Not all meteorites form craters at impact, as the larger Ussuri
fragments did. Even the largest mass of the Norton meteorite merely
buried itself in a funnel-like hole only about 10 feet deep. And the
Russian investigators found a number of the lighter Ussuri fragments at
the bottom of small penetration funnels. Cosmic missiles that are large
enough to blast out craters in the ground are of particular interest to
science, however, not only because of the extraordinarily intense light,
sound, and other effects that accompany their fall, but also because
they produce characteristic and long-lasting basin-like features in the
outer shell of the earth.
Natural processes that change the surface features of the earth have
long been the subjects of field studies by scientists. Geologists have
carefully investigated the major folds formed in the earth’s crust by
mountain-building forces, the clefts and depressions resulting from
earthquake activity and erosion, and the vast plains leveled off by the
scouring action of great ice-sheets. All of these different natural
processes, though, have one thing in common: their source is the
earth-body itself. They take place either _within_ the earth’s crust as
a result of local shifts or changes in pressure (like earthquakes and
volcanic eruptions), or _on_ the surface of the earth as a result of the
action of water or of changes in temperature (like erosion and
glaciation).
On the other hand, meteorite impact craters are not formed by
earth-processes at all. As we have seen, they result when large bodies
of matter from the regions of space _outside_ the earth chance to strike
the surface of our planet at high speed. The study of meteorite craters
is therefore a special field. It is also one of quite recent
development; not until 1905 was the first meteorite crater recognized as
such.
The first thing to be said on this subject is, of course, that not all
holes in the ground, however large and impressive, were necessarily
formed by the impact of meteorites. Features that resemble meteorite
craters may result from certain ordinary earth-processes. For example,
the rock layers underlying a particular area may be dissolved away by
waters circulating beneath the surface of the ground. The overlying
crust will eventually collapse into the empty space, and what geologists
call a “sink hole” or a “sink” is formed. Many such sinks surround the
genuine meteorite crater near Odessa, Texas, and at times have been
mistaken for the real thing.
Since there is some possibility of confusion about whether or not a hole
in the ground is a meteorite crater, it is comforting to know that
scientists have come up with a handy set of rules for reaching a
decision on this point. These rules can be stated in the form of several
questions that crater-investigators should ask themselves:
Have you found meteorites in or near the crater-like feature?
In its vicinity, have you found pieces of country rock that show the
effects of high temperature and pressure (melting or crushing)?
Did people actually see a meteorite come to earth at the point where
the crater is located and where, to their certain knowledge, no crater
existed before?
If the answer to all—or even one—of these questions is yes, then it is
quite likely that the crater-like feature is actually a meteorite
crater. Naturally, if the answer to the _first_ question is yes, the
matter is practically settled in favor of the meteoritic origin of the
feature.
If the impact has taken place in horizontally bedded rock strata—that
is, in flat beds of rock lying one on top of another like the layers in
a stack of griddle cakes—a meteorite crater will have a characteristic
_rim_ of upturned or even overturned rock layers. (None of the ordinary
sink holes near the Odessa crater show such rims.) In addition, pieces
of rock shattered and thrown out by the impact will be found in all
directions around the crater. The amount and size of this fragmented
material will decrease with distance outward from the crater.
A list of the recognized (or genuine) meteorite craters of the world is
given in the table on page 65. All of these craters except the two
Russian ones were formed many thousands of years ago, and, in most
cases, the earth processes of erosion and weathering have by now dimmed
the sharp outlines of their rims and silted up their deep interior
funnels until only basin-like bowls remain.
[Illustration: Cross-section showing the manner in which
horizontally bedded rock strata may be broken and tilted upward by
the impact of a crater-forming meteorite. This schematic diagram is
based on excavations at several meteorite craters.]
You may have visited the very first crater in the world to be recognized
by scientists as a meteorite crater. This huge basin, now known as the
Canyon Diablo meteorite crater (although often referred to incorrectly
as “Meteor Crater”), lies about 20 miles west of Winslow, Arizona. It is
the best known of all the craters listed in the table because in recent
years it has been developed under private ownership as one of the
leading tourist attractions on U.S. Highway 66.
