmosphere of one planet is made up mainly of other matter, that’s hard to say. The type and size of dust varies, and the way light is scattered varies. In other words, if you think the sunset on Earth is as beautiful as a “foreign world”, it’s best to put it another way, because such a sunset is actually a unique feature of a planet with a gaseous atmosphere like Earth, and it’s not a “alien” scene.