Meteoritical and dynamical constraints on the growth mechanisms and formation times of asteroids and Jupiter

Abstract

Peak temperatures inside meteorite parent bodies are closely linked to accretion times. Most iron meteorites come from bodies that accreted <0.5 Myr after CAIs formed and were melted by 26Al and 60Fe, probably inside 2 AU. Chondrite groups accreted intermittently over 4 Myr starting 1 Myr after CAIs formed when planetary embryos may already have formed at ~1 AU. Meteorite evidence precludes accretion of late-forming chondrites on the surface of early-formed bodies; instead chondritic and non-chondritic meteorites probably formed in separate planetesimals. Maximum metamorphic temperatures in chondrite groups correlate inversely with mean chondrule age, as expected for 26Al heating. Dynamical models suggest that asteroids accreted before Jupiter. Therefore Jupiter probably reached its current mass >3-5 Myr after CAIs formed. This precludes formation of Jupiter via a gravitational instability <1 Myr after the solar nebula formed, and strongly favors core accretion. Shocks formed by gravitational instabilities in the disk, proto-Jupiter, or by planetary embryos may have produced some chondrules. The minimum lifetime for the solar nebula of 3-5 Myr inferred from CAI and chondrule ages may exceed the median 3 Myr lifetime for protoplanetary disks, but is well within the total 1-10 Myr range. Shorter formation times for extrasolar planets may help to explain why their orbits are unlike those of solar giant planets.