Asimov's New Guide to Science

by Isaac Asimov

  Meteors and Meteorites

  Even the Greeks knew that shooting stars were not really stars, because no matter how many fell, the celestial population of stars remained the same. Aristotle reasoned that a shooting star, being a temporary phenomenon, had to be something within the atmosphere (and this time he was right). These objects were therefore called meteors, meaning “things in the air.” Meteors that actually reach the earth’s surface are called meteorites.

  The ancients even witnessed fans of meteorites to the earth and found some to be lumps of iron. Hipparchus of Nicaea is said to have reported such a fall.

  The Kaaba, the sacred black stone in Mecca, is supposed to be a meteorite and to have gained its sanctity through its heavenly origin. The Iliad mentions a lump of rough iron being awarded as one of the prizes in the funeral games for Patroclus; this must have been meteoric in origin, because the time was the Bronze Age, before the metallurgy of iron ore had been developed. In fact, meteoric iron was probably in use as early as 3000 B.C.

  During the eighteenth century, with the Age of Reason in full sway, science made a backward step in this respect. The scorners of superstition laughed at stories of “stones from the sky.” Farmers who came to the Académie Française with samples of meteorites were politely, but impatiently, shown the door. When, in 1807, two Connecticut scholars (one of them the young chemist Benjamin Silliman) reported having witnessed a fall, President Thomas Jefferson said that he would sooner believe that two Yankee professors would lie than that stones would fall from heaven.

  Jefferson was actually out of date, for reports of meteorite falls in France had finally stirred the physicist Jean Baptiste Biot, in 1803, to investigate such sightings. His investigation, soberly and thoroughly done, went a long way to convincing the scientific world that stones did indeed fall from heaven.

  Then, on 13 November 1833, the United States was treated to a meteor shower of the type called Leonids because they seem to radiate from a point in the constellation Leo. For some hours it turned the sky into a Roman-candle display more brilliant than any ever seen before or since. No meteorites reached the ground, as far as is known, but the spectacle stimulated the study of meteors, and astronomers turned to it for the first time in all seriousness.

  The very next year, the Swedish chemist Jöns Jakob Berzelius began a program for the chemical analyses of meteorites. Eventually such analyses gave astronomers valuable information on the general age of the solar system and even on the overall chemical makeup of the universe.

  METEORS

  By noting the times of year when meteors came thickest, and the positions in the sky from which they seemed to come, the meteor watchers were able to work out orbits of various clouds of meteors. In this way, they learned that a meteor shower occurs when the earth’s orbit intersects the orbit of a meteor cloud.

  Meteor clouds have elongated orbits as comets do, and it makes sense to consider them as the débris of disintegrated comets. Comets can disintegrate to leave dust and gravel behind according to the Whipple picture of comet structure, and some comets have been actually seen to disintegrate.

  When such comet dust enters the atmosphere, they can make a brave display, as they did in 1833. A shooting star as bright as Venus comes into the atmosphere as a speck weighing only 1 gram (I/28 of an ounce). Some visible meteors are only 1/10,000 as massive as that!

  The total number of meteors hitting the earth’s atmosphere can be computed, and turns out to be incredibly large. Each day there are more than 20,000 weighing at least 1 gram, nearly 200 million others large enough to make a glow visible to the naked eye, and many billions more of still smaller sizes.

  We know about these very small micrometeors because the air has been found to contain dust particles with unusual shapes and a high nickel content, quite unlike ordinary terrestrial dust. Another evidence of the presence of micrometeors in vast quantities is the faint glow in the heavens called zodiacal light (first discovered about 1700 by G. D. Cassin i)—so called because it is most noticeable in the neighborhood of the plane of the earth’s orbit, where the constellations of the zodiac occur. The zodiacal light is very dim and cannot be seen even on a moonless night unless conditions are favorable. It is brightest near the horizon where the sun has set or is about to rise; and on the opposite side of the sky, there is a secondary brightening called the Gegenschein (German for “opposite light”). The zodiacal light differs from the airglow: its spectrum has no lines of atomic oxygen or atomic sodium, but is just that of reflected sunlight and nothing more. The reflecting agent presumably is dust concentrated in space in the plane of the planets’ orbits—in short, micrometeors. Their number and size can be estimated from the the intensity of the zodiacal light.

