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Early Impact History and Dynamical Origin of Differentiated Meteorites and Asteroids

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Early Impact History and Dynamical Origin of Differentiated Meteorites and Asteroids Scott E. R. D., Keil K., Goldstein J. I., Asphaug E., Bottke W. F., and Moskovitz N. A. (2015) Early impact history and dynamical origin of differentiated meteorites and asteroids. In Asteroids IV (P. Michel et al., eds.), pp. 573–595. Univ. of Arizona, Tucson, DOI: 10.2458/azu_uapress_9780816532131-ch030. Early Impact History and Dynamical Origin of Differentiated Meteorites and Asteroids Edward R. D. Scott and Klaus Keil University of Hawai‘i at Mānoa Joseph I. Goldstein University of Massachusetts (now deceased) Erik Asphaug Arizona State University William F. Bottke Southwest Research Institute Nicholas A. Moskovitz Lowell Observatory Differentiated asteroids and igneous meteorites present numerous challenges to our un- derstanding of the impact and dynamical evolution of asteroids and meteorite parent bodies. Igneous meteorites, including irons, achondrites, and stony-iron meteorites, testify to the prior existence of ~100 differentiated bodies. Destruction of these bodies by hypervelocity impact over 4 G.y. would have required numerous giant impacts, although this is inconsistent with the preservation of Vesta’s basaltic crust and the lack of differentiated asteroid families. We review recent advances in elucidating the early chronology of meteorites, spectroscopic observations of likely differentiated asteroids, petrological studies of differentiated meteorites, impact disruption of differentiated planetesimals during accretion, and dynamical scenarios for capturing material into the asteroid belt. Together, these advances suggest a new paradigm in which planetesimals accreted rapidly in the inner solar system and were melted by 26Al less than 2 m.y. after the formation of calcium-aluminum-rich inclusions (CAIs). While molten they were disrupted by grazing hit-and-run impacts during the accretion of planetesimals. Later, when still hot, the survivors were disrupted by hypervelocity impacts. Impact debris from the differentiated bodies was transferred from the newly formed terrestrial planet region to stable orbits in the asteroid belt. This evolutionary history leaves many questions unanswered but suggests new paths for future exploration of the asteroid belt and petrological and isotopic studies of meteorites. 1. INTRODUCTION they were the ultimate source of the differentiated meteorites, one might expect collisions to have created enormous numbers Laboratory studies of igneously formed meteorites suggest of V-type asteroids that were not derived from Vesta, as well that numerous meteorite parent bodies were melted to form as ample numbers of mantle and core fragments compared metallic cores and silicate mantles. Studies by the Dawn to the rest of the main-belt population. This is not observed. spacecraft confirm that (4) Vesta melted in this way (McSween Instead, only limited examples of asteroids composed pre- et al., 2013; see the chapter by Russell et al. in this volume). dominantly of core metallic Fe-Ni (possibly some M-type In principle, one would think the origin of iron and stony- asteroids) or mantle olivine (some A-type asteroids) have been iron meteorites and achondrites would be straightforward to identified. Very few of these bodies are members of asteroid investigate, but this is not the case. If disrupted Vesta-like families, with those in families having orbits and sizes most differentiated bodies were once common in the main belt and consistent with being interlopers. 573 574 Asteroids IV Differentiated asteroids and meteorites provide many 2. MELTING AND SOLIDIFICATION other challenges to our understanding of the formation and OF ASTEROIDS evolution of the asteroid belt. How could core samples have been extracted from numerous large differentiated 2.1. When Were Meteoritic Bodies Melted bodies when Vesta’s basaltic crust was preserved (Davis and Solidified? et al., 1985)? Where are the meteorites from the olivine- rich mantles of the parent bodies of the 12 groups of iron Advances in the use of short-lived and long-lived radio- meteorites and the ~50–70 parent bodies of the ungrouped nuclides for dating meteorites since Asteroids III have revo- irons, the two groups of pallasites, and the type 4–6 un- lutionized our understanding of the timescales for early solar grouped pallasites — “the great dunite shortage” (Bell et system processes (Davis and McKeegan, 2014). The U-Pb al., 1989)? We also lack mantle and core samples from the system provides two isotopic clocks with different half-lives: parent asteroids of the angrites and the ungrouped eucrites 238U decays to 206Pb with a half-life of 4469 m.y., and 235U like Northwest Africa (NWA) 011 and Ibitira (Krot et al., decays to 207Pb with a half-life of 704 m.y. Measurements 2014). Where is the missing Psyche family of silicate-rich of 207Pb/204Pb and 206Pb/204Pb can provide ages with preci- asteroids (Davis et al., 1999)? Why are meteorites derived sions of 0.1–1 m.y. in favorable cases. After allowance for