From the paved road that turns off Highway 66 toward the crater, the
visitor sees the rim as a chain of low, hummocky, tan-colored hills
which contrast sharply with the grayish or reddish hue of the desert
plain.
The outer slopes of the crater rim rise very gently from the level plain
in which the crater was formed, and they are covered with rock fragments
of various sizes thrown out at the time the meteorite struck the earth.
This fragmented material ranges in size from tiny particles of
“rock-flour” as soft as face-powder to gigantic solid masses like
Monument Rock, which is estimated to weigh 4,000 tons.
Field parties have found 50- to 100-pound fragments of the limestone
layer underlying the Canyon Diablo area at distances of 1½ to 2 miles
from the crater. Sizable rock and meteorite fragments out to distances
of 6 miles from the rim have turned up, and smaller fragments of both
materials at even greater distances.
On their first visit to the Canyon Diablo crater, people are always
astonished at the steepness of the inner walls of the crater and at the
very great size of its bowl. This crater is more than 4,000 feet across
and 570 feet deep. It is the largest _recognized_ meteorite crater so
far discovered in the world, although other larger, basin-like features
elsewhere on the surface of the earth have been suspected but not proved
to have a similar origin.
[Illustration: COURTESY OF TRANS-WORLD AIRLINES
Aerial view of the Canyon Diablo, Arizona, meteorite crater.]
When the Canyon Diablo meteorite plunged into the horizontally bedded
rock layers underlying the area of fall, the force of the explosion
following the impact actually bent these layers upward. All around the
inside of the crater, the rock strata tilt away from the center at steep
angles.
Cowboys, ranchers, and scientists have found thousands of solid
nickel-iron meteorite fragments around the crater. The largest of these
weighs 1,406 pounds. The smallest spherules and grains are almost or
quite microscopic in size. (These tiny granules have been well known to
scientists since 1905 in spite of current fables claiming that they are
a recent discovery.) In the rim and on the plain outside the crater,
large and small _shale balls_, composed of weathered meteoritic
material, were found in considerable numbers in the early days. Along
with many solid iron meteorites, shale balls have also been found at
various depths in recent times by field parties from the Institute
employing specially designed meteorite detectors.
In the first two decades of the twentieth century, investigators sank
(at great expense!) a number of shafts and drill holes in the interior
and on the south rim of the crater, in unsuccessful attempts to locate
the supposed “main mass” of the Canyon Diablo meteorite. Most
authorities now believe, however, that the extremely high temperatures,
developed at the time the Canyon Diablo meteorite penetrated into the
earth, changed almost all of the gigantic cosmic missile into vapor.
[Illustration: View of the interior of the Canyon Diablo crater
showing the steep inner slopes of the huge basin.]
No better example of an ancient meteorite crater has been found than
this one near Canyon Diablo. The other craters listed in the table (even
the two recently formed ones), while bearing resemblances to it, also
show individual differences from it.
Some, like Henbury, Campo del Cielo, and Haviland, are not single
craters but rather consist of fields of craters. In these cases, the
earth was struck not by a single large meteoritic body that held
together right down to impact, but either by a “swarm” of meteorites
traveling together through space or by the fragments of a large
meteorite that separated into pieces shortly before it struck the
surface of the ground.
Again, the type of ground into which the meteorite strikes affects the
character of the craters formed. As an illustration, the Wabar, Arabia,
craters were not smashed out of sedimentary, horizontally bedded rock
layers (as was the Canyon Diablo crater) but were formed in clean desert
sand dunes. In this case, the crater rims are composed primarily of
almost pure silica-glass formed by the fusion of the sand at the time of
impact. It is not hard to imagine the terrific boiling and frothing up
of melted sand and meteoritic material that must have accompanied the
formation of the Wabar craters.
Except for Podkamennaya Tunguska and Ussuri, the craters listed in the
table were formed, as we have mentioned, a great many thousands of years
in the past. Just how many thousands is a difficult question to answer,
for all of our estimates must necessarily be made on the basis of
_indirect_ evidence rather than on _direct_ observation.
[Illustration: Before impact of Canyon Diablo meteorite, these rock
layers were horizontal.]
Paleontologists, geologists, and other scientists give us an age of from
20,000 to 70,000 years for the Canyon Diablo crater. The discovery of
the fossil remains of a prehistoric horse buried in the Odessa, Texas,
crater fill has shown that the age of that crater is not less than
200,000 years. The oldest craters known in the United States are the
Haviland group produced by the Brenham, Kansas, meteorites.
Long-continued weathering has almost completely worn down the rims and
covered up the craters of this group. On the basis of the rate at which
nickel-oxide has spread out into the soil about a large deeply buried
Brenham meteorite, calculations carried out at the Institute of
Meteoritics have led to a tentative age of more than 600,000 years for
the Kansas craters.
Perhaps the oldest meteorite crater of all is the one blasted into what
the geologists identify as pre-Cambrian quartzite at Wolf Creek, Western
Australia. Even the highly resistant iron meteorites found around this
crater have almost completely weathered away. Only tiny specks and thin
veinlets of metal are now visible on the cut surfaces of meteorites
that, untold hundreds of thousands of years ago, were solid masses of
nickel-iron.
You may have noticed that the widely publicized circular, water-filled
Chubb crater in the Quebec Province of Canada was not included in the
table. This Canadian feature was left out because the answer to each of
the three questions listed earlier in this chapter is no.
[Illustration: COURTESY OF WILLIAM A. CASSIDY
Two of the deeply weathered meteorites found at Wolf Creek crater
in western Australia.]
The field parties that have carefully searched the Chubb crater and its
surroundings, even when they used one of the Institute’s powerful drag
magnets, were unable to find any trace whatever either of meteorites or
of such weathered remains of meteorites as show the true nature of the
Wolf Creek crater. Furthermore, no searcher has discovered any fragments
of ordinary rock showing the effects of the extreme heat and pressure
that accompany large-scale meteoritic impact. Finally, the meteorite
supposed by some to have produced the Chubb crater was not a recorded
witnessed fall, for the crater is of very ancient origin indeed.
Perhaps further search of the Chubb crater site and especially of the
debris in its deep, water-filled interior will succeed in bringing to
light either specimens of meteorites or of silica-glass or other
products of meteoritic impact. If so, then and only then will
identification of the Canadian crater as a meteorite crater be
justified.
Up to this point, we have talked only of very old meteorite craters. But
two crater-producing meteorite falls have occurred within this century,
both in Siberia. The Ussuri fall was one of these and the more recent of
the two.
The earlier and more unusual fall took place on June 30, 1908, at about
8:00 a.m., approximately 40 miles northwest of the trading post of
Vanovara. A fireball exceeding the sun in brilliance flashed across the
sky and was followed by extremely violent airwaves and earth-tremors.
The pressure wave in the atmosphere set up by this meteorite fall was
strong enough to damage roofs and doors of houses near the point of
impact, as for example, in the village of Vanovara. On both rivers and
lakes in the area of fall, the pressure wave in the air piled up high,
sharp-fronted water waves that resembled the bores on the Seine and
Severn and that upset fishing craft and swamped other small boats.
Throughout a wide region at somewhat greater distances from the impact
point, tidal-like bores were raised on rivers and lakes. So gigantic was
the atmospheric disturbance, that it was detected at almost every
station in the world where sufficiently sensitive barometers were in
operation.
Eyewitnesses of this meteorite fall said that at the time the fireball
passed near them, they felt almost unbearable heat.
A huge “fiery pillar” rose above the point of impact, which by good
fortune was in a desolate and almost uninhabited swampy basin between
the Chunya and the Podkamennaya (i.e., “Stony”) Tunguska rivers. The
meteorite fall takes its name from the latter stream.
The central portion of the region of impact is marked not only by a
number of craters in the swampy terrain, but also by mute evidence of
the extraordinary destructive power of the Podkamennaya Tunguska
meteorite. Over an area of many square miles, the explosion blew down
the standing forest so that the tops of the overthrown trees (estimated
by the Russians to number more than 80,000,000!) all point away from the
impact center. The intense heat charred the trunks and branches of the
trees in this area in much the same way as the heat from the first of
all atomic bomb explosions scorched the desert shrubs around the test
site in south-central New Mexico.
