Broca's Brain (34 page)

The few meteorites that have been tracked as they enter the Earth’s atmosphere originated back in the main asteroid belt, between Mars and Jupiter. Laboratory studies of the physical properties of some meteorites show them to have originated where the temperatures are those of the main asteroid belt. The evidence is clear: the meteorites ensconced in our museums are fragments of asteroids. We have on our shelves pieces of cosmic objects!

But which meteorites come from which asteroids? Until the last few years, answering this question was beyond the powers of planetary scientists. Recently, however, it has become possible to perform spectrophotometry of asteroids in visible and near-infrared radiation; to examine the polarization of sunlight reflected off asteroids as the geometry of the asteroid, the Sun and Earth changes; and to examine the middle-infrared emission of the asteroids. These asteroid observations, and comparable studies of meteorites and other minerals in the laboratory, have provided the first fascinating hints on the correlation between specific asteroids and specific meteorites. More than 90 percent of the asteroids studied fall into one of two composition groups: stony-iron or carbonaceous. Only a few percent of the meteorites on Earth are carbonaceous, but carbonaceous meteorites are very friable and rapidly weather to powder under typical terrestrial conditions. They probably also fragment more readily upon entry into the Earth’s atmosphere. Since stony-iron meteorites are much hardier, they are disproportionately represented in our museum collections of meteorites. The carbonaceous meteorites are rich in organic compounds, including amino acids (the building blocks of proteins), and may be representative of the materials from which the solar system was formed some 4.6 billion years ago.

Among the asteroids which appear to be carbonaceous are 1 Ceres, 2 Pallas, 19 Fortuna, 324 Bamberga and 654 Zelinda. If asteroids that are carbonaceous on the outside are also carbonaceous on the inside, then most of the asteroidal material is carbonaceous. They are generally dark objects, reflecting only a small percent of the light shining on them. Recent evidence suggests that Phobos and Deimos, the two moons of Mars, may also be carbonaceous, and are perhaps carbonaceous asteroids that have been captured by Martian gravity.

Typical asteroids showing properties of stony-iron meteorites are 3 Juno, 8 Flora, 12 Victoria, 89 Julia and 433 Eros. Several asteroids fit into some other
category: 4 Vesta resembles a kind of meteorite called a basaltic achondrite, while 16 Psyche and 22 Kalliope appear to be largely iron.

The iron asteroids are interesting because geophysicists believe that the parent body of an object greatly enriched in iron must have been molten so as to differentiate, to separate out the iron from the silicates in the initial chaotic jumble of the elements in primordial times. On the other hand, for the organic molecules in carbonaceous meteorites to have survived at all they must never have been raised to temperatures hot enough to melt rock or iron. Thus, different histories are implied for different asteroids.

From the comparison of asteroidal and meteoritic properties, from laboratory studies of meteorites and computer projections back in time of asteroidal motions, it may one day be possible to reconstruct asteroid histories. Today we do not even know whether they represent a planet that was prevented from forming because of the powerful gravitational perturbations of nearby Jupiter, or whether they are the remnants of a fully formed planet that somehow exploded. Most students of the subject incline to the former hypothesis because no one can figure out how to blow up a planet—which is just as well. Eventually we may be able to piece together the whole story.

There may also be in hand meteorites which do not come from asteroids. Perhaps there are fragments of young comets, or of the moons of Mars, or of the surface of Mercury, or of the satellites of Jupiter, sitting dusty and ignored in some obscure museum. But it is clear that the true picture of the origin of the meteorites is beginning to emerge.

The Holy of Holies in the Temple of Diana at Ephesus has been destroyed. But the Kaaba has been carefully preserved, although there seems never to have been a true scientific examination of it. There are some who believe it to be a dark, stony rather than metallic meteorite. Recently two geologists have suggested, on admittedly quite fragmentary evidence, that it is instead an agate. Some Muslim writers believe that the color of
the Kaaba was originally white, not black, and that the present color is due to its repeated handling. The official view of the Keeper of the Black Stone is that it was placed in its present position by the patriarch Abraham and fell from a religious rather than an astronomical heaven—so that no conceivable physical test of the object could be a test of Islamic doctrine. It would nevertheless be of great interest to examine, with the full armory of modern laboratory techniques, a small fragment of the Kaaba. Its composition could be determined with precision. If it is a meteorite, its cosmic-ray-exposure age—the time spent from fragmentation to arrival on Earth—could be established. And it would be possible to test hypotheses of origin: such as, for example, the idea that some 5 million years ago, about the time of the origin of the horninids, the Kaaba was chipped off an asteroid named 22 Kalliope, orbited the Sun for ages of geological time, and then accidentally encountered the Arabian Peninsula 2,500 years ago.

