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Multiple and Fast: The Accretion of Ordinary Chondrite Parent Bodies P. Vernazza (LAM), B. Zanda (MNHN), R. Binzel (MIT), T. Hiroi (Brown University), F. E. DeMeo (MIT), M. Birlan (IMCCE) Asteroid Facts: • Most are in the main asteroid belt (between Mars and Jupiter). • Largest is Ceres, discovered 1801 (~ 950 km across). • More than 300,000 asteroids known. • Estimated one-million main-belt asteroids larger than 1 km. (FB) Gaspra (12 km) S S S (FB) Ida (56 km) (RV) Eros (33 km) Dactyl (1 km) (RV) Itokawa (500 m) S (FB) Mathilde (66 km) C Xc (FB) Lutetia (100 km) (RV) Vesta (520 km) (FB) Steins (5 km) Xe V How do planetary systems form and evolve? Asteroids = key population to progress on the following questions: The role of planetary migrations in the evolution The origin of water on a planet (Earth) The formation and evolution of planetesimals Vesta seen by NASA/DAWN Extra-solar systems offer only snapshots of their architecture at a given time Planetary systems with at least 3 planets (Lovis et al. 2011) Evidence of Migrations Deciphering the History of the Solar System Solar nebula Proto disk Planetesimals: asteroids comets TNOs tell us about: Planetesimals The migration of dust prior to the accretion process Solar System The primordial chemical composition from which planets once accreted The migration of bodies during the formation and evolution of the Solar System Radial mixing in the protoplanetary disk Evidence in comets from IR spectra Evidence in asteroids (e.g. Hale-Bopp,Crovisier et al. 1997, Brucato et al. 1999) (chondritic meteorites) 4x5.5 mm Crystalline and amorphous olivine Chondrules formed at T>1600 ºC Matrix formed at T<200 ºC The migration of bodies during the formation and evolution of the Solar System Predicted migrations from dynamical simulations Oort Cloud Jupiter, Saturn Uranus, Neptune The diversity of asteroids is diagnostic and a consequence of planetary migrations 2 spectral classes ~5 spectral classes 24 spectral classes Asteroids = Condensed version of the primordial Solar System Constraints on the Formation of Ordinary Chondrite Parent Bodies Meteorite fall statistics Stony-Irons Irons Achondrites Carbonaceous Chondrites Ordinary Chondrites 80% Ordinary Chondrite meteorites • 3 groups: H (~35%), L (~37%) and LL (~8%) • Constraints on the formation and early evolution of the Solar System : – migration processes in the disk – the post-accretional heating events – the collisional events that have occurred since 4.6 Gyrs Ordinary Chondrites can’t tell us the whole story though • • • • • Formation location of the different classes of OCs? Initial average size of their parent bodies? Amplitude of the bias in our collections? How many parent bodies for a given meteorite class ? Level of radial mixing experienced by their parent bodies after their formation? • Their accretion timescale? S-type asteroids: the parent bodies of Ordinary Chondrites ? S-type asteroid Space Weathering Ordinary Chondrite [Gaffey et al., 1993] The Hayabusa mission has ended a 40 years long debate ! S-type asteroids are the parent bodies of Ordinary chondrites ! Nakamura et al. 2011, Science Location of S-type asteroids Input Data Meteorites Main BeltAsteroids (laboratory measurements) (ground-based observations) • >100 ordinary chondrite spectra (0.4-2.5 µm) 250°C 900°C Expected structure via 26Al & 60Fe heating • 93 S-type asteroid spectra (0.4-2.5 µm) Survey almost complete down to H=8.5 (D~60km) Compositional Modeling Compositional model of Shkuratov et al. (1999). Olivine (Ol), Orthopyroxene (Opx), Clinopyroxene (Cpx) + data + data 11 Parthenope ◊ model ◊ model Olivine: 77% Olivine: 77% Orthopyroxene: 23% Orthopyroxene: 23% Grain Size: 17 Grain Size: 17 Model predicted the recently confirmed link (Nakamura et al. 2011) between 25143 Itokawa and LL chondrites (Vernazza et al. 2008) Asteroid slopes accounted for using weathering model of Brunetto et al. (2006). Composition of OCs Two compositional groups Multiple parent bodies for a given meteorite class Implications: early ring like structures in the disk? Lyra & Kuchner (2013) in Nature Asteroid surfaces as exposed interiors Implications: Present Surface = Primordial surface & interior Coherent with Vesta’s surface composition (as seen from Dawn) ‘Fast’ accretion of the H parent bodies I Paradox: Paucity of ordinary L chondrite parent bodies Fall statistics Implications: Few objects do contribute to the meteorite flux ! O’Brien & Greenberg 2005 Coherent with cosmic ray exposure ages which imply that a few collisional events dominate the meteorite flux Common origin for 2/3 of L chondrites ~2/3 of OLC meteorites (25% of all meteorites !) were heavily-shocked and degassed with 39Ar-40Ar ages 470±5 My (Korochantseva et al. 2007) => common origin => their parent body suffered a major impact ~470 Ma ago and catastrophically disrupted. Limestone beds in the Thorsberg quarry in southern Sweden where the fossil meteorites were deposited in the Ordovician ~470 Myr ago. Strikingly, the timing of the shock event coincides with the stratigraphic age (467 ± 2) of the midOrdovician strata where abundant fossil OLC meteorites were found (Schmitz et al. 2003) I II II Size sorting of chondrules as the origin of ‘reversed’ formation locations ? I Gradient envisioned by meteoricists Conclusion Planetesimal formation must occur from a ring of homogenized chondrule populations: otherwise, how would we get compositional clones? The production of compositional clones is a natural outcome of planetesimal formation: Parent bodies of meteorites are, as a matter of fact, not always unique. The formation process of the H chondrite parent bodies (and by extension, most planetesimals) must have been ‘fast’