from far more differentiated parent bodies than chondritic small variations in the 238U/235U ratio (Wadhwa, 2014), the parent bodies even though the asteroid belt is dominated by Pb-Pb technique gives ages of 4567.2 to 4568.2 Ma for CAIs C and S complex chondritic asteroids (Bottke et al., 2006)? from CV3 chondrites, which provide our best estimate of We reassess ideas about the dynamical origin and early the formation age (t0) of the first solid materials in the solar impact histories of differentiated asteroids and meteorites system. Figure 1, which shows ages of early solar system in the light of major advances since Asteroids III. These materials, was constructed using a mean value of 4567.5 Ma include advances in our understanding of the chronology of for t0. [The small difference between this value and that the early solar system, the distribution of short-lived isotopes preferred by Davis and McKeegan (2014) of 4567.3 ± including 26Al, the identification of pristine chondrites and 0.2 Ma from Connelly et al. (2012) is of no consequence differentiated meteorites that escaped prolonged alteration in this discussion.] or metamorphism, the thermal histories of differentiated Short-lived chronometers like 26Al, which decays to meteorites, the nature of S asteroids, impact modeling during 26Mg with a half-life of 0.71 m.y., and 182Hf, which decays accretion, and dynamical studies of planetesimal accretion. to 182W with a half-life of 8.90 m.y., provide relative ages. Together, these advances suggest a new paradigm (Bottke For example, CAIs have inferred initial ratios of 26Al/27Al et al., 2006; Goldstein et al., 2009) in which planetesimals that are roughly 8 (= 23)× higher than most chondrules, accreted rapidly in the inner solar system and were rapidly and so formed about 2 (3 × 0.71) m.y. before chondrules. melted by 26Al less than 2 m.y. after CAI formation. While Manganese-53, which decays to 53Cr with a half life of they were still molten or semimolten, planetesimals were 3.7 m.y., also provides valuable relative ages but cannot disrupted — first by grazing impacts during the accretion that be used to date CAIs because Mn is depleted in CAIs and harvested the objects making slow, direct hits (Asphaug et al., Cr shows mass-independent isotopic anomalies. The short- 2006), and later by hypervelocity impacts when larger bodies lived and long-lived chronometers are cross-calibrated using excited their orbits. Impact debris from the differentiated bod- angrites — achondrites that crystallized 4–11 m.y. after CAIs ies was tossed into the asteroid belt. Most iron meteorites did and escaped extensive metamorphism and shock heating not cool slowly inside insulating silicate mantles, like Vesta’s (Brennecka and Wadhwa, 2012; Kleine et al., 2012). The core, but rapidly with little or no silicate insulation less than oldest, fine-grained angrites such as D’Orbigny are espe- 5 m.y. after CAI formation. In this scenario, Vesta — the cially important anchors for the short-lived chronometers as dominant single source of differentiated meteorites, viz., they crystallized and cooled very rapidly (in days). the howardites, eucrites, and diogenites (HEDs) — is not a Radioactive isotopic systems record the time at which dif- typical differentiated asteroid. fusion of the parent and daughter isotopes between minerals In this chapter, we first describe the isotopic evidence effectively ceased. Thus ages of slowly cooled rocks depend that has been used to establish a new chronology for melt- on cooling rates (Ganguly and Tirone, 2001). Many igne- ing and solidification of igneous meteorites and to identify ous and metamorphosed meteorites cooled slowly at around the heat source. Then we review the types, properties, and 10–103°C m.y.–1, so that radiometric ages based on isotopic abundances of differentiated asteroids and asteroid families. systems in various minerals with closure temperatures that Next, we describe the various types of igneous meteorites, range from ~300° to 1100°C could differ by ~50 m.y. or focusing on evidence that their parent bodies were disrupted more for slowly cooled meteorites, but only ~1 m.y. for by impacts while they were still partly or wholly molten. quickly cooled meteorites. For example, Hf-W closure Finally, we discuss numerical simulations of grazing impacts temperatures for pyroxene are ~750°–900°C, depending on between differentiated planetesimals during accretion (“hit- grain size and cooling rate (Kleine et al., 2008). However, and-run” impacts), and dynamical studies that suggest how zircons may crystallize from melts above their Hf-W clo- planetesimals could have been scattered into the asteroid sure temperatures and so record the time of crystallization. belt from 1 to 2 AU. Closure temperatures for Pb-Pb ages of phosphates in or- Scott et al.: Early Impact History and Dynamical Origin 575 Age (Ma) 4568 4566 4564 4562 4560 4558 4556 Differentiated Meteorites IIAB IIIAB Phosphate Iron Meteorites IVB IVA FeS Accretion Crystallization Core formation Ureilites Quenched angrites Ungrouped eucrite Shallowater aubrite Chondrites LL CO Chondrule Formation CV CB Accretion Alteration 102 m.y.
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