28.09

Origin of Chondrules at Jovian Resonances

S. J. Weidenschilling (PSI), F. Marzari (U. di Padova), L. L. Hood (LPL, U. of Arizona)

It is widely assumed that chondrules formed before planetesimals, but CAIs, apparently early nebular condensates, are several My older than most chondrules. If CAIs were not promptly incorporated into  km bodies, they would have been lost into the Sun by gas drag. Also, chondrules have I-Xe age differences that imply formation over an interval of several My, while models of planetesimal formation yield short timescales, orbital periods. We suggest planetesimals accreted early, preserving CAIs within them. Chondrules formed later by heating of collisional debris from these bodies, and were accreted into second-generation planetesimals. Jovian resonances were responsible for these events. Jupiter accreted in the gaseous nebula; plausibly gas was present in the asteroid zone after its formation. Its perturbations excited eccentricities at commensurability resonances, with large planetesimals stirred to supersonic velocities relative to the gas. Dust produced by collisions among resonantly stirred planetesimals was melted by passage through bow shocks. Chondrules and debris (including CAIs), drifted inward and settled due to gas drag. This material was captured by non-resonant bodies in smaller orbits, and/or accreted into new planetesimals. Orbital integrations show that resonant planetesimals >20 km diameter would not be damped appreciably by drag in a low-mass nebula. If Jupiter had its present mass (a 10-15 Earth mass core is insufficient), the 2:1 and 3:2 resonances yield eccentricities large enough to produce shocks. The age difference between CAIs and chondrules may constrain Jupiter's formation time. Chondrule production would have continued as long as drag supplied planetesimals to resonances; the spread in chondrule ages may be a measure of the nebula's lifetime.