Mars sunset. According to foreign media reports, sunset is one of the amazing wonders in the universe, which can touch the heartstrings of every human hesitation. From painting to poetry to beautiful photography, the scene of Twilight always awakens our inner art. But if you have a plan to visit Mars and want to end your first day on the red planet with a spectacular sunset, you will be surprised. In 2005, in a photo of a Martian sunset taken by NASA's Spirit rover, a blue and white sun was hanging in the sky, surrounded by a blue halo, which was very different from the sunset scene on earth. There is a wonderful connection between the earth and the Martian sky. The earth is light blue, and the sky is mainly blue. It gradually turns orange at dawn and sunset. Coincidentally, the Martian sky is orange brown and will gradually transition to light blue at sunset. The same sun shines in the sky of two planets. Why are the colors so different? Sunset on Mars Electromagnetic spectrum. Mars is much farther from the sun than the earth, so the sun looks much smaller and darker from Mars. Data collected by multiple Mars rovers show that the sun will appear light blue and white from Mars. In addition, at sunset, the sun seems to be shrouded in a circle of blue halo. The more outward, the lighter the color, and gradually integrate with the grayish red Martian sky. But what causes this phenomenon? The mystery is hidden in the Martian atmosphere. How does the atmosphere affect the sun's rays? The electromagnetic radiation emitted by the sun is distributed in a spectrum, ranging from high-frequency gamma rays to low-frequency radio waves. Part of it can be detected by the naked eye, so it is called visible spectrum segment. We usually call this part of light "white light", but they can be further decomposed into seven different wavelengths, namely "red orange yellow green blue purple". In short, the intensity of the same electromagnetic radiation reaching different planets is different, but the colors of the sky and the sun on each planet are quite different. This is because, in addition to the sunlight itself, the color we see depends on another factor: the medium through which light passes. Light interacts differently with different particles. When encountering a particle, light may be absorbed, reflected, or scattered. The extent to which these phenomena occur depends on the nature of the particles that interact with light. Some particles are more likely to scatter longer wavelength red light, while others are more likely to scatter shorter wavelength blue light. Therefore, some wavelengths of light will be eliminated in the process of light propagation, and the remaining light determines what color we see. For example, there are no particles in outer space that can scatter or absorb light, so the sun looks white to us. Why is the sunset red on earth? This is related to a phenomenon called Rayleigh scattering. This phenomenon occurs when the particle size is very small compared with the wavelength of light. Tiny nitrogen and oxygen molecules in the earth's atmosphere will scatter blue light. When the light enters our eyes, there will be only red light left. Therefore, to understand why the sunset on Mars is blue, we must first understand the composition of the Martian atmosphere. Martian atmosphere The scattering of light in the earth's atmosphere. The density of Martian atmosphere is only one eighth of that of earth. In the thin Martian atmosphere, about 95% is carbon dioxide, 3% is nitrogen, 1.6% is argon, and less than 1% is oxygen. But the Martian atmosphere is also full of dust particles. The surface of Mars is covered with zeolite, hematite, olivine and magnetite particles, which are the main reasons for the blue sunset on Mars. Depends on the size of the scattered particles, as described earlier. The particle volume in the earth's atmosphere is smaller, so Rayleigh scattering is more likely to occur. But Mars is not. The diameter of dust particles suspended in its atmosphere is between 400 and 700 nm, which is almost the same as the wavelength of visible light. Therefore, these particles cannot undergo Rayleigh scattering, but will produce another optical phenomenon - Mie scattering. Mie scattering Mie scattering is the main scattering form of large particles. Unlike Rayleigh scattering, the relationship between Mie scattering and wavelength is not large, but depends more on the direction of light. The probability of forward scattering is greater than that of side or back scattering. Generally speaking, the probability of Mie scattering of light at each wavelength is equal, but the scattering wavelength will be different depending on the particle size and the direction of incident light. For example, dust particles in the Martian atmosphere are more likely to scatter red light than blue light. Coupled with a large amount of red iron oxide in the Martian atmosphere, the Martian sky naturally appears red. Scientific principles behind blue sunset An artist's topographic map of Mars. When analyzing the sunset on Mars, we will notice two characteristics: one is that the sun itself is blue and white, and the other is that there is a light blue halo around the sun. What are the reasons for this phenomenon? Why is the sun blue? The reason why the sun looks blue on Mars is that the Martian atmosphere will filter out the reddish light. The size of dust particles in the Martian atmosphere is suitable for scattering red light, so more red light will be scattered by these particles than blue light. Since sunlight travels the longest at sunrise and sunset, when the light reaches the surface, the red light has almost been eliminated, leaving only blue light with shorter wavelength. This phenomenon is called wavelength selective extinction, which is why the sun is blue. Simply put, you can think of the atmosphere as a "filter" of sunlight. The earth's atmosphere is easier to filter out blue light and leave red light, while the Martian atmosphere is easier to filter out red light and leave blue light. How do blue halos form around the sun? The blue halo around the sun cannot be explained simply by wavelength selective extinction. The scattering law also plays an important role here. As mentioned earlier, Mie scattering depends largely on the direction of light and is easier to scatter forward. Therefore, when light passes through the Martian atmosphere, most light will scatter forward at a small deflection angle. Therefore, what we see is not the uniform dispersion of blue light along the horizon, but a small and concentrated blue halo around the sun. Mie scattering is more likely to occur in the forward direction (i.e. in the direction of incident light). From left to right are Rayleigh scattering, intermediate Mie scattering and complete Mie scattering respectively. In addition, the scattering law of light with different colors is also different. In the forward direction, the intensity of blue light is almost six times that of red light, so in our opinion, the closer we are to the sun, the brighter the blue light is. The intensity of blue light reaches the maximum when the scattering angle is 10 °, and the dominant position of blue wavelength will decrease with the increase of scattering angle. When the scattering angle exceeds 28 °, the intensity of red light will be more dominant, and this halo will gradually integrate into the dark red Martian sky. In short, the blue of the sun is caused by wavelength selective extinction, and the blue halo is the result of angular scattering of solar light under the action of Michaelis scattering. Can we see a blue sunset on earth? Don't think you can only see a blue sunset on Mars. In fact, such a scene can be seen occasionally on earth. After the eruption of karakato volcano in Indonesia in 1883, locals reported that the sun and moon in the region turned blue for more than a month. At the same time, bishop Sereno of Hawaii also reported that he saw a blue halo around the sun, surrounded by a brown halo. This phenomenon is also called "bishop's ring". In addition to volcanic eruptions, blue sunsets are also occasionally reported in the Arabian desert where sandstorms are frequent. In addition, forest fires sometimes create red skies and blue sunsets. conclusion In addition to the above factors, the color of the Martian sunset depends largely on our eyes. Only after human beings witness it, can we describe the real color of the sunset on Mars. Until then, we had to rely on photos from Mars rovers or wait to see the occasional blue sunset on earth. (Xinhua News Agency)