Why does the earth look mostly blue when seen from space?

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This is an interesting question and could be answered in a few different ways. The easiest answer is to say that Earth, when seen from space, looks blue to the human eye because that is the color that Earth mainly reflects into the color receptive cones of the eye. Students are always shocked to hear that chlorophyll in plants doesn't absorb green light. It reflects green light. That green light is what enters the eye, and that is why you see green. By the same reasoning, Earth looks blue from space because it is reflecting a lot of blue light. Some people say that Earth is blue from space because Earth contains a lot of water. While that does play a part in the answer, it can't be the only reason. If ocean water were critical for a planet to be blue, then Neptune and Uranus would be a different color. Earth is blue because of what that water does with the light coming into it from the sun. The ocean is made up of water molecules and those molecules  absorb and scatter wavelengths of light. The easiest wavelengths for water to absorb are infrared, ultraviolet, and red light. This is why the sun doesn't provide much warmth to deep waters. As you go deeper the orange and yellow light go away. Blue light carries the most energy, so it has the highest chance of being reflected, scattered, and re-emitted back out into space where it will enter the eye of the human observer, and that person will see blue.

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The easy response would be that the earth looks blue from space because it is more than two-thirds water. By this reasoning, because water is blue, the earth looks blue. This would be an over-simplification of a complex question that isn't exactly true. Have you ever dipped a clear cup in the ocean and looked at the water? It's not blue.

Because the ocean is made up of water molecules, these molecules absorb wavelengths of light from the sky. The wavelengths that are most easily absorbed are those that are ultraviolet and infrared. This means the deeper you go in the ocean, the less warming from the sun there will be and the more bluish hue you will see as the red light wavelength is absorbed elsewhere.

In addition to the appearance of blue water on earth, the sky also is blue. The reason why the sky appears blue is because the earth's atmosphere is able to more efficiently spread blue light rather than red, thanks to the fact that blue light boasts a shorter wavelength.

 

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