Geoscience Reference
In-Depth Information
body of theoretical investigations and numerical simulations published in
the literature. Now, look at the real bodies that are today present in the
asteroid main belt, and in particular focus on the two “giant” asteroid
(1) Ceres and (4) Vesta. Having a diameter of about 1,000 km (Ceres) and
500 km (Vesta), these two asteroids are indeed giant with respect to the
rest of the asteroid population, apart from a few bodies like (2) Pallas
or (10) Hygiea. In fact, asteroids like Ceres and Vesta are fundamentally
different with respect to normally-sized asteroids in several respects, and
are in some sense real “planets”. A fundamental property which is shared by
both Ceres and Vesta is that they have sizes (and consequently masses) that
make them practically indestructible by the process of collisional evolution
which steadily grinds the rest of the asteroid population into smaller pieces.
Not surprisingly, the overall shapes of Ceres and Vesta are mostly oblate
spheroids (with some more pronounced triaxiality in the case of Vesta),
compatible with the fact that self-gravitation starts to be dominant for
objects of this size.
If we consider the current observational evidence about Ceres and Vesta,
however, we easily realize that we could hardly imagine two asteroids more
different with respect to each other. Ceres belongs to the
G
taxonomic class,
which is related to the wider
C
complex, that dominates the asteroid pop-
ulation, specially in the outer regions of the belt. Its reflectance spectrum
is mostly flat and featureless, and is diagnostic of a primitive composi-
tion, likely similar to that of carbonaceous chondrites. Some observations
have suggested the presence of hydrated minerals on its surface, compati-
ble with an overall primitive composition.
2
Bodies like Ceres are generally
interpreted in terms of a mineralogical composition that has never been
affected by substantial heating episodes. Ceres should be therefore strongly
reminiscent of the overall composition of the planetesimals located in the
region where it was grown.
In contrast, Vesta is a classical example of an asteroid whose reflectance
properties are clearly diagnostic of an overall basaltic crust. As a con-
sequence, Vesta is thought to be a fully differentiated body, including a
metallic core, a mantle and a lighter crust. Bodies like this must have been
produced by early planetary growth at these heliocentric distances, to jus-
tify the existence of iron meteorites and asteroids having a mostly metallic
composition (at least a fraction of today's
M
-type asteroids), interpreted
as exposed cores of differentiated parent bodies disrupted by collisions and
consistent with meteoritic evidence. The taxonomic classification of Vesta,
the so-called
V
-type, has long been unique among asteroids. The reflectance