MUCH OF HUMAN HISTORY
can, I think, be described as a gradual and sometimes painful liberation from provincialism, the emerging awareness that there is more to the world than was generally believed by our ancestors. With awesome ethnocentrism, tribes all over the Earth called themselves “the people” or “all men,” relegating other groups of humans with comparable accomplishments to subhuman status. The high civilization of ancient Greece divided the human community into Hellenes and barbarians, the latter named after an uncharitable imitation of the languages of non-Greeks (“Bar Bar …”). That same classical civilization, which in so many respects is the antecedent of our own, called its small inland sea the Mediterranean—which means the middle of the Earth. For thousands of years China
called itself the Middle Kingdom, and the meaning was the same: China was at the center of the universe and the barbarians lived in outer darkness.

Such views or their equivalent are only slowly changing, and it is possible to see some of the roots of racism and nationalism in their pervasive early acceptance by virtually all human communities. But we live in an extraordinary time, when technological advances and cultural relativism have made such ethnocentrism much more difficult to sustain. The view is emerging that we all share a common life raft in a cosmic ocean, that the Earth is, after all, a small place with limited resources, that our technology has now attained such powers that we are able to affect profoundly the environment of our tiny planet. This deprovincialization of mankind has been aided powerfully, I believe, by space exploration—by exquisite photographs of the Earth taken from a great distance, showing a cloudy, blue, spinning ball set like a sapphire in the endless velvet of space; but also by the exploration of other worlds, which have revealed both their similarities and their differences to this home of mankind.

We still talk of “the” world, as if there were no others, just as we talk about “the” Sun and “the” Moon. But there are many others. Every star in the sky is a sun. The rings of Uranus represent millions of previously unsuspected satellites orbiting Uranus, the seventh planet. And, as space vehicles have demonstrated so dramatically in the last decade and a half, there are other worlds—nearby, relatively accessible, profoundly interesting, and not a one closely similar to ours. As these planetary differences, and the Darwinian insight that life elsewhere is likely to be fundamentally different from life here, become more generally perceived, I believe they will provide a cohesive and unifying influence on the human family, which inhabits, for a time, this unprepossessing world among an immensity of others.

Planetary exploration has many virtues. It permits us to refine insights derived from such Earth-bound sciences as meteorology, climatology, geology and biology,
to broaden their powers and improve their practical applications here on Earth. It provides cautionary tales on the alternative fates of worlds. It is an aperture to future high technologies important for life here on Earth. It provides an outlet for the traditional human zest for exploration and discovery, our passion to find out, which has been to a very large degree responsible for our success as a species. And it permits us, for the first time in history, to approach with rigor, with a significant chance of finding out the true answers, questions on the origins and destinies of worlds, the beginnings and ends of life, and the possibility of other beings who live in the skies—questions as basic to the human enterprise as thinking is, as natural as breathing.

Interplanetary unmanned spacecraft of the modern generation extend the human presence to bizarre and exotic landscapes far stranger than any in myth or legend. Propelled to escape velocity near the Earth, they adjust their trajectories with small rocket motors and tiny puffs of gas. They power themselves with sunlight and with nuclear energy. Some take only a few days to traverse the lake of space between Earth and Moon; others may take a year to Mars, four years to Saturn, or a decade to traverse the inland sea between us and distant Uranus. They float serenely on pathways predetermined by Newtonian gravitation and rocket technology, their bright metal gleaming, awash in the sunlight which fills the spaces between the worlds. When they arrive at their destinations, some will fly by, garnering a brief glimpse of an alien planet, perhaps with a retinue of moons, before continuing on farther into the depths of space. Others insert themselves into orbit about another world to examine it at close range, perhaps for years, before some essential component runs down or wears out. Some spacecraft will make landfall on another world, decelerating by atmospheric friction or parachute drag or the precision firing of retrorockets before gently setting down somewhere else. Some landers are stationary, condemned to examine a single spot on a world awaiting exploration. Others are self-propelled, slowly wandering to a distant horizon which
holds no man knows what. And still others are capable of remotely acquiring rock and soil—a sample of another world—and returning it to the Earth.

All these spacecraft have sensors that extend astonishingly the range of human perception. There are devices that can determine the distribution of radioactivity over another planet from orbit; that can feel from the surface the faint rumble of a distant planetquake deep below; that can obtain three-dimensional color or infrared images of a landscape like none ever seen on Earth. These machines are, at least to a limited degree, intelligent. They can make choices on the basis of information they themselves receive. They can remember with great accuracy a detailed set of instructions which, if written out in English, would fill a good-sized book. They are obedient and can be reinstructed by radio messages sent to them from human controllers on Earth. And they have returned, mostly by radio, a rich and varied harvest of information on the nature of the solar system we inhabit. There have been fly-bys, crash-landers, soft-landers, orbiters, automated roving vehicles, and unmanned returned sample missions from our nearest celestial neighbor, the Moon—as well as, of course, six successful and heroic manned expeditions in the Apollo series. There has been a fly-by of Mercury; orbiters, entry probes and landers on Venus; fly-bys, orbiters and landers to Mars; and fly-bys of Jupiter and Saturn. Phobos and Deimos, the two small moons of Mars, have been examined close up, and tantalizing images have been obtained of a few of the moons of Jupiter.