Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies

Abstract

Although petrologic, chemical, and isotopic studies of ordinary chondrites and meteorites in general have largely helped establish a chronology of the earliest events of planetesimal formation and their evolution, there are several questions that cannot be resolved via laboratory measurements and/or experiments alone. Here, we propose the rationale for several new constraints on the formation and evolution of ordinary chondrite parent bodies (and, by extension, most planetesimals) from newly available spectral measurements and mineralogical analysis of main-belt S-type asteroids (83 objects) and unequilibrated ordinary chondrite meteorites (53 samples). Based on the latter, we suggest that spectral data may be used to distinguish whether an ordinary chondrite was formed near the surface or in the interior of its parent body. If these constraints are correct, the suggested implications include that: (1) large groups of compositionally similar asteroids are a natural outcome of planetesimal formation and, consequently, meteorites within a given class can originate from multiple parent bodies; (2) the surfaces of large (up to ∼200 km) S-type main-belt asteroids mostly expose the interiors of the primordial bodies, a likely consequence of impacts by small asteroids (D < 10 km) in the early solar system; (3) the duration of accretion of the H chondrite parent bodies was likely short (instantaneous or in less than ∼10 5 yr, but certainly not as long as 1 Myr); (4) LL-like bodies formed closer to the Sun than H-like bodies, a possible consequence of the radial mixing and size sorting of chondrules in the protoplanetary disk prior to accretion.

3D shape of asteroid (6) Hebe from VLT/SPHERE imaging: Implications for the origin of ordinary H chondrites

“…It was further proposed that Hebe could be the parent body of an ancient asteroid family (Gaffey & Fieber-Beyer, 2013). The idea of H chondrites mainly originating from Hebe, however, was recently weakened by the discovery of a large number of asteroids (including several asteroid families) with similar spectral properties (hence composition, Vernazza et al, 2014). Here, we challenge this hypothesis by studying the three-dimensional shape and topography of Hebe derived from disk-resolved observations.…”

“…For comparison, the five known S-type families spectrally analogous to Hebe (therefore to H chondrites; Vernazza et al 2014) and located close to the main-belt 3:1 and 5:2 mean-motion resonances, namely Agnia (located at semi-major axis a=2.78 AU and eccentricity e=0.09), Koronis (a=2.87 AU, e=0.05), Maria (a=2.55 AU, e=0.06), Massalia (a=2.41 AU, e=0.14) and Merxia (a=2.75 AU, e=0.13) encompass a total volume of respectively ∼ 2.4 × 10 4 km 3 , 5.6 × 10 5 km 3 , 3.6 × 10 5 km 3 , 5.7 × 10 4 km 3 and 1.8 × 10 4 km 3 when the larger member of each family is removed. Family membership was determined using Nesvorný (2015)'s Hierarchical Clustering Method (HCM)-based classification (http://sbn.psi.edu/pds/resource/nesvornyfam.html) and rejecting possible interlopers that do not fit the "V-shape" criterion as defined in Nesvorný et al (2015).…”

The Creston, California, meteorite fall and the origin of L chondrites

“…Gefion is the only known family with large members of L (as opposed to H and LL) chondrite composition (Vernazza et al. ) and some asteroids found in the 3:1 mean‐motion resonance, e.g., (355) Gabriella, (14470) Utra, and (1722) Goffin, have L chondrite‐like spectra (Fieber‐Beyer and Gaffey ). In more recent years, however, the reflection spectra of some Gefion family members were found to resemble that of H chondrites and basaltic achondrites (McGraw et al.…”

Potentially hazardous Asteroid 2007 LE: Compositional link to the black chondrite Rose City and Asteroid (6) Hebe

“…This small subset of H-chondrite like asteroids were suggested as a possible old, dispersed asteroid family belonging to (6) Hebe (Gaffey and Fieber-Beyer, 2013). Complementing the existing and ever growing list of H-chondrite candidates near the 3:1 Kirkwood Gap, Vernazza et al (2014) identified several additional asteroids as viable H-chondrite candidates. Furthermore, Burbine et al (2002) make the point that there may be several H chondrite asteroids from the original parent body.…”

“…It is probable that the H-chondrites and IIE irons came from (6) Hebe (Gaffey and Gilbert, 1998), though caveats exist (e.g., Rubin and Bottke, 2009). The identification of many more large asteroids with H chondrite-like spectra also means that more candidates are now available to serve as a source for these meteorites (Vernazza et al, 2014).…”