Within the area of fall, countless reindeer belonging to the native
Tunguse herdsmen were killed, only their charred carcasses remaining.
How great the heat released at impact was may be judged by the
well-established fact that the prized silver samovars of the nomads were
found melted amid the debris of their flattened camps. In at least one
instance, a Tunguse was so overcome by the terrible event he had
witnessed that he was “sick for a long time.” The whole impact-region
came to be considered as accursed by the natives, who abandoned the use
of all trails crossing it.
For many years the Podkamennaya Tunguska fall was neglected, partly
because of the remoteness of the area in which it occurred, partly
because of unsettled conditions in Russia; but chiefly because, in
general, the Russian scientific and governmental officials simply did
not believe the “fantastic” tales concerning the fall told by the native
Tunguses, from which we have given a few details above.
Belated study established, however, both the truthfulness of the Tunguse
reports and the exceedingly unusual character of the meteorite fall
itself. In spite of the overwhelming and, in fact, worldwide evidence
that the Podkamennaya Tunguska fall was one of the greatest and most
violent in history, no meteorites have ever been recovered from any part
of the region devastated by its impact. It is the one and only true
meteorite crater that is meteoriteless!
This strange circumstance led the senior author to suggest, in 1941,
that the almost incredible Podkamennaya Tunguska incident had resulted
from the infall of a meteorite that, together with an equivalent mass of
the earth-target, was transformed into energy upon contact with our
planet. How can such extraordinary behavior be accounted for?
[Illustration: LEONID A. KULIK PHOTO. SOVFOTO
Infall of meteorite, June 30, 1908, had this effect on a Siberian
forest. See p. 55.]
The most obvious explanation involves a new and wider concept of matter.
Ordinary terrestrial matter is regarded as composed of atoms having
positively charged nuclei around which negatively charged electrons
revolve.
Suppose that the situation shown in the first diagram were reversed so
that the nucleus of the atom were negatively charged and the charges of
the particles revolving about it were positive, as in the second
diagram. Matter built up from atoms like those in this diagram would
bear somewhat the same relation to ordinary matter that -2 does to +2.
Such matter is now known variously as _reversed matter_, _anti_-matter,
or, as it was first called by V. Rojansky, _contraterrene_ matter. In
recent years, scientists at the University of California Radiation
Laboratory have produced experimentally all the fundamental particles
necessary for the creation of contraterrene matter.
What would happen now if a contraterrene meteorite penetrated into the
ordinary matter of the earth? The answer is that just as an electron and
a positron mutually annihilate each other when they collide, so the
meteorite and an equal mass of the earth-target itself would vanish at
the instant of impact. The nearest simple analogy to the actual complex
physical situation is represented by the familiar equation -2 + 2 = 0.
Unlike “summing to zero” in simple arithmetic, however, the
disappearance of mass, technically called its annihilation, results in a
release of energy, as was long ago observed in the case of
electron-positron annihilation. Where considerable masses are
annihilated, as in an A-bomb explosion, the amount of energy released is
tremendous, as is now well known to everyone.
[Illustration: A. Representation of the structure of an atom of
ordinary terrestrial matter. The nucleus is positively charged and
around it circle negatively charged electrons.
B. Representation of the structure of an atom of contraterrene
matter. This is the reverse of the situation in (A). The nucleus
here is negatively charged, and around it revolve positively charged
electrons, also called positrons.]
The effect of such an energy release as would accompany the infall of a
contraterrene meteorite would be a _natural_ nuclear explosion of vast
power. Such an explosion would account for all the sensational phenomena
observed at the time of the Podkamennaya Tunguska incident; and,
furthermore, would explain why the Russian investigators have never
succeeded in recovering meteorites from this fall. (Further details, p.
102.)
