by Carl Sagan
The other reason we see in visible light is because that’s where the Sun puts out most of its energy. A very hot star emits much of its light in the ultraviolet. A very cool star emits mostly in the infrared. But the Sun, in some respects an average star, puts out most of its energy in the visible. Indeed, to remarkably high precision, the human eye is most sensitive at the exact frequency in the yellow part of the spectrum at which the Sun is brightest.
Might the beings of some other planet see mainly at very different frequencies? This seems to me not at all likely. Virtually all cosmically abundant gases tend to be transparent in the visible and opaque at nearby frequencies. All but the coolest stars put out much, if not most, of their energy at visible frequencies. It seems to be only a coincidence that the transparency of matter and the luminosity of stars both prefer the same narrow range of frequencies. That coincidence applies not just to our Solar System, but throughout the Universe. It follows from fundamental laws of radiation, quantum mechanics, and nuclear physics. There might be occasional exceptions, but I think the beings of other worlds, if any, will probably see at very much the same frequencies as we do.*
Vegetation absorbs red and blue light, reflects green light, and so appears green to us. We could draw a picture of how much light is reflected at different colors. Something that absorbs blue and reflects red light appears to us red; something that absorbs red light and reflects blue appears to us blue. We see an object as white when it reflects light roughly equally in different colors. But this is also true of gray materials and black materials. The difference between black and white is not a matter of color, but of how much light they reflect. The terms are relative, not absolute.
Perhaps the brightest natural material is freshly fallen snow. But it reflects only about 75 percent of the sunlight falling on it. The darkest material that we ordinarily come into contact with—black velvet, say—reflects only a few percent of the light that falls on it. “As different as black and white” is a conceptual error: Black and white are fundamentally the same thing; the difference is only in the relative amounts of light reflected, not in their color.
Among humans, most “whites” are not as white as freshly fallen snow (or even a white refrigerator); most “blacks” are not as black as black velvet. The terms are relative, vague, confusing. The fraction of incident light that human skin reflects (the reflectivity) varies widely from individual to individual. Skin pigmentation is produced mainly by an organic molecule called melanin, which the body manufactures from tyrosine, an amino acid common in proteins. Albinos suffer from a hereditary disease in which melanin is not made. Their skin and hair are milky white. The irises of their eyes are pink. Albino animals are rare in Nature because their skins provide little protection against solar radiation, and because they lack protective camouflage. Albinos tend not to last long.
In the United States, almost everyone is brown. Our skins reflect somewhat more light toward the red end of the visible light spectrum than toward the blue. It makes no more sense to describe individuals 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 of stars, 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 this argument: Beings like us who see only in visible light deduce that everyone in the entire Universe must see in visible light. Knowing how our history is rife with chauvinisms, I can’t help being suspicious of my conclusion. But as nearly as I can see, it follows from physical law, not human conceit.
* These are among the reasons that “African-American” (or equivalent hyphenations in other countries) is a much better descriptive than “black” or—the same word in Spanish—“Negro.”
CHAPTER 5
FOUR COSMIC
QUESTIONS
When on high the heaven had not been named,
Firm ground below had not been called by name …
No reed hut had been matted, no marsh land had appeared,
When no god whatever had been brought into being,
Uncalled by name, their destinations undetermined—
Then it was that the gods were formed …
Enuma Elish,
the Babylonian creation myth (late third millennium B.c.)*
Every culture has its creation myth—an attempt to understand where the Universe came from, and all within it. Almost always these myths are little more than stories made up by story tellers. In our time, we have a creation myth also. But it is based on hard sci
entific evidence. It goes something like this …
We live in an expanding Universe, vast and ancient beyond ordinary human understanding. The galaxies it contains are rushing away from one another, the remnants of an immense explosion, the Big Bang. Some scientists think the Universe may be one of a vast number—perhaps an infinite number—of other closed-off universes. Some may grow and then collapse, live and die, in an instant. Others may expand forever. Some may be poised delicately and undergo a large number—perhaps an infinite number—of expansions and contractions. Our own Universe is about 15 billion years past its origin, or at least its present incarnation, the Big Bang.
There may be different laws of Nature and different forms of matter in those other universes. In many of them life may be impossible, there being no suns and planets, or even no chemical elements more complicated than hydrogen and helium. Others may have an intricacy, diversity, and richness that dwarfs our own. If those other universes exist, we may never be able to plumb their secrets, much less visit them. But there is plenty to occupy us about our own.
