362 Encyclopedia of the Solar System
shows a major transition in asteroid mineralogy to less free
metal, more oxidized silicates, important low-temperature
carbon minerals, and significant amounts of volatiles such
as water. The P asteroids peak at about 4 AU, and the D
asteroids, which peak at 5.2 AU, are probably richer in low-
temperature materials such as carbon compounds, complex
organics, clays, water, and volatiles and represent the tran-
sition between the rocky asteroids of the main belt and
the volatile-rich comets in the Kuiper Belt and the Oort
cloud.
Several processes have blurred the taxonomic imprint
from the original condensation. Apparently, a thermal event
heated much of the Asteroid Belt soon after accretion. Ev-
idence from meteorites shows that some parent asteroids
were completely melted (basaltic achondrites, irons, stony
irons), some asteroids were strongly metamorphosed (or-
dinary chondrites), and some were heated only enough to
boil off volatiles and produce aqueous alteration (CI and
CM carbonaceous chondrites). This event seems to have
been much more intense in the inner Asteroid Belt and
strongly affected the E-, S-, A-, R-, V-, and M-class aster-
oids. The dynamical interaction of asteroids with each other
and the planets, particularly Jupiter, has altered and blurred
the original orbital distribution of the asteroids and cleared
whole sections of the belt. The net result probably has been
to expand the original compositional zones and produce or-
bital overlaps of zones that once may have been distinct
from each other.
4. Puzzles and Promise
4.1 Telescopic Searches and Exploration
It is a rare but exciting event in science when a single idea
by a small group of scientists ignites an entirely new field
of study and redefines the scientific debate. That is exactly
what happened to such diverse fields as impact physics,
asteroid observations, and paleontology after Alvarez and
colleagues hypothesized that the iridium anomaly found
in Cretaceous–Tertiary (K/T) boundary sediments was the
mark of an impact event that destroyed the dinosaurs. [See
Planetary Impacts.]
Asteroid impacts are a consistent and steady-state fact
in the solar system. One just has to look at the extensively
cratered surface of any solid body to realize that impacts
happen. To some extent, the fact that the Earth has active
geological processes that erase the scars of impact craters
rapidly and a thick atmosphere that filters out the smaller
impactors has lulled us into a false sense of security.
The real question is not whether asteroids hit the Earth,
but rather how often does it happen. Before they hit, these
impactors are comets and asteroids with the same power
law distribution of sizes that we see in the Asteroid Belt,
so small impacts will be more frequent and large “species-
killing” impacts will be much rarer. However, as those who
live near dormant volcanoes should realize, rare events on
human timescales can be common and frequent events on
geologic timescales.
There is plenty of evidence in the geologic and fossil
record for repeated major impacts, some of which are as-
sociated with mass extinctions. For instance, there were 5
mass extinctions during the last 600 million years, about
what would be predicted by a purely impact-driven extinc-
tion model. The bottom line is that asteroid impacts should
be treated as one of the steady-state processes that results
from a dynamic solar system. Although the chances of a cra-
tering event like the one that dug the almost 1 mile diam-
eter Meteor Crater in Arizona happening on any random
day are small, the probability is 100% that it will happen
sometime. The only question is when? When faced with
predictable dangers, it is sensible to take precautions. In the
same way that people who live on the Gulf coast of North
America track hurricanes and people who live in tornado-
prone Oklahoma build houses with cellars, it seems a rea-
sonable precaution to identify, track, and study the asteroids
in near-Earth space. [SeeNear-Earth Objects.]4.2 Origins of Asteroids
As pieces of a planet that was never formed, the asteroids
represent important chemical and physical clues about the
origin of the planets. But these clues can only be interpreted
by having a reliable theory for how the asteroids themselves
were formed. The key questions to be addressed for the
asteroids include: How much material was originally in the
region of the solar system where the asteroids were formed?
What interrupted the formation of a planet here? Where did
all the missing material go? What processes shaped both the
structure of the individual asteroids and the characteristics
of the Asteroid Belt as a whole?
We do have a reasonably complete census of asteroids in
the main belt, down to a size of a few kilometers, and from
that we can infer how the perturbing gravity of Jupiter and
Saturn has shaped the distribution of asteroids today. We
know some asteroids come in distinct spectral classes, and
that there is a tendency for S-type asteroids to be found in
the inner belt and C-types to be found in the outer belt. But
while we can infer compositions for those types, based on
the meteorite sample, we recognize that those inferences
are very uncertain, and that there could well be material
in the Asteroid Belt that is not sampled in our meteorite
collections. Still, with the data in hand, we can sketch out
a testable scenario for the formation and evolution of the
Asteroid Belt, knowing that this is not a final answer but
rather a best-guess, which we will continue to test and refine
as we learn more about the asteroids.