Mathematics – Logic
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.p31c..05g&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #P31C-05
Mathematics
Logic
5455 Origin And Evolution
Scientific paper
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: 26Al 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 26Al in various chondrites and achondrites. The case for 26Al 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 26Al 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 244Pu fission tracks and 40Ar-39Ar 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 26Al, where the heat source is distributed uniformly per unit mass. Nebular homogeneity and detectable excesses of fossil 26Mg are necessary but not sufficient conditions for 26Al 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 26Al 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 ( ˜1027 g) planetary embryos in < 1 Myr. Runaway growth generates a bimodal size distribution with numerous ˜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 26Al 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.
Amelin Yu.
Ghosh Arpita
McSween Harry Y.
Weidenschilling Stuart J.
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