12. THE MODERN VIEW
After the L’Aigle shower of 1803, a whole new era opened in the study of meteorites. No longer did scientists hold these objects up to ridicule and scorn. Instead, they came to regard meteorites as well worth collection and careful study.
The Vienna Museum, the British Museum, the Paris Museum, the Academy of Science of St. Petersburg (now Leningrad), and the U.S. National Museum began to build up splendid meteorite collections. Scientists in Germany, England, France, and Russia engaged in the painstaking mineralogical study and classification of individual meteorite specimens.
The modern science of meteoritics is rooted deep in the nineteenth century. Many special fields of investigation had their beginnings then. Scientists became interested in the chemistry, the mineralogy, and the metallurgy of meteorites; in the orbits of meteorites and the trajectories they follow through the earth’s atmosphere down to impact with the ground; and in the distribution of meteorite falls in space and time.
From this period we can date such milestones of progress in meteoritics as:
The discovery of the beautiful and significant Widmanstätten patterns characteristic of the majority of the irons, and the less spectacular but equally important lines named for J. G. Neumann, the German meteoriticist who discovered them, in 1848, in the Braunau meteorite.
The realization that there were many different kinds of meteorites and that these diverse objects were very important to an understanding of the internal structure and origin of the earth, and perhaps of the Solar System and the wider cosmos as well.
Tentative explanations of the violent and terrifying light and sound effects connected with meteorite falls.
Tentative explanations of such oval-shaped areas as shown above.
Typical distribution of meteorite fragments according to size, within oval-shaped area of fall. The larger masses of the shower carry farther on, in the direction of the motion of the meteorite. As early as 1814, investigators had noted this peculiarity of meteorite-shower distributions. See pp. [32], [89], [94].
By 1850, A. Boisse, an early French geologist and meteoriticist, had put forth the basic meteorite-planet hypothesis. According to this theory of his, meteorites are the fragments of a planet[12] that formerly orbited between Mars and Jupiter in what is now called the “asteroid belt.” And untold millions of years ago, this planet was shattered by some unknown but very great force, possibly collision with another celestial body.
The structure of the meteorite-planet was considered to have been very much like that of the earth. The various divisions of recognized meteorites were believed to be representatives of the several concentric, or nested, shells of material originally making up the destroyed planet. These shells were progressively less dense with increasing distance from the center of the planet.
Today Boisse’s theory is one of the most widely accepted as an explanation of at least one major category of the meteorites. Some modern investigators would insist that the meteorite-planet had a thin outer glassy shell from which the tektites came.
Most of the larger fragments of the meteorite-planet, now called the asteroids, move so that the average asteroidal orbit very closely approximates the orbit of the original planet. But many of the smaller fragments follow paths in space that differ considerably from the original meteorite-planet’s orbit. Even some of the asteroids behave this way, either because of the high speeds they acquired at the time of disruption of the meteorite-planet, or because of the later influence of the major planets and particularly of the giant planet, Jupiter.
A diagram (not drawn to scale) showing position of asteroid-belt with respect to the orbits of Mars and Jupiter. The asteroids with average orbits move within this belt. The non-typical asteroid indicated follows an orbit that brings it well inside that of the earth. There are a number of asteroids with such peculiar orbits. It is possible that in the past a nickel-iron asteroid in one of these orbits collided with the earth and produced the Canyon Diablo meteorite crater.
In fact, at the present time, several asteroids move well within the orbits of the earth and Venus. It is quite possible therefore that such a large meteorite crater as the one at Canyon Diablo, was produced by the prehistoric fall of one of these small members of our Solar System. If so, we have reason to believe that a core-fragment of the meteorite-planet came to earth at Canyon Diablo. For the extensive mining operations carried out there during the last half-century have shown that the projectile responsible for this greatest of all meteoritic shell-holes in the face of Mother Earth was a mass of solid nickel-iron, which in all likelihood was core material.
The lengthy and costly series of mining operations at Canyon Diablo were all undertaken in the hope of locating the “main mass” of this huge projectile and thus of opening up what might be called a cosmic-lode of quite valuable metals. Unfortunately, the miners overlooked the fact that impacts at meteoritic speeds produced almost incredible amounts of heat. Even the solid iron meteorites are vaporized and widely dispersed at the temperatures resulting from such impacts, as we have seen was the case at Wabar (see [Chapter 4]). So it was at Canyon Diablo.
The idea of a cosmic-metal mine might at first strike some readers as too futuristic to take seriously. But the necessity for catching a core-fragment before it enters the consuming atmosphere of our planet is really nothing new. As far back as 1939, the senior author had occasion to point out that if we wish to start a successful cosmic-metal mine, we must catch our core-fragment before it is turned into unminable vapor. This point will come up again in the [next chapter].
Cross-section of Boisse’s hypothetical meteorite-planet. Fragmentation of this sphere was believed to have given rise to the following divisions of meteorites:
The iron meteorites came from A, the dense nickel-iron core.
The stony-iron meteorites came from B, the intermediate zone of cellular nickel-iron and silicate minerals.
The stony meteorites came from C, the outer zone of silicate minerals in which relatively little or no nickel-iron is present. The chondrites were believed to come from the inner portion of this zone; the achondrites, from the outer portion.
There are several other theories of the origin of meteorites interesting enough to mention. The early view that the meteorites were debris thrown out by ancient volcanoes on the moon or recent ones on the earth came to be discredited largely on physical grounds. On the other hand, extremely violent primordial volcanoes on the earth (not the weak ones of historic times, like Aetna or Vesuvius) could have ejected material that in much later times fell, and continues to fall back on our globe. This theory has not been ruled out and it still receives support, for example, from some authorities in the U.S.S.R. These same Russian scientists take most seriously a suggestion that the meteorites (and comets as well) were thrown out by volcanoes believed to exist on the planet, Jupiter—a theory dating back almost a century to the English astronomer, R. A. Proctor.
Some scientists believe that meteorites represent the congealed remains of gaseous bolts of matter ejected by the sun. Others interpret them as fragments of comets that have been torn apart by passing too close to the sun, which is the most powerful gravitational center in the Solar System.
Chemists, geologists, astronomers, and physicists—as well as the meteoriticists themselves—are constantly working toward a solution of the problem of the meteorites. Where do these bodies come from? What can we learn from them about their age and origin and about the age and origin of our Solar System? Years may be required, but eventually the riddle of the meteorites will be solved by the patient, concerted efforts of men and women of science.
Collapsed mine buildings in the bottom of the Canyon Diablo meteorite crater. A shaft was put down here in one of several unsuccessful attempts to locate the main mass of the meteorite. See pp. [44]-52.