We have all seen the photographs of the Earth take from space.
The Earth has a brilliant blue color? Has that always been true? NO!|
In order to consider this question, we need to first establish just why the Earth appears blue now! We know it's not because of the land areas, and the photographs usually clearly show that the ocean areas are the parts with the blue color.
Is water blue? No! It is clear, and colorless. If it should have ANY color on its own, it would probably be greenish, because of the algae and seaweed and other green vegetative matter that live in the oceans near its surface. But it's NOT green, it's clearly blue!
The wavelengths of red light are somewhat bigger, and they tend to be able to get past the oxygen and nitrogen molecules without being entirely blocked. This allows the red light part of sunlight to more easily pass through the atmosphere, while blue light doesn't get very far before being scattered.
By the way, this also explains why the Sun looks reddish at sunset, because the sunlight had to pass through much more of the Earth's atmosphere on its way to you (over the heads of a LOT more people before it got to being over your head!), so it lost nearly all the blue light it contained. Primarily only red light remains IN THE DIRECT LIGHT FROM THE SUN by the time the light gets to you.
All this is a LOT more complicated than this, of course, but this is the general reason why the sky looks blue to us.
When sunlight passes through the sky over an ocean, the same scattering of blue light occurs. Some of that scattered blue light goes downward. That light then becomes reflected off the surface of the ocean, which would give a bluish tint to the oceans. At the same time, some of the original scattered light is scattered UPWARD toward space! (directly from the air molecules) The two effects add together, to create a very distinct and brilliant blue color for the oceans from space.
Over land, by the way, the UPWARD scattering still occurs, so there IS a bluish contribution to the overall color even over the continents. But the intensities of the colors of the browns and greens of the mountains and grasslands are much greater, and they make the blue contribution generally unnoticeable. Over the oceans, where there IS no actual background color, except for the same blue light being reflected up from the surface of the ocean, the blue seems to predominate, mostly because there is no competition!
SEE why no other planet looks blue? BOTH the size of the oxygen and nitrogen atoms in our atmosphere AND the lack of other colors from the ocean are important. I have never heard any Teacher or even College Professor explain that fully and correctly!
Early in the Earth's existence, there was no atmosphere and there were no oceans. The Earth probably looked generally brownish or reddish, like Mars or Mercury. Later, an atmosphere of methane and carbon dioxide and some other simple chemicals probably existed. At this point, the Earth may have had faint colored stripes or banding like we see on Jupiter, but FAR less noticeable. Later yet, volcanoes spewed out a lot of stuff from the inside of the Earth. The two most obvious color affectors would be smoke/volcanic ash, which would likely make the Earth look black or dark gray, and water vapor, which when condensed into droplets would likely make the earth look white like steam!
LONG after these activities had occurred, plant life developed, which generally had the effect of converting the carbon dioxide in the atmosphere into oxygen. As this oxygen content increased, the bluish color of the Earth would have increased, as described above.
We have not mentioned the nitrogen in the atmosphere. At whatever point nitrogen became prominent in the atmosphere, this also would have enhanced the bluish color. There seem to be different opinions about where and when the nitrogen got into the atmosphere. One popular opinion was that it came out of the volcanoes at the much earlier time, so the Earth may have been somewhat bluish soon after the oceans formed, and well before there was much free oxygen in the atmosphere.
The point is, most people just ASSUME that a planet with oceans will look blue, because the Earth does, now! I just wanted to bring thinking people to realize the many hidden assumptions we tend to make in many of the things we investigate. Rather than being sloppy and blindly accepting such assumptions, we need to cautiously think about all matters that might alter the findings we are seeking!
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago