Linking Meteorites to Asteroids: How Many Parent Bodies Do We Sample in Our Meteorite Collections?

Introduction: Meteorites provide us with a great diversity of extraterrestrial materials. However, to interpret this record effectively we need to evaluate its relationship, both to the contemporary asteroid population and to how that population has evolved with time. This involves addressing a number of key issues: i) how many asteroids/parent bodies are represented in the worldwide meteorite collection? [1,2,3]; ii) how representative is the meteorite record of both the NEO (near-Earth object) and main belt populations? [1,4,5]; iii) how useful are contemporary meteorites and asteroids as indicators of the composition and structure of first generation planetesimals; those that accreted within 1-2 Myr of Solar System formation? [6]. Relevant to this final point are the proposals that: (i) giant planet migration was a major control on main belt structure [7] and (ii) that early planetesimal fragmentation resulted in a differential loss of mantle materials [8]. Previous parent body estimates: Burbine et al. [2] estimated that meteorites could sample as few as ~100 asteroids (~27 chondritic, ~2 primitive achondritic, ~6 differentiated achondritic, ~4 stony irons, ~10 iron groups, ~50 ungrouped irons). Hutchison [3] suggested that meteorites are sourced from approximately 120 asteroids, with about 80 being ungrouped irons. In contrast, Wasson [9] argued that only 17 asteroids are sampled by the ungrouped irons, making a total of 26 asteroids for the irons as a whole. Evidence from O-isotope studies: Here we use primarily the results from high-precision O-isotope studies, to reassess the likely number of parent bodies represented in the meteorite record [10]. Primitive achondrites. With the exception of the brachinites, the main primitive achondrite groups (acapulcoite-lodranite clan, ureilites and winonaites/IABIIICD irons) are each derived from a single parent body (Fig. 1). Considerable uncertainty exists about the number of parent bodies sampled by the brachinites and brachinite-like achondrites [10]. A conservative estimate would require 2, one for the “main-group” brachinites, and a second for Mg-rich, brachinite-like samples such as Divnoe, NWA 4042, NWA 4518, RBT 04255, RBT 04239 and Zag (b) (Fig.2). Differentiated achondrites and stony-irons. Apart from the pallasites, which appear to be derived from 6 distinct parent bodies [10] and the aubrites which are probably samples from 2 [11], the other main differentiated groups (angrites, HEDs, main-group pallasites, mesosiderites) are each derived from unique parent bodies (Fig. 1). Mesosiderites and HEDs may be from the same parent body [10], but here we adopt a conventional approach and assign each to a distinct source.