Organisateurs : Daniela Lazzaro, Marcello Fulchignoni

Until quite recently, only (4) Vesta [1], its family members and small asteroids in its neighborhood [2], were reckoned to have a basaltic composition. This led to what can be called a classical scenario in which only (4) Vesta achieved to preserve an almost intact basaltic crust, while all the other were disrupted and dispersed over the time. Alternatively, (4) Vesta might have been the unique asteroid able to completely differentiate, probably due to its large size. The discovery of the basaltic nature of the small asteroid (1459) Magnya [3] in the exterior region of the Main Belt can, thus, be considered as a breakthrough in these scenarios. Further strengthen by the discovery of an increasingly large number of small asteroids with similar composition all around the Main Belt [4], [5], [6]. All these objects, classified as V-type in the diverse taxonomies, have a surface composition similar to that of the Howardites, Eucrites and Diogenites (HED) meteorites. A common origin for all these objects has been object of diverse studies in the last years.
The observational data on a large sample of V-type small asteroids indicate a variety of basaltic compositions which make it difficult to link them to a unique body. Moreover, while the Dawn preliminary spectroscopic analysis indicate a predominance of Eucrite-like composition, most of the observed V-type asteroids show an Howardite- and Diogenite-like composition. Last but not least, dynamical considerations makes it almost impossible for diverse V-type asteroids in the sample to have a common origin. It seems that only distinct origins can better fit all the observational and dynamical results, although a large fraction of the inner main belt V-type most probably were formed by one or more cratering events on (4) Vesta. The presence of isolated V-type asteroids, not linked to any dynamical family, or with an highly improbable linkage, raise the hypothesis of the formation of small basaltic asteroids, without the necessity for these to be fragments of larger differentiated objects. The differentiation of small bodies would have occurred in the early stages of the Solar nebula, when the 26Al was abundant and in accordance with recent works [10] which seem to indicate that the HED parent body formed before the H chondrite parent body.
Other important issues related to de differentiation process that took place in the asteroid belt are : the lack of "differentiated" families, i.e., with material coming from the distinct layers of the parent body ; the existence of isolated, not related to a dynamical family, M-type asteroids ; very few Olivine-rich asteroids although the classical model predicts that a differentiated body has a very large olivine-rich mantle ; detailed study of iron meteorites indicate their provenience from more than a dozen distinct bodies ; no Dunites in our meteorite collections.

Publications
[1] McCoord et al. (1970) Science 168, 1445.
[2] Binzel R.P. and Xu S. (1993) Science 260, 186.
[3] Lazzaro, D. et al. (2000) Science, 288, 2030.
[4] Florczak et al. (2002) Icarus 159, 178.
[5] Binzel R.P. et al. (2006) DPS, Abstract #71.06.
[6] Hammergren et al. (2006) astro-ph/0609420.
[7] Russel et al. (2012) Science 336, 684.
[8] De Sanctis et al. (2012) Science 336, 697.
[9] Marchi et al. (2012) Science 336, 690.
[10] Scott (2006) Icarus 185, 72.