Main-Belt Asteroids 361
60 different parent bodies indicating a wide variety of dif-
ferentiated bodies in the Asteroid Belt. However, some M-
class asteroids have been shown to have hydrated minerals
on their surfaces. The spectral characteristics of M aster-
oids can also be characteristics of some clay-rich silicates,
and this raises the possibility that the “wet” M asteroids are
assemblages of clays, like the CI carbonaceous chondrites,
but without the carbon-rich opaques that darken the CIs.
The W (or “wet”) class was coined to classify these unusual
objects.
The E-class asteroids are another example of the perils
of extrapolation from limited information to a convenient
meteorite analog. Looking at the spectrum of the “type”
asteroid for the E-class, 44 Nysa, it was easy to assume
that these asteroids were excellent analogs for the enstatite
achondrites. The only problem was that enstatite mete-
orites are entirely anhydrous, and 44 Nysa was observed
to be strongly hydrated. Although some E-class asteroids
are probably composed of the same differentiated enstatite
assemblages as the enstatite achondrites, about half of the
observed Es are hydrated and cannot be composed of an-
hydrous enstatite. The “wet” E asteroids like Nysa may be
related to the W asteroids and have surfaces rich in hydrated
silicate clays.
Perhaps the most complex class of asteroids is the very
large S class. S-class spectra, on average, indicate varying
amounts of olivine and pyroxene with a substantial metallic
component, but the mineralogy of these asteroids varies
from almost pure olivine to almost pure pyroxene, to a
variety of mixtures of these two end-members. With this
wide range of mineralogies comes a wide range of mete-
orite analogs and possible formation scenarios. The S class
probably represents a range of asteroid material from core–
mantle boundary, the mantle, and the lower crust of differ-
entiated asteroids and includes undifferentiated but meta-
morphosed asteroids that are the parent bodies of ordinary
chondrite meteorites. Ordinary chondrites are by far the
largest meteorite type, accounting for approximately 80%
of observed meteorite falls, but so far only a few small aster-
oids have been identified as Q class, direct analogs for ordi-
nary chondrites. A number of S-class asteroids have spectral
absorption bands roughly similar to those of ordinary chon-
drites, but S asteroids typically have a moderate spectral
red slope that is not seen in ordinary chondrites. However,
it has been shown in laboratory experiments that ordinary
chondrite material can redden in response to “space weath-
ering” by micrometeorite bombardment. The small ordi-
nary chondrite parent bodies are probably relatively young
fragments that have not had enough time to redden their
surfaces. The larger ordinary chondrite parents have older,
reddened surfaces and are members of the S class.
In general, the differentiated asteroids of the V, A, R, S,
and M classes may represent examples of a geologic tran-
sect from the crust to the core of differentiated asteroids
FIGURE 9 The distribution of taxonomic classes from Bell et al.
(1989). Reproduced courtesy of Bell, Davis, Hartmann, and
Gaffey, 1989, in “Asteroids II” (R. P. Binzel, T. Gehrels, and M.
S. Matthews, eds.), Univ. Arizona Press, Tucson, p. 925.and can tell us a great deal about the geochemical evolu-
tion of a differentiated body. In this scenario, the V-class
asteroids would be the surface and crustal material. The A
asteroids would be from a completely differentiated man-
tle, while the R asteroids would represent a mantle that
experienced only partial differentiation. Some S asteroids,
particularly the olivine-rich members, would be either ma-
terial from some region in the mantle or the core–mantle
boundary. And finally, M-class materials represent samples
of the metallic cores of these asteroids. From the preceding
discussion, it is clear that the asteroid classes were not uni-
formly distributed throughout the Asteroid Belt. The S class
dominates the inner asteroid belt, while the C class is far
more abundant in the outer Asteroid Belt. The most popu-
lous taxonomic classes (the E, S, C, P, and D classes) peak
in abundance at different heliocentric distances. Shown
in Fig. 9 is the distribution of taxonomic classes. If we
assume that the spectral and albedo differences between
the asteroid classes reflect real differences in mineralogy,
then we are seeing rough compositional zones in the As-
teroid Belt. According to models of solar system conden-
sation, the high-to-moderate-temperature silicate minerals
would tend to dominate the inner solar system, while lower-
temperature carbonaceous minerals would be common in
the cooler, outer regions of the solar system. The transition
between moderate- and low-temperature nebular conden-
sates is apparently what we are seeing in the taxonomic
zonation of the Asteroid Belt. The innermost major group
of asteroids, peaking at 2 AU, is the E class, which is rich
in iron-free silicate enstatite, indicating formation under
high-temperature, relatively reducing conditions. The next
group out is the S class, thought to be rich in the moderate-
temperature silicates olivine and pyroxene and to have large
amounts of free iron–nickel, which indicate more oxidizing
conditions. The C class peaks in abundance at 3 AU and