Encyclopedia of the Solar System 2nd ed

(Marvins-Underground-K-12) #1
360 Encyclopedia of the Solar System

TABLE 2 Meteorite Parent Bodies

Asteroid Class Inferred Major Surface Minerals Meteorite Analogs

D Organics+anhydrous silicates? (+ice??) None (cosmic dust?)
P Anhydrous silicates+organics? (+ice??) None (cosmic dust?)
C (dry) Olivine, pyroxene, carbon (+ice??) “CM3” chondrites, gas-rich/blk chondrites?
K Olivine, orthopyroxene, opaques CV3, CO3 chondrites
Q Olivine, pyroxene, metal H, L, LL chondrites
C (wet) Clays, carbon, organics CI1, CM2 chondrites
B Clays, carbon, organics None (highly altered CI1, CM2??)
G Clays, carbon, organics None (highly altered CI1, CM2??)
F Clays, opaques, organics None (altered CI1, CM2??)
W Clays, salts???? None (opaque-poor CI1, CM2??)
V Pyroxene, fledspar Basaltic achondrites
R Olivine, pyroxene None (olivine-rich achondrites?)
A Olivine Brachinites, pallasites
M Metal, enstatite Irons (+EH, EL chondrites?)
T Troilite? Troilite-rich irons (Mundrabilla)?
E Mg-pyroxene Enstatite achondrites
S Olivine, pyroxene, metal Stony irons, IAB irons, lodranites, winonites,
siderophyres, ureilites, H, L, LL chondrites

zones of the main belt and working inward toward the Sun.
The outer asteroid belt is dominated by the low-albedo P
and D classes. The analogs most commonly cited are cos-
mic dust or CI carbonaceous chondrites that are enriched
in organics like the Tagish Lake meteorite. However, the
spectral characteristics of these asteroids are difficult to
duplicate with material that is delivered to the inner solar
system. Probably P and D asteroids are composed of primi-
tive materials that have experienced different geochemical
evolution than cosmic dust or CI chondrites. Their spectra
indicate increasing amounts of complex organic molecules
with increasing distance from the Sun. These objects are
also probably very rich in volatiles including water ice.
Dark inner asteroid belt asteroids include the B, C, F,
and G classes whose meteorite analogs are the dark CI and
CM carbonaceous chondrite meteorites. The spectral dif-
ferences between these classes are thought to represent
varying histories of aqueous alteration or thermal metamor-
phism. The CI carbonaceous chondrites, rich in water, clay
minerals, volatiles, and carbon, represent primitive mate-
rial that has been mildly heated and altered by the action
of water. [SeeMeteorites.]
Sunward of 3 AU, differentiated bright asteroids become
much more common. This zone was strongly affected by the
early solar system heating event and contains those classes
most likely to represent differentiated and metamorphosed
meteorites. Perhaps the best asteroid/meteorite spectral
matches are the V-class asteroids with the basaltic achon-
drite meteorites. V-types are interpreted to be a differen-


tiated assemblage of primarily orthopyroxene with vary-
ing amounts of plagioclase, which makes them very close
analogs to the basaltic howardite–eucrite–diogenite (HED)
association of meteorites. These meteorites are basaltic par-
tial melts, essentially surface lava flows and near-surface in-
trusions originating on asteroids that underwent extensive
heating, melting, and differentiation.
While the V-class asteroids represent the surface and
near-surface lava flows of a differentiated asteroid, the
A-class asteroids are thought to represent the next zone
deeper. These asteroids are interpreted to be nearly pure
olivine and may be derived from the mantle of extensively
differentiated parent bodies. The Earth’s mantle is domi-
nated by olivine and theoretical studies show that differ-
entiation of asteroids with a bulk composition similar to
ordinary chondrite meteorites should produce olivine-rich
mantles. Another possible mantle-derived asteroid is the
R class, which is a single-member class made up of the as-
teroid 349 Dembowska. Analysis of its reflectance spectra
suggests a mineralogy that contains both olivine and py-
roxene and may be a partial melt residue of incomplete
differentiation.
A more common asteroid class is the M class, which
has the spectral characteristics of almost pure iron-nickel
metal and several show high radar reflections consistent
with metal. These objects are thought to be direct analogs
to the metallic meteorites and may represent the cores of
differentiated asteroids. Isotopic and chemical studies in-
dicate that iron meteorites could come from as many as
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