When scientists are faced with extremely difficult theoretical problems, there is always the possibility of performing experiments. In studies of the climate of an entire planet, however, experiments are expensive and difficult to perform, and have potentially awkward social consequences. By the greatest good fortune, nature has come to our aid by providing us with nearby planets with significantly different climates and significantly different physical variables. Perhaps the sharpest test of theories of climatology is that they be able to explain the climates of all the nearby planets, Earth, Mars and Venus. Insights gained from the study of one planet will inevitably aid the study of the others. Comparative planetary climatology appears to be a discipline, just in the process of birth, with major intellectual interest and practical applications.
CHAPTER 15
We imagine them
flitting
cheek to jowl,
these driftrocks
of cosmic ash
thousandfold afloat
between Jupiter and Mars.
Frigga,
Fanny,
Adelheid
Lacrimosa.
Names to conjure with,
Dakotan black hills,
a light-opera
staged on a barrier reef.
And swarm they may have,
crumbly as blue-cheese,
that ur-moment
when the solar system
broke wind.
But now
they lumber
so wide apart
from each
to its neighbor’s
pinprick-glow
slant millions
and millions
of watertight miles.
Only in the longest view
do they graze
like one herd
on a breathless tundra.
DIANE ACKERMAN,
The Planets (New York, Morrow, 1976)
ONE OF THE seven wonders of the ancient world was the Temple of Diana at Ephesus, in Asia Minor, an exquisite example of Greek monumental architecture. The Holy of Holies in this temple was a great black rock, probably metallic, that had fallen from the skies, a sign from the gods, perhaps an arrowhead shot from the crescent moon, the symbol of Diana the Huntress.
Not many centuries later-perhaps even at the same time-another great black rock, according to the belief of many, fell out of the sky onto the Arabian Peninsula. There, in pre-Islamic times, it was emplaced in a Meccan temple, the Kaaba, and offered something akin to worship. Then, in the seventh and eight centuries A.D., came the stunning success of Islam, founded by Muhammed, who lived out most of his days not far from this large dark stone, the presence of which might conceivably have influenced his choice of career. The earlier worship of the stone was incorporated into Islam, and today a principal focus of every pilgrimage to Mecca is that same stone-often called the Kaaba after the temple that enshrines it. (All religions have shamelessly coopted their predecessors-e.g., consider the Christian festival of Easter, where the ancient fertility rites of the spring equinox are today cunningly disguised as eggs and baby animals. Indeed the very name Easter is, according to some etymologies, a corruption of the name of the great Near Eastern Earth mother goddess, Astarte. The Diana of Ephesus is a later and Hellenized version of Astarte and Cybelle.)
In primitive times, a great boulder falling out of a clear blue sky must have provided onlookers with a memorable experience. But it had a greater importance: at the dawn of metallurgy, iron from the skies was, in many parts of the world, the purest available form of this metal. The military significance of iron swords and the agricultural significance of iron plowshares made metal from the sky a concern of practical men.
Rocks still fall from the skies; farmers still occasionally break their plows on them; museums still pay a bounty for them; and, very rarely, one falls through the eaves of a house, narrowly missing a family in its evening hypnogogic ritual before the television set. We call these objects meteorites. But naming them is not the same as understanding them. Where, in fact, do meteorites come from?
Between the orbits of Mars and Jupiter are thousands of irregularly shaped, tumbling little worlds called asteroids or planetoids. “Asteroid” is not a good term for them because they are not like stars. “Planetoid” is much better because they are like planets, only smaller, but “asteroid” is the more widely used term by far. Ceres, the first asteroid to be found, was discovered [11] telescopically on January 1, 1801-an auspicious finding on the first day of the nineteenth century-by G. Piazzi, an Italian monk. Ceres is about 1,000 kilometers in diameter and is by far the largest asteroid. (By comparison, the diameter of the Moon is 3,464 kilometers.) Since then, more than two thousand asteroids have been discovered. Asteroids are given a number indicating their order of discovery. But following Piazzi’s lead, a great effort was also made to give them names-female names, preferably from Greek mythology. However, two thousand asteroids is a great many, and the nomenclature becomes a little ragged toward the end. We find 1 Ceres, 2 Pallas, 3 Juno, 4 Vesta, 16 Psyche, 22 Kalliope, 34 Circe, 55 Pandora, 80 Sappho, 232 Russia, 324 Bamberga, 433 Eros, 710 Gertrud, 739 Mandeville, 747 Winchester, 904 Rockefelleria, 916 America, 1121 Natasha, 1224 Fantasia, 1279 Uganda, 1556 Icarus, 1620 Geographos, 1685 Toro, and 694 Ekard (Drake [University] spelled backwards). 1984 Orwell is, unfortunately, a lost opportunity.
Many asteroids have orbits that are highly elliptical or stretched-out, not at all like the almost perfectly circular orbits of Earth or Venus. Some asteroids have their far points from the Sun beyond the orbit of Saturn; some have their near points to the Sun close to the orbit of Mercury; some, like 1685 Toro, live out their days between the orbits of Earth and Venus. Since there are so many asteroids on very elliptical orbits, collisions are inevitable over the lifetime of the solar system. Most collisions will be of the overtaking variety, one asteroid nudging up to another, making a soft splintering crash. Since the asteroids are so small, their gravity is low and the collision fragments will be splayed out into space into slightly different orbits from those of the parent asteroids. It can be calculated that such collisions will produce, on occasion, fragments that by accident intercept the Earth, fall through its atmosphere, survive the ablation of entry, and land at the feet of a quite properly astonished itinerant tribesman.
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.
[11] Unexpected discoveries are useful for calibrating pre-existing ideas. G. W. F. Hegel has had a very powerful imprint on professional philosophy of the nineteenth and early twentieth centuries and a profound influence on the future of the world because Karl Marx took him very seriously (although sympathetic critics have argued that Marx’s arguments would have been more compelling had he never heard of Hegel). In 1799 or 1800 Hegel confidently stated, using presumably the full armamentarium of philosophy available to him, that no new celestial objects could exist within the solar system. One year later, the asteroid Ceres was discovered. Hegel then seems to have returned to pursuits less amenable to disproof.