692 Encyclopedia of the Solar System
FIGURE 18 Geometric albedo versus heliocentric distance of
known asteroids detected by IRAS. The high albedo asteroids
are located almost exclusively in the inner portion of the main
asteroid belt.
Trojan populations had a very different collisional history,
or that their origins are different.
Albedo provides insight into composition. Meteorite
studies show that very dark surfaces arise from largely car-
bonaceous materials, while high albedo surfaces are as-
sociated with silicic compositions lacking such carbona-
ceous material.IRASconfirmed that most C-type asteroids
(thought to be carbonaceous) are indeed dark compared to
the “stony” S-type asteroids, and that there is a trend toward
darker asteroid surfaces with increasing heliocentric dis-
tance (Fig. 18). This is consistent with the view that there is
not only a residual primordial composition gradient through
the Asteroid Belt, but that inner belt asteroids (predomi-
nantly S-type) were significantly processed by heating in the
early solar system, while the outermost asteroids have expe-
rienced little heating and have retained a more “primitive”
mineralogy.
New observations often result in as many new questions
as new answers, and theIRASasteroid observations are no
exception. Prior toIRAS, ground-based thermal observa-
tions had been preferentially made of the largest asteroids.
It was noticed that there was a bimodal distribution in the
inferred albedos, which was consistent with the main-belt
asteroid population being dominated by dark C-type and
bright S-type asteroids.IRASadded large numbers of ob-
servations of smaller asteroids and it was found that they had
an albedo distribution quite different from the larger aster-
oids. Small asteroids have a unimodal distribution spanning
the total range of albedos inferred for the large asteroids
(Fig. 19). Since the small asteroids are fragments of larger
asteroids, this might imply that surface minerologies are
not representative of interior minerologies or that signifi-
cant “space weathering” may affect the surface spectra of
the larger bodies.
There has been some question as to whether all asteroid
families mark the site of a past catastrophic disruption or
whether in some cases asteroids might be clumped together
FIGURE 19 Large and small asteroids evidence different
albedo distributions.due to dynamical forces such as gravitational perturbations
on their orbits by Jupiter. Albedo distributions can also pro-
vide clues to the origin of some asteroid families. Assuming
the parent to have been compositionally homogeneous, the
fragments should exhibit similar spectral properties. On the
other hand, members of purely dynamical clusters would
not be expected to have similar compositions.IRASscanned
enough of the members of the largest families to show that
family members had albedos more similar to each other
than to the background asteroids nearby, giving support, in
those cases, to the asteroid breakup hypothesis (Fig. 20).
Thermal spectra can provide information including
thermal inertia and composition.Spitzerobservations of
Jupiter Trojan asteroids have yielded results suggesting
fine-grained silicates in a relatively low-density, perhaps
“fairy-castle” matrix. Does this indicate a possible cometary
origin? Evidence for complex carbon compounds on prim-
itive asteroids has also been detected. ASpitzersurvey of
M-class asteroids, spectrally similar at visible wavelengths,
indicates major differences in thermal inertia among them,
which could indicate significant differences in surface den-
sities (due to relative age and collisional processing) or
conductivity (some M-types are metallic, some may be
stony). Thermal infrared observations of asteroids from
space-based facilities are revealing an increasingly diverse
population of objects.FIGURE 20 A comparison of the albedo histograms of the Eos
asteroid family and non-family members near the same location
in the asteroid belt supports the hypothesis that the family
members derived from a single parent body.