[Creative Cultivation Program] The sky and sea are one color on Earth, are there also blue skies on Mars and Venus?

When the weather is fine, the sky and the sea are blue. Have you ever wondered why? In addition, humans are exploring space. If we go to other planets, will the sky we see be blue? Or other colors?

First, let's look at how our eyes recognize colors. Our eyes observe the world and recognize colors by receiving light from the outside world. The essence of light is electromagnetic waves, and different wavelengths of light correspond to different colors. When we use our eyes to see things, we are actually using our eyes to receive visible light of various wavelengths.

When we see an object with a certain color, it means that the object reflects a certain wavelength of light. For example, when we see green leaves, it is because the chlorophyll in the leaves absorbs the red and purple light in the visible light, making the leaves reflect green light. Then our eyes receive the green light, and we feel that the leaves are green.

To discuss the topic of sky color, we must first know that the light we see mainly comes from the main star. The star in our solar system is the sun. The sky seen on all celestial bodies in the solar system is the effect of sunlight. If there is no sun, the sky will always be black. This is a simple principle.

So, when the sun shines on the earth or other planets, will different colors be produced? Let's first look at the composition of sunlight. Sunlight itself is white light, which is composed of seven colors: red, orange, yellow, green, cyan, blue, and purple. If there is no obstruction, the light we see, or the space, is colorless. The blue sky we see is because when sunlight passes through the atmosphere, part of the seven colors of light is absorbed, and part is emitted, and only the remaining part can enter our sight.

Specifically, it is because after the sun's rays enter the atmosphere, they encounter atmospheric molecules and particles suspended in the atmosphere and are scattered. According to scientists' measurements, green, blue and purple light have higher frequencies and shorter wavelengths; red, orange and yellow light have lower frequencies and longer wavelengths. When encountering obstacles in the air, the blue light is "scattered" everywhere, covering the entire sky. This is how the sky is "scattered" to become blue.

In the morning and evening, because sunlight is oblique and has a long propagation path, after passing through the thick atmosphere, the short-wavelength blue light is basically blocked by the particles suspended in the atmosphere and cannot go far. Red light, due to its long wavelength, can easily cross obstacles and is not easily blocked. It can reach farther places. Therefore, the morning and evening glow we see at sunrise and sunset are often red.

In short, the clear sky is blue, not because the atmosphere itself is blue, nor because there are blue substances in the atmosphere, but because of the scattering of sunlight by atmospheric molecules and tiny particles suspended in the atmosphere. The phenomenon that light deviates from its original propagation direction and scatters to the side due to the inhomogeneity of the medium is called medium scattering of light.

What about the sky on a rainy day? The sky on a rainy day is grayish white, because the thick clouds reflect the sunlight, and the light is weaker than on a sunny day, so the sky appears darker than on a sunny day. When the sunlight passes through the thick clouds again, due to the high water content and more dust in the clouds, the sunlight is reflected and shows the color of dust and small water droplets or small ice crystals, so the sky on a rainy day looks grayish white.

From this we can see that the sky color we see will be different if the atmospheric density and composition are different. From this we can imagine the situation on other planets.

For example, Mars has a very thin atmosphere, with carbon dioxide as the main component, and the surface atmospheric pressure is less than 1 atmosphere. Due to frequent sandstorms, there is a lot of dust in the atmosphere, and there are many iron oxide particles in the size of microns to nanometers. Therefore, if you look at the sky on Mars, you can see a very light blue sky when it is sunny, but most of the time it is yellowish.

Venus is at the other extreme. The atmosphere of Venus is very dense, with a surface pressure of 93 atmospheres, which is 93 times the atmospheric pressure. The main components are carbon dioxide, as well as a small amount of nitrogen, water and sulfur dioxide. The surface temperature of Venus reaches more than 400 degrees Celsius. If we stand on Venus and look at the sky, we can't see through it at all. Only a small amount of light can pass through its atmosphere. The sky we see is a very turbid earthy yellow, a bit like what it looks like when encountering a severe sandstorm.

As for celestial bodies such as the moon and Mercury, they have almost no atmosphere, so when light reaches their surface, there is no change, and the sky we see is colorless and transparent. Let's talk about the ocean. Why is the color of the Earth's ocean also blue?

The reason why the seawater looks blue is also due to the effect of sunlight. The longer the wavelength of sunlight, the easier it is to penetrate into the seawater and be absorbed by the seawater or marine life. Most of the short wavelength light is reflected and scattered and cannot enter the seawater.

The color of water changes with the environment around it. The dissolved elements or suspended impurities in the water may cause the water to have different colors. The water we usually see seems to be colorless and transparent. That is because the amount of water is relatively small, and the water molecules that make up the water absorb so little visible light that it can be ignored. Therefore, almost all visible light will penetrate through the water, and in our eyes, the water is naturally transparent.

But in fact, pure water will appear light blue. When there is a lot of water and it is very deep, we can easily find this. Although it was mentioned earlier that water molecules absorb very little visible light, there is something called "hydrogen bonds" in water molecules. Under its effect, water absorbs red and green light slightly more than blue light, making blue light easier to show.

Therefore, when pure water reaches a certain depth, it will appear blue. The deeper the water, the bluer it is, such as the huge ice blocks in the ocean and glaciers. When light penetrates deep sea water or ice, more and more red light is lost along the way. When it reaches a depth of several meters, most of the red light has been consumed, while blue light can penetrate very far. Before being completely absorbed, blue light can scatter and reflect freely, and part of it eventually returns to the surface of the sea water or ice, shining blue light into our eyes. In addition, the surface of the ocean and lakes often reflects the blue sky, which also makes it appear bluer, showing a scene of the sea and sky being one color.