Early and sequential formation of planetesimals in an outer region of the protoplanetary disc
Authors: Bouvier Audrey , Ma Ning , Neumann Wladimir , Neri Adrien , Schwarz Winfried H. , Ludwig Thomas , Trieloff Mario , Klahr Hubert 
Published: July 2021

Isotopic anomalies in meteorites have revealed two reservoirs of planetesimal formation corresponding to non-carbonaceous (NC) and carbonaceous (C) compositions sampling the inner and more outer regions of the disc respectively (Kruijer et al. 2020), or possibly formed at different periods (Lichtenberg et al. 2021). Chronological studies provide constraints on the timing of such separation lasting between 1 and 4 Myr after CAIs. Generally, differentiated planetesimals are found to be prevalent in the NC region and accreted within 50 km. This age is before the formation of two other CR-related differentiated achondrite and the CR2 chondrite PBs at ~1.5, ~1.7, and ~4 Myr, respectively (Sanborn et al. 2019). Our results indicate that TPB formed concurrently with NC primitive achondrite PB (Neumann et al. 2018).Based on geochemistry and reported 17O-54Cr-50Ti isotopic anomalies (Sanborn et al. 2019), Tafassites can be further distinguished from CR2 chondrite and NWA 011 and NWA 6704 differentiated achondrite parent bodies (PB). While a high pebble flux is a preferred mechanism for regulating early planetesimal formation (Lenz et al. 2019; 2020), the close relationship of isotopic anomalies for 54Cr among these CR-related planetesimals suggest instead that limited radial mixing of pebbles as building material occurred after 1 Myr of disc evolution. The late formation and compositional observations of CR chondrites require both a change to less efficient planetesimal formation and a pebble storage mechanism, e.g., in disc ring structures (Hartlep and Cuzzi 2020). Our results therefore demonstrate that rocky parent bodies accreted early and sequentially, from 1 to 4 Myr after CAIs, and depict different accretion mechanisms within a limited region of the outer solar system.References:Hartlep and Cuzzi. 2020. Cascade Model for Planetesimal Formation by Turbulent Clustering, The Astrophysical Journal, 892: 120.Kruijer, Kleine and Borg. 2020. The great isotopic dichotomy of the early Solar System, Nature Astronomy, 4: 32&#8211;40.Lenz, Klahr and Birnstiel. 2019. Planetesimal population synthesis: Pebble flux-regulated planetesimal formation, The Astrophysical Journal, 874: 36.Lenz, Klahr, Birnstiel, Kretke and Stammler. 2020. Constraining the parameter space for the solar nebula-The effect of disk properties on planetesimal formation, Astronomy &amp; Astrophysics, 640: A61.Lichtenberg, Dra&#807;&#380;kowska, Sch&#246;nb&#228;chler, Golabek and Hands, 2021. Bifurcation of planetary building blocks during Solar System formation, Science, 371: 365-70.Neumann et al. 2018. Modeling the evolution of the parent body of acapulcoites and lodranites: A case study for partially differentiated asteroids, Icarus, 311: 146-69.Sanborn et al. 2019. Carbonaceous achondrites Northwest Africa 6704/6693: Milestones for early Solar System chronology and genealogy, Geochimica et Cosmochimica Acta, 245: 577-96.Schwarz et al. 2021. Pb-Pb Ages of Chondritic Phosphates. Lunar and Planetary Science Conference, Abstract # 1981.