Meteorites 255
FIGURE 4 Concentrations of cosmic ray–produced radioactive
and stable nuclides during cosmic ray exposure and after the
meteorite’s fall to Earth.
disruption in space—but this is very controversial. Evidence
from temperature-sensitive components indicates that, in
their orbits about the Sun, some meteorites have perihelia
within 0.5 AU resulting in detectable solar heating.
Some meteorites contain regolithic material bombarded
by very energetic particles. Once material is ejected from its
parent body by an impact until it falls on Earth, meter-sized
meteoroids are irradiated by cosmic rays (mainly protons)
of solar or galactic origin. Solar cosmic rays have a power–
law energy distribution with the particle flux increasing
rapidly with decreasing energy: most solar particles have
energies<1 MeV. Galactic and some solar particles have
energies of hundreds of MeV to GeV and can induce nu-
clear reactions producing cosmogenic radioactive or stable
nuclides. In larger meteoroids, cosmogenic nuclear reac-
tions occur only in the meter-thick shell that cosmic ray
primaries and secondaries penetrate. As discussed in Sec-
tions 6.1 and 6.2, levels of nuclides produced during cosmic
ray exposure (CRE) establish the duration of energetic par-
ticle bombardment (the CRE age) and the time spent by a
meteoritic find on Earth, the terrestrial age (Fig. 4).
1.3 Impact on Earth
If a meteoroid is small enough to be decelerated sig-
nificantly during atmospheric passage, it may land as an
individual or as a shower. A recovered individual can have
a mass of≤1 g [as in the 1965 fall of the Revelstoke stone
(CI1) in British Columbia], or up to 60 metric tons (e.g., the
Hoba IVB iron meteorite found in 1920 on a Namibian farm
where it remains). A meteorite shower results from a mete-
oroid fragmenting high in the atmosphere, usually leaving
a particle trail down to dust size. Shower fragments striking
the Earth define an ellipse whose long axis—perhaps ex-
tending for tens of kilometers—is a projection of the origi-
nal trajectory. Typically, the most massive fragments travel
farthest and fall at the farthest end of the ellipse.
Some falls are signaled by both light and sound dis-
plays; others, like the Peekskill meteorite (Fig. 5), exhibit a
spectacular fireball trail observed over many states. Small
falls, like Noblesville (Fig. 1a), fall silently and unspectacu-
larly and, when recovered immediately, have cold to slightly
warm surfaces. Meteorites can fall anywhere at any time.
The 500-g Borodino stone (H5) fell on 5 September 1812—
two days before the famous battle there—and was recov-
ered by a Russian sentry.
U.S. Department of Defense data demonstrate that, at
least since 1975, reconnaissance satellites have detected
large explosions at random locations in the Earth’s atmo-
sphere. On average, about 9 of these mysterious explosions
[with energies up to 1 megaton (Mt) equivalent of TNT]
occur annually: no meteorite falls or fireballs are associated
with any of these events.
Large meteoroids—tens of meters or larger—are not
decelerated much by atmospheric transit and, with an
appropriate trajectory, may ricochet off the Earth’s at-
mosphere (Fig. 5a) or strike it at full geocentric veloc-
ity,>11 km/s. (Obviously, distinguishing a large meteoroid
from a small asteroid is arbitrary.) Such explosive, crater-
forming impacts can do considerable damage. The 1-km-
diameter Meteor Crater (Fig. 5b) in northern Arizona,
which formed 50,000 years (i.e., 50 ka) ago by the impact
of a 25- to 86-m meteoroid, yielded fragments now surviv-
ing as Canyon Diablo iron meteorites. At least 40 terrestrial
craters exhibit features believed to be produced only by
intensive explosive impact of a large meteoroid (e.g., as in
the 1908 event at Tunguska in Siberia) or perhaps even a
comet nucleus. Another 269 features on Earth may be of
impact origin. One expert classed 130 of them as definite
impact craters. The 180-km-diameter Chicxulub feature in
Yucatan, Mexico, is suspected as the impact site of a 10-km
meteoroid. By consensus, this impact generated the climatic
consequences responsible for the extinction of∼60% of
then-known species of biota—including dinosaurs—ending
the Cretaceous period and beginning the Tertiary, 65 Ma
ago (the K-T event). Other, less well-established events are
suggested as having caused extinctions at other times.
Some meteorites have struck man-made objects. The
Peekskill stone meteorite (H6), with a recovered mass of
12.4 kg, ended its journey on the trunk of a car (Fig. 5d).
Its descent in 1992 was videotaped over a five-state area of
the eastern United States by many at Friday evening high
school football games (Fig. 5c), yielding a well-determined
orbit. Two authenticated reports of humans hit by meteorite
falls exist. The first involved a 3.9 kg (H4) stone (the larger