Billions & Billions by Carl Sagan Read Free Book Online Page B

with high melanin content as “colored” than it does to describe individuals with low melanin content as “bleached.”
    Only at visible and immediately adjacent frequencies are any significant differences in skin reflectivity manifest. People of Northern European ancestry and people of Central African ancestry are equally black in the ultraviolet and in the infrared, where nearly all organic molecules, not just melanin, absorb light. Only in the visible, where many molecules are transparent, is the anomaly of white skin even possible. Over most of the spectrum, all humans are black. *
    Sunlight is composed of a mixture of waves with frequencies corresponding to all the colors of the rainbow. There is slightly more yellow light than red or blue, which is partly why the Sun looks yellow. All of these colors fall on, say, the petal of a rose. So why does the rose look red? Because all colors other than red are preferentially absorbed inside the petal. The mixture of light waves strikes the rose. The waves are bounced around helter-skelter below the petal’s surface. As with a wave in the bathtub, after every bounce the wave is weaker. But blue and yellow waves are absorbed at each reflection more than red waves. The net result after many interior bounces is that more red light is reflected back than light of any other color, and it is for this reason that we perceive the beauty of a red rose. In blue or violet flowers exactly the same thing happens, except now red and yellow light is preferentially absorbed after multiple interior bounces and blue and violet light is preferentially reflected.

    There’s a particular organic pigment responsible for the absorption of light in such flowers as roses and violets—flowers so strikingly colored that they’re named after their hues. It’s called anthocyanin. Remarkably, a typical anthocyanin is red when placed in acid, blue in alkali, and violet in water. Thus, roses are red because they contain anthocyanin and are slightly acidic; violets are blue because they contain anthocyanin and are slightly alkaline. (I’ve been trying to use these facts in doggerel, but with no success.)
    Blue pigments are hard to come by in Nature. The rarity of blue rocks or blue sands on Earth and other worlds is an illustration of this fact. Blue pigments have to be fairly complicated; the anthocyanins are composed of about 20 atoms, each heavier than hydrogen, arranged in a particular pattern.
    Living things have inventively put color to use—to absorb sunlight and, through photosynthesis, to make food out of mere air and water; to remind mother birds where the gullets of their fledglings are; to interest a mate; to attract a pollinating insect; for camouflage and disguise; and, at least in humans, out of delight in beauty. But all this is possible only because of the physics ofstars, the chemistry of air, and the elegant machinery of the evolutionary process, which has brought us into such superb harmony with our physical environment.
    And when we’re studying other worlds, when we’re examining the chemical composition of their atmospheres or surfaces—when we’re struggling to understand why the high haze of Saturn’s moon Titan is brown and the cantalouped terrain of Neptune’s moon Triton pink—we’re relying on the properties of light waves not very different from the ripples spreading out in the bathtub. Since all the colors that we see—on Earth and everywhere else—are a matter of which wavelengths of sunlight are best reflected, there is still more than poetic merit to think of the Sun as caressing all within its reach, of sunlight as the gaze of God. But you have a much better shot at understanding what’s happening if you think instead of a dripping faucet.
    * And one octave above Middle C is 526 hertz; two octaves, 1052 hertz; and so on.
    * I know, I know. I can’t help it: that’s how many there are.
    * I still worry that some kind of visible light chauvinism plagues