Origin and history of ureilitic material in the solar system: The view from asteroid 2008 TC₃ and the Almahata Sitta meteorite

Abstract

Abstract-Asteroid 2008 TC 3 (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth's atmosphere, shattering at 44-36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50-70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre-atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC 3 probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC 3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC 3 was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re-evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5-0.6 Ma after formation of calcium-aluminum-rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC 3 . It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti-type chondrites were implanted by low-velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth-crossing orbits. 2008 TC 3 comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC 3 and other polymict ureilites.

Predictions for impactor contamination on Ceres based on hypervelocity impact experiments

“…The exogenic material in ureilites (e.g., Almahata Sitta) [Goodrich et al, 2015] and howardites supports this experimentally driven hypothesis of ubiquitous exogenic debris on asteroids. Because projectile contamination is driven by the impact process, which has dramatically modified all the asteroids, projectile contamination should be ubiquitous among bodies in the main belt.…”

“…As discussed in section 3.1, vertical impacts into veneered targets yielded lower projectile retention efficiencies than 60°i mpacts. Hewins, 1979;Lorenz et al, 2001Lorenz et al, , 2007Warren et al, 2009] and the Almahata Sitta ureilite [Goodrich et al, 2015] provide direct evidence for retention of both stony and metallic impactors on other bodies in the asteroid belt; the same should be true for Ceres. (see also Schultz et al [2005]).…”

“…Some impacts may remove more material than they deliver [e.g., Gault et al, 1963;Svetsov, 2011], possibly including the projectile component delivered by previous impacts. To assess the balance between projectile delivery and target removal, we combine well-established ejecta scaling relationships from Housen et al [1983] with projectile retention efficiencies reported here.…”

Timing of the formation and migration of giant planets as constrained by CB chondrites

“…In addition, the ureilites (carbon-bearing ultramafic achondrites) could be related to the heightened impact velocities associated with giant planet migration. Before it reaccreted, the ureilite parent body was disrupted by an energetic impact around the same time the CB chondrites formed ( 21 , 22 ). Nominally, for a similar size distribution and dynamical excitement, the number of similar collisions would scale linearly with the total mass of the belt, such that a population twice as massive or half as massive may produce too many or too few high-velocity collisions, respectively.…”

Thermophysical properties of Almahata Sitta meteorites (asteroid 2008 TC3) for high‐fidelity entry modeling

“…Based on other known polymict ureilites, it is suspected that the nonureilitic materials survived entry more efficiently and most of the initial asteroid was composed of ureilites, with perhaps 1–2% of enstatite and ordinary chondrites mixed in (Goodrich et al. ).…”