Meteoritic, Asteroidal, and Theoretical Constraints on the 500 Ma Disruption of theLChondrite Parent Body

The high abundance of heavily shocked and degassedLchondrites with Ar–Ar ages around 500 Myr shows that theLchondrite parent body suffered a major impact 500 Myr ago. We infer from constraints on the thermal evolution of impact heated rocks after the 500-Myr event and the high abundance of shockedLchondrites that the parent body was catastrophically disrupted. The slow cooling rates of some shocked and degassedLchondrites (0.01–1°C year−1) show that they were derived from kilometer-sized impact-heated fragments or rubble piles that were ejected from near the impact point. We suggest that the catastrophic dispersion of the parent body provided some fragments with sufficiently high velocities to put them into resonances and that this initiated the orbital evolution which resulted in the high flux ofLchondrite meteoroids impacting the Earth at present. It appears likely that this was a “slow-track” type of dynamical evolution, with most objects avoiding drastic resonant changes of orbital eccentricity, and undergoing a slow random walk in orbital element space, driven by a sequence of random encounters with Mars and, later on, with the Earth. The ν6secular resonance provides a plausible mechanism to start this evolution, since fragments inserted into it in the main belt frequently have their eccentrici_ties raised to values of about 0.4, sufficient for Mars-crossing but not for Earth-crossing orbits; on the other hand, recent numerical work has shown that the 3:1 mean motion resonance leads almost always to a fast-track evolution, ending up with a solar collision or a hyperbolic ejection within a few Myr.

[1]  D. Heymann.  On the origin of hypersthene chondrites: Ages and shock effects of black chondrites , 1967 .

[2]  H. Voshage.  Bestrahlungsalter und Herkunft der Eisenmeteorite , 1967 .

[3]  J. Goldstein,et al.  The metallic microstructures and thermal histories of severely reheated chondrites , 1976 .

[4]  K. Ernstson,et al.  The Ries impact crater. , 1977 .

[5]  F. Hoerz,et al.  Shock metamorphism of granulated lunar basalt , 1979 .

[6]  J. Wood.  Review of the metallographic cooling rates of meteorites and a new model for the planetesimals in which they formed , 1979 .

[7]  D. Bogard,et al.  Ar-40/Ar-39 dating, Ar diffusion properties, and cooling rate determinations of severely shocked chondrites , 1980 .

[8]  L. Schultz,et al.  Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy , 1981 .

[9]  A. Fujiwara.  Complete fragmentation of the parent bodies of Themis, Eos, and Koronis families , 1982 .

[10]  Asteroid Collisions; Effective Body Strength and Efficiency of Catastrophic Disruption , 1983 .

[11]  F. Hörz,et al.  Bunte Breccia of the Ries: Continuous deposits of large impact craters , 1983 .

[12]  K. Keil,et al.  FRAGMENTAL BRECCIAS AND THE COLLISIONAL EVOLUTION OF ORDINARY CHONDRITE PARENT BODIES , 1983 .

[13]  A. Vickery,et al.  Size-velocity distribution of large ejecta fragments , 1984 .

[14]  H. Melosh.  Impact ejection, spallation, and the origin of meteorites , 1984 .

[15]  P. Farinella,et al.  Collisional origin of the asteroid families: Mass and velocity distributions , 1984 .

[16]  H. Melosh.  Impact Cratering: A Geologic Process , 1986 .

[17]  G. J. Taylor,et al.  Original structures, and fragmentation and reassembly histories of asteroids - Evidence from meteorites , 1987 .

[18]  Kevin R. Housen,et al.  Some recent advances in the scaling of impact and explosion cratering , 1987 .

[19]  D. Sears,et al.  Overview and classification of meteorites. , 1988 .

[20]  Mean motion resonances. , 1989 .

[21]  Paolo Farinella,et al.  A semiempirical model of catastrophic breakup processes , 1989 .

[22]  J. Bell,et al.  Asteroid families - Physical properties and evolution , 1989 .

[23]  N. Nakamura,et al.  Young asteroid melting event indicated by Rb–Sr dating of the Point of Rocks meteorite , 1990, Nature.

[24]  H. Haack,et al.  Effects of regolith/megaregolith insulation on the cooling histories of differentiated asteroids , 1990 .

[25]  K. Keil,et al.  Shock metamorphism of ordinary chondrites , 1991 .

[26]  K. Marti,et al.  COSMIC-RAY EXPOSURE HISTORY OF ORDINARY CHONDRITES , 1992 .

[27]  D. Fink,et al.  Cosmogenic nuclides in core samples of the Chico L6 chondrite - Evidence for irradiation under high shielding , 1992 .

[28]  T. Fujiwara,et al.  Additional Evidence of a Young Impact-Melting Event on the L-Chondrite Parent Body , 1992 .

[29]  Robert A Kolvoord,et al.  Collision lifetimes and impact statistics of near-Earth asteroids , 1993 .

[30]  Alessandro Morbidelli.  Asteroid Secular Resonant Proper Elements , 1993 .

[31]  Fragment jets from catastrophic break-up events and the formation of asteroid binaries and families , 1993 .

[32]  R. S. Hudson,et al.  Shape of Asteroid 4769 Castalia (1989 PB) from Inversion of Radar Images , 1994, Science.

[33]  A. Rubin.  Metallic copper in ordinary chondrites , 1994 .

[34]  Giovanni B. Valsecchi,et al.  Asteroids falling into the Sun , 1994, Nature.

[35]  Paolo Farinella,et al.  Asteroid collisional evolution: results from current scaling algorithms , 1994 .

[36]  H. Haack,et al.  Catastrophic fragmentation of asteroids: evidence from meteorites , 1994 .

[37]  D. Bogard.  Impact ages of meteorites: A synthesis , 1995 .

[38]  M. Moons,et al.  Secular Resonances in Mean Motion Commensurabilities: The 4/1, 3/1, 5/2, and 7/3 Cases , 1995 .

[39]  H. Haack,et al.  Fragmentation and Reaccretion of Differentiated Asteroids: Evidence from Thermal Histories of Meteorites and Theoretical Constraints , 1995 .

[40]  P. Farinella,et al.  Secular Resonances and the Dynamics of Mars-Crossing and Near-Earth Asteroids , 1995 .

[41]  (Erratum) Long-term dynamical evolution of the brightest bolides. , 1995 .

[42]  P. Farinella,et al.  Dynamical evolution of NEAs: Close encounters, secular perturbations and resonances , 1996 .