Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23c1725s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P23C-1725
Mathematics
Logic
[6040] Planetary Sciences: Comets And Small Bodies / Origin And Evolution
Scientific paper
High precision measurements of isotopic abundances in meteorites show that refractory grains known as calcium-aluminium-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in CV carbonaceous chondrite meteorites formed within a 4000 kyr time interval, i.e. 'instantaneously', defining the start of the solar system. The first planetesimals accreted at about the same time. Molten iron cores had separated by about 500 kyr later. Chondrules were probably made 1 to 2 Myr later, presumably in the manner advanced recently by Asphaug, namely low velocity disruption of a molten impactor planetesimal during sloppy collisional accretion. Chondritic planetesimals clearly formed later still, evidently too late to be melted by the decay of 26Al. Basaltic meteorites, like irons, probably come from planetesimals that accreted very early because the basalt had already crystallized 3 to 4 Myr after the start. These temporal constraints are broadly consistent with published thermal modelling of planetesimals, assuming 26Al decay was the heat source. However, current models are poor predictors of the cooling phase, largely because there is no general agreement on how molten planetesimals lost their heat. Did they solidify 'top down', with the outer insulating carapace becoming steadily thicker through time? Or did they solidify from the centre outwards, as crystal cumulates settled, and the residual magma ocean remained at, or just beneath, the surface? The latter mechanism would result in far more rapid cooling than the former. Did the bulk of the molten interior reach the liquidus, or was it buffered as a sub-liquidus slurry due to accelerated heat loss through a thinned down insulating cap? Another consideration is the thermal consequence of collisional accretion when, according to Asphaug, much of the disrupted impactor falls back as chondrules to form a veneer wrapping the enlarged target body, thus producing a planetesimal with a chondritic outer layer and a molten interior. Our goal here is to set out possible ways of modelling these and other cooling scenarios, to elucidate which of them may be plausible in light of the known temporal and petrological constraints on early planetesimal evolution.
Jones Steven M.
Sanders Ian
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