Pale Blue Dot - Carl Sagan [60]
It is sometimes said that scientists are unromantic, that their passion to figure out robs the world of beauty and mystery. But is it not stirring to understand how the world actually works—that white light is made of colors, that color is the way we perceive the wavelengths of light, that transparent air reflects light, that in so doing it discriminates among the waves, and that the sky is blue for the same reason that the sunset is red? It does no harm to the romance of the sunset to know a little bit about it.
Since most simple molecules are about the same size (roughly a hundred millionth of a centimeter), the blue of the Earth’s sky doesn’t much depend on what the air is made of—as long as the air doesn’t absorb the light. Oxygen and nitrogen molecules don’t absorb visible light; they only bounce it away in some other direction. Other molecules, though, can gobble up the light. Oxides of nitrogen—produced in automotive engines and in the fires of industry—are a source of the murky brown coloration of smog. Oxides of nitrogen (made from oxygen and nitrogen) do absorb light. Absorption, as well as scattering, can color a sky.
OTHER WORLDS, OTHER SKIES: Mercury, the Earth’s Moon, and most satellites of the other planets are small worlds; because of their feeble gravities, they are unable to retain their atmospheres—which instead trickle off into space. The near-vacuum of space then reaches the ground. Sunlight strikes their surfaces unimpeded, neither scattered nor absorbed along the way. The skies of these worlds are black, even at noon. This has been witnessed firsthand so far by only 12 humans, the lunar landing crews of Apollos 11, 12, and 14–17.
A full list of the satellites in the Solar System, known as of this writing, is given in the accompanying table. (Nearly half of them were discovered by Voyager) All have black skies—except Titan of Saturn and perhaps Triton of Neptune, which are big enough to have atmospheres. And all asteroids as well.
Venus has about 90 times more air than Earth. It isn’t mainly oxygen and nitrogen as here—it’s carbon dioxide. But carbon dioxide doesn’t absorb visible light either. What would the sky look like from the surface of Venus if Venus had no clouds? With so much atmosphere in the way, not only are violet and blue waves scattered, but all the other colors as well—green, yellow, orange, red. The air is so thick, though, that hardly any blue light makes it to the ground; it’s scattered back to space by successive bounces higher up. Thus, the light that does reach the ground should be strongly reddened—like an Earth sunset all over the sky. Further, sulfur in the high clouds will stain the sky yellow. Pictures taken by the Soviet Venera landers confirm that the skies of Venus are a kind of yellow-orange.
SIXTY-TWO WORLDS FOR THE THIRD MILLENNIUM: KNOWN MOONS OF THE PLANETS (AND ONE ASTEROID)—LISTED IN ORDER OF DISTANCE FROM THEIR PLANET
Mars is a different story. It is a smaller world than Earth, with a much thinner atmosphere. The pressure at the surface of Mars is, in fact, about the same as the altitude in the Earth’s stratosphere to which Simons rose. So we might expect the Martian sky to be black or purple-black. The first color picture from the surface of Mars was obtained in July 1976 by the American Viking 1 lander—the first spacecraft to touch down successfully on the surface of the Red Planet. The digital data were dutifully radioed from Mars back to Earth, and the color picture assembled by computer. To the surprise of all the scientists and nobody else, that first image, released to the press, showed the Martian sky to be a comfortable, homey blue—impossible for a planet with so insubstantial an atmosphere.