Geoscience Reference
In-Depth Information
spectrum of Vesta turns out to be very similar to achondrite meteorites
having an overall basaltic composition (HED meteorites). Only after the
reliable identification of a dynamical family associated to Vesta,
3
it was
shown that the other members of the family share the same reflectance
properties of Vesta.
4
More recently, a number of
V
-type objects have been
discovered also among the population of near-Earth asteroids. As a conse-
quence, it is now widely believed that the Vesta family was produced by a
violent impact on Vesta's surface, which produced a very large crater and
excavated a substantial amount of material, that escaped the gravitational
well of Vesta and produced a swarm of fragments that we see today as fam-
ily members. The presence of an hemispheric-sized crater on Vesta's surface
had been first suggested by polarimetric observations,
5
and had been also
confirmed by an extensive analysis of available lightcurves of the asteroid.
6
The crater has been more recently directly seen by means of HST images.
7
It is also believed that Vesta is the origin of a number of other small objects
which have been found to belong to the
V
taxonomic class, and are located
in the space of orbital elements in the region between Vesta and the loca-
tions of the 3/1 mean-motion resonance with Jupiter, and of the so-called
ν
6
secular resonance. These resonances are known to be ecient transfer
pathways to the region of the inner planets. Thus, it is believed that frag-
ments of Vesta can reach these resonances under the effect of dynamical
evolution likely dominated by the Yarkovsky effect.
8
It is now commonly
accepted that
V
-type near-Earth asteroids and HED meteorites originate
from Vesta, although some HED meteorites exhibit a variety of spectral
features that are not immediately understood in terms of a common origin
from a unique parent body.
9
What we call the “Ceres-Vesta” paradox, is the fact that, if we accept
the (1)-(3) assumptions quoted above, it is very dicult to explain why
and how the properties of Ceres and Vesta can be as they appear to be
from observations. In particular, what is intriguing is that, among these
two bodies, the bigger one seems to be primitive, whereas the smaller one
is fully differentiated. If we accept assumption (2), Ceres, being twice as
large as Vesta and having a similar composition, should have contained
in its interior an amount of radiogenic nuclei more than sucient to com-
pletely melt it. But in this case we should expect also for Ceres a surface of
basaltic composition. One could explain this apparent paradox by assum-
ing that the growth of Ceres took place later with respect to Vesta, and in
particular when a substantial fraction of the short-lived radiogenic nuclei
had already decayed. This possible explanation, however, is in conflict with
