Linking asteroids and meteorites to the primordial planetesimal population

Ian A Franchi,Richard C Greenwood,Thomas H Burbine

doi:10.1016/j.gca.2020.02.004

Ian A Franchi, Richard C Greenwood + Show 1 more

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https://doi.org/10.1016/j.gca.2020.02.004

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Abstract

Meteorites provide a unique insight into early Solar System processes. However, to fully interpret this record requires that these meteorites are related back to their source asteroids and ultimately to the original planetesimal population that formed early in Solar System history. As a first step in this process an assessment has been undertaken of the likely number of distinct source asteroids sampled by meteorites and related extraterrestrial materials. The results of this survey indicate that there are between 95 and 148 parent bodies represented in our sample collections. This number has been steadily increasing as new “anomalous” meteorites are characterized. Attempts to link these parent bodies to identified asteroidal sources has so far been of limited success, due to the non-unique reflectance spectra of almost all known asteroids. Asteroid (4) Vesta and the HED (howardites, eucrite, diogenite) meteorites are the best example of a relatively non-disputed asteroid-meteorite linkage.As part of this study the “parent body” concept has been examined and was found to be a widely, but loosely, used term in the literature to designate “a body that supplies meteorites to Earth.” This concept could be rendered more meaningful by discriminating between primary and secondary parent bodies. A primary parent body is the source asteroid from which the meteorite is ultimately derived, and a secondary parent body is an asteroid derived through impact or break-up of the primary body. A clear example of this usage is provided by (4) Vesta, with the main asteroid being the primary parent body and the Vestoids representing secondary parent bodies. The concept of primary vs. secondary parent bodies may have important implications for early Solar System evolution. Chondritic parent bodies are known to have accreted between 1 and 4 Myr after CAIs. This timing difference may reflect the fact that their source asteroids, particularly those of the carbonaceous chondrites, are secondary bodies, with the original CAI-bearing primary bodies destroyed during early collisional processing.The number of primary parent bodies represented by meteorites (95–148) appears low when compared to the estimated number of asteroids in the main belt (>100,000 with diameters exceeding ∼2 km). A range of potential reasons may explain this apparent mismatch: (i) meteorites provide an unrepresentative sampling of the main belt, (ii) the belt may only contain a limited number of primary parent bodies, (iii) meteorites may be preferentially derived from the ∼120 identified asteroid families, (iv) loosely consolidated types are filtered by Earth’s atmosphere, (v) multiple, near-identical, “clone” parent bodies may be present in the belt. At present, it is not possible to determine which of these potential mechanisms are dominant and all may be operating to a greater or lesser extent.Based on classical accretion models the meteorite record appears to be highly unrepresentative of the primordial asteroid population. In contrast, pebble accretion models suggest that these first-generation bodies may have been relatively large, in which case meteorites may provide a more unbiased record of early Solar System processes.

Highlights

Meteorites provide us with a great diversity of extraterrestrial materials
This involves addressing a number of key issues: (i) how many asteroids/parent bodies are represented in the worldwide meteorite collection? (Wasson, 1995; Burbine et al, 2002a; Hutchison, 2004); (ii) how well can we link meteorites to particular asteroids (e.g., Burbine, 2016); (iii) how useful are contemporary meteorites and asteroids as indicators of the composition and structure of the first-generation of planetesimals, those that accreted early in the evolution of the Solar System? (e.g. Weidenschilling 1988; Ruzicka et al, 1997; Day et al, 2009, 2015, 2019; Scott et al, 2018)
Dynamic models suggest that inward--outward migration of the gas giants first cleaned out the main belt, repopulated its inner regions with planetesimals that accreted in the inner Solar System ($1–3 AU) and repopulated its outer regions with bodies that formed between and beyond the orbits of the giant planets (Walsh et al, 2011)

Summary

Introduction

Meteorites provide us with a great diversity of extraterrestrial materials. to interpret this record effectively we need to link these meteorites to their source asteroids and relate both to the original asteroidal population. Dynamic models suggest that inward--outward migration of the gas giants first cleaned out the main belt, repopulated its inner regions with planetesimals that accreted in the inner Solar System ($1–3 AU) and repopulated its outer regions with bodies that formed between and beyond the orbits of the giant planets (Walsh et al, 2011). This migration has been postulated to have occurred over 4480 Myr ago due to the lack of widespread crustal reset ages of various parent bodies after $4450 Myr ago (Mojzsis et al, 2019). Such isotopic differences may reflect secular changes in the composition of the materials from which Solar System bodies were formed (Schiller et al, 2018)

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