If the Podkamennaya Tunguska meteorite was contraterrene, then the soil
in the impact area must have been made radioactive in the same way that
the earth around the “ground zero” of a nuclear explosion is
contaminated by radioactivity. After the senior author had repeatedly
urged Russian scientists (who are the only ones that have been permitted
to visit the site of the Podkamennaya Tunguska fall) to try to detect
any long-lasting radioactivities that might still be present in the
ground at Podkamennaya Tunguska, such a radioactivity survey was finally
carried out in the summer of 1960. According to an official report of
the Soviet news agency TASS, the investigators obtained “abnormally high
radioactivity readings” which the Russians tentatively considered to be
the result of “a natural nuclear explosion” occurring in the
Podkamennaya Tunguska area on June 30, 1908.
Science-fiction fans in the U.S.S.R. would like to believe that this
“nuclear explosion” resulted from the impact of a Martian spaceship
rather than a contraterrene meteorite. Reputable Russian scientists,
however, have shown how completely absurd this “fable” of a Martian
landing really is.
When and where will the next crater-producing fall occur? Perhaps on the
earth, perhaps on the moon, for our nearest neighbor in space has also
been the target of meteorites of huge size. The effects of this
meteoritic bombardment are shown by the rarest and most striking type of
lunar crater: that which exhibits long, bright rays extending outward
from the crater itself as the spokes of a wheel radiate from its hub.
These so-called _ray-craters_ show to best advantage at or near the time
of full moon, when they become one of the most remarkable features
visible on our satellite.
[Illustration: G. W. RICHEY PHOTO. COURTESY OF YERKES OBSERVATORY
The lunar ray-crater Tycho.]
In earlier days, most scientists believed that the craters on the moon
had _all_ been formed by volcanic action. Now the pendulum of scientific
opinion seems to have swung toward the view that _all_ the thousands of
lunar craters are the result of meteorite impacts that took place in the
long distant past. Both views are better examples of how scientific
“fashions” control men’s minds than they are of explanations that really
account for all of the observed facts—as any acceptable explanation must
do.
Those who have studied the moon most carefully from an uncomfortable
seat in a cold observatory rather than from a warm, comfortable armchair
are well aware that instead of just one type of lunar crater, there are
really _two_ quite distinct types. No single “explanation” can be
expected to explain satisfactorily lunar features as strikingly
different as:
First, the rare and distinctive _ray-craters_ described above, which are
scattered at random over the moon, just as the points of impact of
meteorites are upon our own globe. (Roughly defined, a random
distribution is one showing no apparent pattern. For example, if you
were to throw a handful of rice up in the air, the points where the
grains of rice finally came to rest on the floor would be randomly
distributed or very nearly so.)
Second, the ordinary or “run-of-the-mill” craters sprinkled in profuse
but non-random fashion over the visible face of our satellite.
The ray-craters on the moon are the counterparts of the meteorite
craters on the earth. This fact is shown not only by their random
distribution, but by the long, bright rays which gave them their name.
On the earth, rays of similar appearance, composed of thrown-out
material, are one of the most characteristic features of explosion
craters, whether the cause of the explosion is the high-speed impact of
a great meteorite or the detonation of a charge of high explosive
(either conventional or nuclear).
The hypothesis that meteorite craters do exist on the moon is therefore
justified even though it applies to far fewer craters than its
supporters believe.
As for the ordinary, non-ray lunar craters, these features are not at
all volcanic craters in the usual sense. One of the few good things to
come out of World War II was the first satisfactory explanation of the
“run-of-the-mill” craters on the moon. Jeremi Wasiutynski, a brilliant
Polish scientist forced to take refuge in Norway, sought to explain
these craters as originating in _convection_ processes.
While the term “convection” may not be familiar, the role convection
plays in filling the sky with beautiful clouds on a hot summer’s day is
well known. Such cloud formation results from convection in the gaseous
free atmosphere. Much more remarkable and regular are the results of
_controlled_ convection in layers of _liquids_ rather than gases.
Laboratory investigation of the effects produced by convection processes
in heated liquids formed the basis for Wasiutynski’s new theory.
According to this theory, convection processes in the only partially
solidified outer shell of the youthful moon could have given rise to
great numbers of surface features having the size, shape, and
distribution of the common lunar craters. In far more satisfactory
fashion than any other theory so far proposed, the convection-current
hypothesis of Wasiutynski explains the many and distinctive
characteristics of the non-ray craters on the moon.