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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.]