Meteorites 269
3. Meteorites of Asteroidal Origin
and their Parent Bodies
3.1 The Meteorite–Asteroid Connection
Two links have already been noted that suggest or imply an
asteroidal origin for most meteorites. These are:
1.Photographically determined orbits for seven ordi-
nary chondrites, one unique carbonaceous one and
an EL6 (Fig. 3).
2.Mineralogic evidence indicating origin of most me-
teorites in asteroidal-sized objects. (Some chondrites
could come from much smaller primary objects.) This
evidence includes iron meteorite cooling rates (im-
plying formation depths of asteroidal dimensions),
the presence of minerals (e.g., tridymite), and phase
relations (e.g., the Widmanst ̈atten pattern) indicative
of low-pressure (1 GPa) origin, and the absence of
any mineral indicating highlithostatic(generated by
the rocky overburden)—rather thanshock-pressures.Another property linking meteorites and certain aster-
oidal types, spectral reflectance, is a research area of strong
current interest. The reflectivity (albedo)-wavelength vari-
ation for an asteroid, involving white (solar) incident light,
can characterize its mineralogy and mineral chemistry
somewhat [seeMain-BeltAsteroids]. To uncover pos-
sible links, asteroidal spectral reflectance can be compared
with possible meteoritic candidates, both as-recovered or
treated in the laboratory to simulate effects of extraterres-
trial processes (Fig. 13).
The best matches exist between the HED association
and rare V-class asteroids (4 Vesta and its smaller progeny),
iron meteorites and numerous M-class asteroids; CI and
CM chondrites thermally metamorphosed at temperatures
up to 900◦C with the very numerous C-class and appar-
ently related B-, F-, and G-class asteroids; aubrites with the
somewhat unusual E-class asteroids; pallasites with a few of
the very abundant, and diverse S-class—which constitute a
plurality of all classified asteroids—and/or rare A-type as-
teroids; and ordinary chondrites with the very rare Q-type
asteroids, which are near-Earth asteroids, or 6 Hebe, an
inner Belt object belonging to the S(IV) subclass of S aster-
oids. A typical good news/bad news situation results. The
good news is that specific meteorite types are similar to (de-
rive from) surface regions of identifiable asteroid types. The
bad news is that relative frequencies with which meteorites
of a given type and asteroids of a supposedly similar type are
encountered do not agree. Specifically, there is the ordinary
chondrite-S asteroid paradox (cf. Table 1): Why are there
so few asteroidal candidates for the very numerous ordi-
nary chondrites and so few olivine-dominated stony-irons
from the very numerous S asteroids? One obvious answer is
that “space weathering” (energetic dust impingement on a
meteoroid surface causing metal reduction and dispersion)
FIGURE 13 Spectral reflectances of the Coopertown IIIE
coarse octahedrite, Juvinas eucrite and V-class asteroid 4 Vesta,
and Vigarano C3V chondrite and G-class asteroid 1 Ceres. The
albedo scale for all but Coopertown is on the left: Coopertown’s
is at right. Solid and dashed lines delineate meteorite and
asteroid spectra, respectively. (Courtesy of Dr. Lucy-Ann
McFadden, University of Maryland.)could mask ordinary chondrite-like interiors. Another, is
that Earth collects a biased meteorite sampling compared
with the asteroid population, in either near-Earth space or
in the Asteroid Belt. This may also account for the near
absence of meteorites from the numerous D or P aster-
oids, located at>3 AU from the Sun. Alternatively, ejecta
from such asteroids might not survive atmospheric passage
because D and P surface materials are inferred to be very
organic-rich and, presumably, very friable. Tagish Lake, the
only meteorite that spectrally resembles a D-class asteroid,
contains organic globules and is extremely friable.3.2 Sampling Bias
The contemporary flux of meteorites is biased and unrepre-
sentative of the meteoroid population in near-Earth space,
let alone in the Asteroid Belt, so generalizations about par-
ent body formation and evolution from studies of mete-
orites falling today may be incomplete. The contemporary
flux of meteorites includes not only observed falls during
mainly the past 200 years, but also by non-desert-cluster
finds (i.e., omitting the many from hot and cold deserts).
Most finds contain metallic iron, which should be readily
oxidized on Earth, but they are surprisingly resistant to de-
structive oxidation, even in temperate climates. The smaller
iron–nickel grains of chondrite finds are more readily oxi-
dized.