Constraints on planetesimal formation from asteroid compositions

International audienceOrdinary chondrite meteorites (OCs) are by far the most abundant meteorites (80% of all falls). Their study along with that of other chondrite classes has provided numerous constraints on the formation and early evolution of the solar system, including a) the migration processes that occurred in the protoplanetary disk prior to primary accretion (i.e. planetesimal formation) and their associated timescales, b) the post- (and syn-) accretional heating events, and c) the collisional events that occurred since 4.6 Gyrs. Although petrologic, chemical and isotopic studies of OCs and meteorites in general have largely helped establish a chronology of the earliest events of planetesimal formation, there are several questions that cannot be resolved via laboratory measurements and/or experiments only. These include the formation location of the different classes of ordinary chondrites (and meteorites in general); the initial average size of their parent bodies; the amplitude of the bias in our collections with respect to the compositional distribution of OC-like material in the Asteroid Belt; the number of parent bodies for a given meteorite class (it is typically proposed that each meteorite class has only one parent body); the level of radial mixing experienced by parent bodies after their formation; and their accretion timescale. To investigate answers to these questions, we conducted an extensive spectroscopic survey of 83 main belt S-type asteroids and 5 S-type families as it was recently established unambiguously that these asteroids encompass the parent bodies of OCs. In parallel, we also obtained for the first time spectral measurements for a representative number (53) of unequilibrated ordinary chondrites (UOCs) as those were lacking in current databases (e.g. RELAB; http://www.planetary.brown.edu/relab/). We will present evidence for establishing several new constraints on the planetesimal formation process from our broadened spectral survey