The chemical structure of the asteroid belt

The asteroid main belt between Mars and Jupiter holds evidences fromthe early Solar System history. Built from the originally thin andstratified accretion disk, it has been scrambled by planetary migrations,resulting in a dynamical excitement and radial mixing of compositions.Since the 1970s,spectral surveys have characterized the surface compositions of thelargest members first, then of smaller bodies, slowly progressing throughthe size distribution. These surveys led to majordiscoveries, including the succession of dominating compositional classesalong heliocentric distances, and the presence of interlopersin this over-arching structure.In the 2000s, these results havesustained the emergence of the current paradigm of Solar Systemformation in which planets migrated from theirformation locations to their current orbits (the Nice model).Since then, the Sloan Digital Sky Surveyand the NASA WISE mission have observed over a hundred thousands ofasteroids in the visible and mid-infrared,characterizing their surface composition and providing their diameter. Simultaneously, our knowledge on asteroid densitypresented a tenfold increase. This dataset allowed to push the description of asteroidcompositions to smaller sizes and to study the distribution of material in the main belt by mass, rather than bynumbers.The resulting picture go back over the previous view,and the few interlopers seem to be rule. The largescale structure discovered in the 1980s seen on the largest bodiesholds, but mixing increases at smaller sizes. This detailed picture supports the main results fromrecent dynamical models of planetary migration and radial mixing ofsmaller bodies, albeit several observed structures remain yet to beexplained: numerous primitive D-type in the inner belt, apparentlymissing mantle counterpart (A-types) to the crustal and iron core-like(V- and M-types) material. In this talk, observational evidences from past decadewill be reviewed, current picture of the compositional distribution ofmaterial in the main belt presented; open questions, inherited frompast spectral surveys, summarized; and prospectives drawn.