  Micrometeors have now been counted with new precision by means of such satellites as Explorer XVI, launched in December 1962, and Pegasus I, launched 16 February 1965. To detect them, some of the satellites are covered with patches of a sensitive material that signals each meteoric hit through a change in electrical resistance. Others record the hits by means of a sensitive microphone behind the skin, picking up the “pings.” The satellite counts have indicated that 3,000 tons of meteoric matter enter our atmosphere each day, five-sixths of it consisting of micrometeors too small to be detected as shooting stars. These micrometeors may form a thin dust cloud about the earth, one that stretches out, in decreasing density, for 100,000 miles or so before fading out to the usual density of material in interplanetary space.

  The Venus probe Mariner 2 showed the dust concentration in space generally to be only 1/10,000 the concentration near Earth—which seems to be the center of a dustball. Fred Whipple suggests that the moon may be the source of the cloud, the dust being flung up from the moon’s surface by the meteorite beating it has had to withstand. Venus, which has no moon, also has no dustball.

  The geophysicist Hans Petterson, who has been particularly interested in this meteoric dust, took some samples of air in 1957 on a mountaintop in Hawaii, which is as far from industrial dust-producing areas as one can get on the earth. His findings led him to that about 5 million tons of meteoric dust fall on the earth each year. (A similar measurement by James M. Rosen in 1964, making use of instruments borne aloft by balloons; set the figure at 4 million tons, though still others find reason to place the figure at merely 100,000 tons per year.) Hans Petterson tried to get a line on this fall in the past by analyzing cores brought up from the ocean bottom for high-nickel dust. He found that, on the whole, there was more in the upper sediments than in the older ones below; thus—though the evidence is still scanty—the rate of meteoric bombardment may have increased in recent ages. This meteoric dust may possibly be of direct importance to all of us, for, according to a theory advanced by the Australian physicist Edward George Bowen in 1953, this dust serves as nuclei for raindrops. If so, then the earth’s rainfall pattern reflects the rise and fall of the intensity with which micrometeorites bombard us.

  METEORITES

  Occasionally pieces of matter that are larger than tiny bits of gravel, even substantially large, penetrate Earth’s atmosphere. They may be large enough to survive the heat of air resistance as they race through the atmosphere at anywhere from 8 to 45 miles per second, and to reach the ground. These, as I have said, are meteorites. Such meteorites are thought to be small asteroids—specifically, earth grazers that have grazed too closely and come to grief.

  Most of the meteorites found on the ground (about 1,700 are known altogether, of which 35 weigh over a ton each) have been iron, and it seemed that iron meteorites must far outnumber the stony type. This theory proved to be wrong, however. A lump of iron lying half-buried in a stony field is very noticeable, whereas a stone among other stones is not; a stony meteorite, once investigated, however, shows characteristic differences from earthly stones.

  When astronomers made counts of meteorites found that were actually seen to fall, they discovered that the stony meteorites outnumbered iron ones 9 to 1. (For a
time, most stony meteorites were discovered in Kansas, which may seem odd until one realizes that, in the stoneless, sedimentary soil of Kansas, a stone is as noticeable as a lump of iron would be elsewhere.)

  These two types of meteorites are thought to arise in the following manner: Asteroids, in the youth of the solar system, may have been larger, on the average, than they now are. Once formed, and prevented from further consolidation by the perturbations of Jupiter, they underwent collisions among themselves and breakups. Before that happened, however, the asteroids may have grown hot enough, on forming, to allow a certain separation of components, with iron sinking to the center and stone forced into the outer layer. Then, when such asteroids were fragments, there were both stony and metallic débris, making for meteorites of each type on Earth now.

  There is a third type of meteorite—carbonaceous chondrites—that is quite rare. These will be discussed, more appropriately, in chapter 13.

