General, Research, Technology

Why is the earth blue from space? (And it's not the color of the sky at all)

Mars is red. The moon is ash gray. Saturn is yellow. The sun is dazzling white. But our planet, even if you look at it from the depths of space, even if we rise slightly above the atmosphere, in low Earth orbit, or if we fly to the outer edges of the solar system, our planet is blue. Why? What makes her blue? Obviously, not the whole planet is blue. The clouds are white, reflect white, direct sunlight at the viewer from above. Ice - for example, at the polar poles - is white for the same reason. Continents are brown or green when viewed from afar, depending on the season, topography and vegetation.


An important conclusion can be drawn from this: The earth is blue not because the sky is blue. If this were so, all the light reflected from the surface would be blue, but we do not observe it. But there is a hint that the truly blue parts of the planet leave: the seas and oceans of the Earth. The shade of blue that water has depends on its depth. If you look closely at the picture below, you can see that the water regions that frame the continents (along the continental shelf) have a lighter shade of blue than the deep, dark places of the ocean.

You may have heard that the ocean is blue becausethe sky is blue and the water reflects the sky. The sky is blue, that's for sure. And the sky is blue, because our atmosphere more efficiently scatters blue (with a short wavelength) than red light (with a longer wave). From here:

  • The sky appears blue during the day, as short-wave light entering the atmosphere is scattered in all directions, and more “blue” gets into our eyes, compared to the rest.
  • The sun and moon look red at sunrise andsunset, because blue light, passing through the thick layers of the atmosphere, is scattered, and mostly saturated red light remains, which gets into our eyes.
  • The moon turns red during a total lunar eclipse: red light passing through our atmosphere will fall on the surface of the moon, while blue light is easily scattered.


But if the explanation were that - that the oceanreflects the sky - we would not see these shades of blue when we looked at the deeper water. In fact, if you took a picture underwater in natural light, without additional light sources, you would see - even at the most modest depth - that everything has a bluish tint.

You see, the ocean is made up of water molecules, and water- like all molecules - selectively absorbs light of certain wavelengths. The easiest way to absorb water is infrared, ultraviolet and red light. This means that if you dip your head in water even at a modest depth, you will be protected from the Sun, from ultraviolet radiation and everything will appear blue: red light will be excluded.

Dive deeper - the orange will also disappear.

Even lower - yellow, green, purple.

Having plunged for many kilometers, we find that blue has disappeared, although it will disappear last.

That is why the depths of the ocean are dark blue: all other wavelengths are absorbed, and the blue one has the highest probability of reflecting and re-entering the Universe. For the same reason, if the Earth were completely covered by the ocean, only 11% of visible sunlight would be reflected: the ocean absorbs sunlight perfectly.

Since 70% of the surface of the world is covered by the ocean, and for the most part the ocean is deep, our world seems blue from afar.

Uranus and Neptune, two other blue worlds of the Solarsystems possess atmospheres consisting primarily of hydrogen, helium and methane. (Neptune is richer in ice and has a wider variety of components, therefore, has a different shade). In sufficiently high concentrations, methane absorbs red light a little better and reflects blue a little better than other wavelengths, while hydrogen and helium are practically transparent for all frequencies of visible light. In the case of blue gas giants, the color of the sky really matters.

But on earth? Our atmosphere is subtle enough not to affect the color of the planet. The sky and ocean are blue not at all due to reflections; they are blue, blue, but each of their own free will. If you remove the oceans, a person on the surface will still see a blue sky, and if you remove our sky (and at the same time leave incomprehensible liquid water on the surface), our planet will also remain blue.