Our Universe is composed of some hundred billion galaxies, one of which is the Milky Way. “Our Galaxy,” we like to call it, although we certainly do not have possession of it. It is composed of gas and dust and about 400 billion suns. One of them, in an obscure spiral arm, is the Sun, the local star—as far as we can tell, drab, humdrum, ordinary. Accompanying the Sun in its 250 million year journey around the center of the Milky Way is a retinue of small worlds. Some are planets, some are moons, some asteroids, some comets. We humans are one of the 50 billion species that have grown up and evolved on a small planet, third from the Sun, that we call the Earth. We have sent spacecraft to examine seventy of the other worlds in our system, and to enter the atmospheres or land on the surfaces of four of them—the Moon, Venus, Mars, and Jupiter. We have been engaged in a mythic endeavor.
—
Prophecy is a lost art. Despite our “eager desire to pierce the thick darkness of futurity,” in Charles McKay’s words, we’re often not very good at it. In science the most important discoveries are often the most unexpected—not a mere extrapolation from what we currently know, but something completely different. The reason is that Nature is far more inventive, subtle, and elegant than humans are. So in a way it’s foolish to attempt to anticipate what the most significant findings in astronomy might be in the next few decades, the future adumbration of our creation myth. But on the other hand, there are discernible trends in the development of new instrumentation that indicate at least the prospect of goosebump-raising new discoveries.
Any astronomer’s choice of the four most interesting problems will be idiosyncratic and I know many would make choices different from mine. Among other candidate mysteries are what 90 percent of the Universe is made of (we still don’t know); identification of the nearest black hole; the bizarre putative result that the distances of galaxies are quantized—that is, galaxies are at certain distances and their multiples but not at intermediary distances; the nature of gamma ray bursters, in which the equivalent of whole solar systems episodically blow up; the apparent paradox that the age of the Universe may be less than the age of the oldest stars in it (probably resolved by the recent conclusion, using Hubble Space Telescope data, that the Universe is 15 billion years old); the investigation in Earth laboratories of returned cometary samples; the search for interstellar amino acids; and the nature of the earliest galaxies.
Unless there are major cuts in the funding for astronomy and space exploration worldwide—a doleful possibility by no means unthinkable—here are four questions* of enormous promise:
1. Was There Ever Life on Mars? The planet Mars is today a bone-dry frozen desert. But all over the planet there are clearly preserved ancient river valleys. There are also signs of ancient lakes and perhaps even oceans. From how cratered the terrain is, we can make a rough estimate of when Mars was warmer and wetter. (The method has been calibrated by cratering on our Moon and radioactive dating from the half-lives of elements in lunar samples returned by Apollo astronauts.) The answer is about 4 billion years ago. But 4 billion years ago is just the epoch in which life was arising on Earth. Is it possible that there were two nearby planets with very similar environments, and life arose on one but not the other? Or did life arise on early Mars, only to be wiped out when the climate mysteriously changed? Or might there be oases or refuges, perhaps subsurface, where some forms of life linger into our own time? Mars thus raises two fundamental enigmas for us—the possible existence of past or present life, and the reason that an Earth-like planet has become locked into a permanent ice age. This latter question may be of practical interest to us, a species that is busily pushing and pulling on its own environment with a very poor understanding of the consequences.
When Viking landed on Mars in 1976, it sniffed the atmosphere, finding many of the same gases as in the Earth’s atmosphere—carbon dioxide, for example—and a paucity of gases prevalent in the Earth’s atmosphere—ozone, for example. What’s more, the particular variety of molecule, its isotopic composition, was determined and was in many cases different from the isotopic composition of the comparable molecules on Earth. We had discovered the characteristic signature of the Martian atmosphere.
A curious fact then transpired. Meteorites—rocks from space—had been found in the Antarctic ice sheet, sitting directly on top of the frozen snows. Some had been discovered by the time of Viking, some after; all had fallen to Earth before the Viking mission, often tens of thousands of years before. On the clean Antarctic ice shelf, they were not difficult to discern. Most of the meteorites so collected were brought to what in the Apollo days had been the Lunar Receiving Laboratory in Houston.