  Meteorites seldom do damage. Although about 500 substantial meteorites strike the earth annually (with only some 20 recovered, unfortunately), the earth’s surface is large, and only small areas are thickly populated. No human being has ever been killed by a meteorite so far as is known, although a woman in Alabama reported being bruised by a glancing blow on 30 November 1955. In 1982, a meteorite flashed through a home in Wethersfield, Connecticut, without hurting the occupants. Oddly enough, Wethersfield had been struck eleven years earlier without harm.

  Yet meteorites have a devastating potentiality. In 1908, for instance, a strike in central Siberia gouged out craters up to 150 feet in diameter and knocked down trees for 20 miles around. Fortunately, the meteorite fell in a wilderness and, while it destroyed a herd of deer, did not kill a single human being. Had it fallen from the same part of the sky five hours later in the earth’s rotation, it might have hit St. Petersburg (Leningrad), then the capital of Russia. If it had, the city would have been wiped out as thoroughly as by a hydrogen bomb. One estimate is that the total weight of the meteorite was 40,000 tons.

  This Tunguska event (so-called from the locality of the strike) has presented mysteries. The inaccessibility of the locality, and the confusion of war and revolution that took place soon after, made it impossible to investigate the area for many years. Once investigated, it offered no trace of meteoric material. In recent years, a Soviet science-fiction writer invented radioactivity at the site as part of a story—an invention that was taken as a sober finding by many people who had a natural affection for the sensational. As a result, many wild theories evolved—from a strike by a mini-black hole to an extraterrestrial nuclear explosion. The most likely rational explanation is that the incoming meteor was icy in nature, and probably a very small comet, or a piece of a larger one (possibly Comet Encke). It exploded in air before striking and did immense damage without producing any meteoric matter of stone or metal.

  The largest strike since then, near Vladivostok (again in Siberia), was in 1947.

  There are signs of even heavier strikes in prehistoric times. In Coconino County in Arizona, there is a round crater about four-fifths of a mile across and 600 feet deep, surrounded by a lip of earth 100 to 150 feet high. It looks like a miniature crater of the moon. It was long assumed to be an extinct volcano, but a mining engineer named Daniel Moreau Barringer insisted it was the result of a meteoric collision, and the hole now bears the name Barringer Crater. The crater is surrounded by lumps of meteoric iron—thousands (perhaps millions) of tons of it altogether. Although only a small portion has been recovered so far, more meteoric iron has already been extracted from it and its surroundings than in all the rest of the world. The meteoric origin of the crater was also borne out by the discovery there, in 1960, of forms of silica that could have been produced only by the momentary enormous pressures and temperatures accompanying meteoric impact.

  Barringer Crater, formed in the desert an estimated 25,000 years ago by an iron meteorite about 150 feet across, has been preserved fairly well. In most parts of the world, similar craters would have been obliterated by water and plant overgrowth. Observations from airplanes, for instance, have sighted previously unnoticed circular formations, partly water-filled and partly overgrown, which are almost certainly meteoric. Several have been discovered in Canada, including Brent Crater in central Ontario and Chubb Crater in northern Quebec, each of which is 2 miles or more in diameter; and Ashanti Crater in Ghana, which is 6 miles in diameter. These are perhaps more than a million years old. Some seventy such fossil craters are known, with diameters of up to 85 miles or so.

  The craters of the moon range from tiny holes to giants 150 miles or more across. The moon—lacking air, water, or life—is a nearly perfect museum for craters since they are subject to no wear except from the very slow action of temperature change resulting from the two-week alternation of lunar day and lunar night. Perhaps the earth would be pockmarked like the moon were it not for the healing action of wind, water, and growing things.

  It had been felt, at first, that the craters of the moon were volcanic in origin, but they do not really resemble earthly volcanic craters in structure. By the 1890s, the view that the craters had originated from meteoric strikes came into prominence and has gradually become accepted.

