HR: 09:00h
AN: P31C-05    [Abstracts]
TI: Planet Formation and Early Solar System heating: Recent advancements
AU: * Ghosh, A
EM: aghosh@cs.utk.edu
AF: University of Tennessee, Dept. of Earth and Planetary Sciences, Knoxville, TN 37996 United States
AU: * Ghosh, A
EM: aghosh@cs.utk.edu
AF: NASA Goddard, Goddard Spaceflight Center, Greenbelt, MD 20771 United States
AU: Weidenschilling, S J
AF: Planetary Science Institute, 1700 E. Ft. Lowell Rd. Suite 106, Tucson, AZ 85719 United States
AU: Amelin, Y
AF: Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8 Canada
AU: McSween, H Y
AF: University of Tennessee, Dept. of Earth and Planetary Sciences, Knoxville, TN 37996 United States
AB: In a seminal review in 1991, Wood and Pellas laid out the basic characteristics of protoplanetary heating as preserved in the meteorite record and pointed out the strengths and drawbacks of the two plausible heat sources: $^{26}$Al and electromagnetic induction. A decade and a half later, considerable advances have been made, in our theoretical understanding of planet formation and its effect on protoplanetary heating. In addition, remarkable progress has been made in meteoritic studies, including geochronological measurements and the search for fossil remains of $^{26}$Al in various chondrites and achondrites. The case for $^{26}$Al heating of asteroids has become increasingly robust. Its decay product has been found in most classes of chondrites and some achondrites. Reasons as to why evidence for $^{26}$Al might be obscured in other achondrites have been suggested. The heat source appears capable of explaining the full range of temperature excursions of asteroids within the main belt. The process of accretion has been shown to be intrinsically related to the thermal evolution of a protoplanetary body, since the timescale of accretion is comparable to the half life of the heat source. Thus, discerning characteristics of the parent body like volume proportion of various petrologic types, thermal-model cooling rates and closure ages, have been shown to be a function of accretion rate. A study of $^{244}$Pu fission tracks and $^{40}$Ar-$^{39}$Ar thermochronology of unshocked H chondrites seems to provide support to a layered onion shell parent body, where the outer layers cooled faster than the asteroidal interior. This fits the pattern of heating generated by $^{26}$Al, where the heat source is distributed uniformly per unit mass. Nebular homogeneity and detectable excesses of fossil $^{26}$Mg are necessary but not sufficient conditions for $^{26}$Al heating to be viable. It is critical for the heat source to be live at the time, and after, the formation of the planetary building blocks. Thus, the time, relative to CAI formation, at which accretion initiates, and the duration of the accretionary process strongly influence the efficacy of $^{26}$Al as a heat source. The oldest chondrule Pb-Pb dates measured are 4566.6 +/- 1.0 Myrs for Allende: this denotes a time interval of 0.6 +/- 1.2 Myrs between CAI and chondrule formation. If initiation of accretion is assumed to postdate formation of the oldest chondrules, time of initiation of accretion relative to CAI formation can be interpreted as the time difference between CAI and chondrule formation: thus, accretion can initiate between 0 to 1.8 Myrs after CAI formation. Recent accretional models produce Mars-sized ($\sim$10$^{27}$ g) planetary embryos in $<$ 1 Myr. Runaway growth generates a bimodal size distribution with numerous $\sim$100 km bodies that accrete on a comparable timescale. Melting and metarmorphism in the asteroid belt is likely to have occurred if accretion initiates between 0 to 1.8 Myrs and if duration of accretion is of the order of a few Myrs. The issue of possible nebular heterogeneity of $^{26}$Al clouds its efficacy as a heat source. Recent work, on electromagnetic induction, suggests that electromagnetic induction could cause melting in asteroids: however, the input parameter sets for the models remain largely unconstrained. Thus, the issue, of whether electromagnetic induction could have been the protoplanetary heat source in the early Solar System, remains unresolved.
DE: 5455 Origin and evolution
SC: Planetary Sciences [P]
MN: 2004 AGU Fall Meeting