But funding is very meager at NASA these days, and not even a preliminary look at all these meteorites had been performed for years. Some turned out to be from the Moon—a meteorite or comet impacting the Moon, spraying Moon rocks out into space, one or some of which land in Antarctica. One or two of these meteorites come from Venus. And astonishingly, some of them, judging by the Martian atmospheric signature hidden away in their minerals—come from Mars.
In 1995—96, scientists at NASA’s Johnson Space Flight Center finally got around to examining one of the meteorites—ALH84001—that proved to come from Mars. It looked in no way extraordinary, resembling a brownish potato. When the microchemistry was examined, certain species of organic molecules were discovered, chiefly polycyclic aromatic hydrocarbons (PAHs). These are not in themselves all that remarkable. Structurally they resemble the hexagonal patterns on bathroom tiles with a carbon atom at each vertex. PAHs are known in ordinary meteorites, in interstellar grains, and are suspected on Jupiter and Titan. They do not by any means indicate life. But the PAHs were arranged so that there were more of them deeper in the Antarctic meteorite, suggesting that this was not contamination from Earthly rocks (or automobile exhaust), but intrinsic to the meteorite. Still, PAHs in uncontaminated meteorites do not indicate life. Other minerals sometimes associated with life on Earth were also found. But the most provocative result was the discovery of what some scientists are calling nanofossils—tiny spheres attached one to another, like very small bacterial colonies on Earth. But can we be sure that there are no terrestrial or Martian minerals that have a similar form? Is the evidence adequate? For years I’ve been stressing with regard to UFOs that extraordinary claims require extraordinary evidence. The evidence for life on Mars is not yet extraordinary enough.
But it’s a start. It points us to other parts of this particular Martian meteorite. It guides us to other Martian meteorites. It suggests the search for quite different meteorites in the Antarctic ice field. It hints that we search not just for other deeply buried rocks obtained from or on Mars, but for much shallower rocks. It urges upon us a reconsideration of the enigmatic results from the biology experiments on Viking, some of which were argued by a few scientists to indicate the presence of life. It suggests sending spacecraft missions to special local
es on Mars which may have been the last to surrender their warmth and wetness. It opens up the entire field of Martian exobiology.
And if we are so lucky as to find even a simple microbe on Mars, we have the wonderful circumstance of two nearby planets, each with life on it in the same early epoch. True, maybe life was transported by meteorite impact from one world to another and does not indicate independent origins on each world. We should be able to check that by checking the organic chemistry and morphology of the life-forms uncovered. Maybe life arose on only one of these worlds, but evolved separately on both. We then would have an example of several billion years of independent evolution, a biological bonanza available in no other way.
And if we are most lucky, we will find really independent life-forms. Are they based on nucleic acids for their genetic coding? Are they based on proteins for their enzymatic catalysis? What genetic code do they use? Whatever the answers to these questions, the entire science of biology is the winner. And whatever the outcome, the implication is that life may be much more widespread than most scientists had thought.
In the next decade there are vigorous plans by many nations for robot orbiters, landers, roving vehicles, and subsurface penetrator spacecraft to be sent to Mars to lay the groundwork for answering these questions; and—maybe—in 2005 a robotic mission to return surface and subsurface samples from Mars to Earth.
2. Is Titan a Laboratory for the Origin of Life? Titan is the big moon of Saturn, an extraordinary world with an atmosphere ten times denser than the Earth’s and made mainly of nitrogen (as here) and methane (CH4). The two U.S. Voyager spacecraft detected a number of simple organic molecules in the atmosphere of Titan—carbon-based compounds that have been implicated in the origin of life on Earth. This moon is surrounded by an opaque reddish haze layer, which has properties identical to a red-brown solid made in the laboratory when energy is supplied to a simulated Titan atmosphere. When we analyze what this stuff is made of we find many of the essential building blocks of life on Earth. Because Titan is so far from the Sun, any water there should be frozen—and so you might think it is at best an incomplete analog of the Earth at the time of the origin of life. However, occasional impacts by comets are capable of melting the surface, and it looks as if an average place on Titan has been underwater for a millennium, more or less, in its 4.5 billion year history. In the year 2004, a NASA spacecraft called Cassini will arrive in the Saturn system; an entry probe built by the European Space Agency called Huygens will detach itself and slowly sink through the atmosphere of Titan toward its enigmatic surface. We may then learn how far Titan has gone on the path to life.