  The large “seas” or maria, which are vast, roughly circular stretches that are relatively craterfree, would, in this view, result from the impact of particularly large meteors. This view was bolstered in 1968 when satellites placed in orbit about the moon showed unexpected deviations in their circumlunar flights. The nature of these deviations forced the conclusion that parts of the lunar surface are denser than average and produce a slight increase in gravitational attraction, to which the satellite flying over them responded. These denser-than-average areas, which seemed to coincide with the maria, received the name mascons (short for “mass concentration”). The most obvious deduction was that the sizable iron meteors that formed the seas are still buried beneath them and are considerably denser than the rocky material that generally makes up the moon’s crust. At least a dozen mascons were detected within a year of their initial discovery.

  The view of the moon as a “dead world” where no volcanic action is possible is, on the other hand, overdrawn. On 3 November 1958, the Russian astronomer N. A. Kozyrev observed a reddish spot in the crater Alphonsus. (William Herschel had reported seeing reddish spots on the moon as early as 1780.) Kozyrev’s spectroscopic studies seemed to make it clear that gas and dust had been emitted. Since then, other red spots have been momentarily seen, and it seems certain that volcanic activity does occasionally take place on the moon. During the total lunar eclipse in December 1964, it was found that as many as 300 craters were hotter than the surrounding landscape—although, of course, they were not hot enough to glow.

  Airless worlds generally, such as Mercury and the satellites of Mars, Jupiter, and Saturn, are thickly spread with craters which commemorate the bombardment that took place 4 billion and more years ago when the worlds were formed by accretion of planetesimals. Nothing has occurred since to remove those markings.

  Venus is poor in craters, perhaps because of the erosive effects of its thick atmosphere. One hemisphere of Mars is poor in craters, perhaps because volcanic action has built up a fresh crust. 10 has virtually no craters because of the lava built up by its active volcanoes. Europa has no craters because meteoric impacts break through the encircling glacier into the liquid beneath, whereupon the liquid exposed quickly refreezes and “heals” the break.

  Meteorites, as the only pieces of extraterrestrial matter we can examine, are exciting not only to astronomers, geologists, chemists, and metallurgists but also to cosmologists, who are concerned with the origins of the universe and the solar system. Among the meteorites are puzzling glassy objects found in several places on earth. The first were found in 1787 in what is now western Czechoslovakia. Australian examples were detected in 1864. They received the name tektites, from a Greek word for “molten,” because they appear to have melted in thei
r passage through the atmosphere.

  In 1936, the American astronomer Harvey Harlow Ninninger suggested that tektites are remnants of splashed material forced away from the moon’s surface by the impact of large meteors and caught by Earth’s gravitational field. A particularly widespread strewing of tektites is to be found in Australia and southeast Asia (with many dredged up from the floor of the Indian Ocean). These seem to be the youngest of the tektites, only 700,000 years old. Conceivably, these could have been produced by the great meteoric impact that formed the crater Tycho (the youngest of the spectacular lunar craters) 011 the moon. The fact that this strike seems to have coincided with the most recent reversal of Earth’s magnetic field has caused some speculation that the strikingly irregular series of such reversals may mark other such earth-moon catastrophes.

  Another unusual classification of meteorites are those that may be found in Antarctica. For one thing, any meteorite, whether stony or metallic, if lying Oil the vast Antarctica icecap is inevitably noticeable. In fact, any solid object anywhere on that continent, if not ice and not of human origin, is bound to be a meteorite. And once it lands, it remains untouched (at least over the last 20 million years) unless it is buried in snow or stumbled over by an emperor penguin.

  Not many human beings are present in Antarctica at any time, and not much of the continent has been peered at closely, so that up to 1969, only four meteorites were found—all by accident. In 1969, a group of Japanese gologists came across nine closely spaced meteorites. These roused the interest of scientists generally, and ever more meteorites were found. By 1983, more than 5,000 meteoric fragments had been found on the frozen continent, more by far than in all the rest of the world. (Antarctica is not especially chosen out for strikes, but meteorites are much more easily spotted there.)