The chemical structure of the Main-Belt

The asteroid main belt between Mars and Jupiter holds evidences from the early Solar System history. The original chemical stratification of the accretion disk has been scrambled by planetary migrations, resulting in a radial mixing of compositions. Since the 1970s, spectral surveys have characterized the surface compositions of the largest members first, then of smaller bodies, slowly tapering into the size-frequency distribution. These surveys led to major discoveries, including the succession of dominating taxonomic classes along heliocentric distances, stained by the presence of interlopers in this over-arching structure. In the 2000s, these results have sustained the emergence of the current paradigm of Solar System formation: the Nice model, in which planets migrated from their formation locations to their current orbits.Since then, all-sky surveys in the visible and mid-infrared, the Sloan Digital Sky Survey and NASA WISE mission, have observed tens of thousands of asteroids, allowing characterization of their surface composition and estimation of their diameter. Simultaneously, our knowledge on asteroid density greatly improved: the sample of density determinations presented a tenfold increase. Such a rich dataset opened the possibility to scrutinize asteroid compositions to smaller sizes and to study the distribution of material in the main belt by mass, rather than by numbers. The picture resulting from these data go back over the previous view, and the few interlopers seem to be rule. The large scale structure seen on the largest bodies holds, but mixing increases at smaller sizes. This detailed picture supports the main results from recent dynamical models of planetary migration and radial mixing of smaller bodies, albeit several observed structures remain yet to be explained: numerous primitive D-type in the inner belt, apparently missing mantle counterpart (A-types) to the crustal and iron core-like (V- and M-types) material.Observational evidences from past decade will be reviewed, current picture of the compositional distribution of material in the main belt presented; open questions, inherited from past spectral surveys, summarized; and prospectives drawn.