The Other Blue Planet

There are some who call our Earth "The Blue Marble", because from space, it appears to be a rich, deep blue. But Earth is not alone in that aspect. As different as two planets can be, Neptune shares the blue hue with our own planet. Pictures from the ground show the distant gas giant planet to be a bluish tint, but it was pictures from the space probe Voyager 2 that, to me, brought home Neptune's deep blue coloring.

The Earth appears blue because the gases in the atmosphere scatter incoming sunlight, and blue light is scattered the most. [Oops! (Feb 1, 1999): That ain't right. The reason the Earth looks blue from space is because water absorbs red light, so water preferentially gives off blue light upon reflection. The blue light scattered from the air gets mixed back in with the other light passing through the air, so it doesn't add much if any color to the Earth. Most of the real estate of the Earth is actually water, so the Earth looks blue from space.] Neptune, however, has a trace amount of methane in its atmosphere. Methane absorbs red light very strongly, so when sunlight reflects off Neptune's atmosphere, the red light is basically sucked out of it. That leaves only the bluer light free to be seen by us back here on Earth, so the planet looks blue to us.

Neptune is very far away, and so even to large telescopes it doesn't reveal much detail (except perhaps to a very few, like Hubble, or to some on top of very high mountains). However, I once had the extremely satisfying opportunity to see Neptune myself through my own small telescope. I could just barely see that what I was looking at was not a star, since it had a tiny, tiny disk. But the color is what threw me. Many stars are blue, but not this color I saw in my eyepiece! Even from my driveway in light polluted Washington DC I could see the currently most distant planet, and tell something about it. From four billion kilometers away, Neptune, the last massive guardian between the warm fires of the Sun and the vast frigid depths of interstellar space, reminded me of a warm blue breeze against the cold black night.