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The melting of planetesimals heated by 26 Al has been modelled using a new finite difference method that incorporates convection. As an example, we consider a planetesimal with a radius of 64 km, which accretes instantaneously at t = 0.75 Myr (after the formation of calcium- aluminum-rich inclusions) from cold (250 K) nebular dust with 50% porosity. At t = 0.9 Myr (T = 700 K), the planetesimal shrinks to a radius of 50 km due to sintering. At t = 1.2 Myr (T = 1425 K), the fully insulated interior, deeper than a few kilometers, starts to melt, and at t = 1.5 Myr (T = 1725 K), with 50% melting, convection starts. By t = 2 Myr, the planetesimal is a globe of molten, convecting slurry inside a thin residual crust. From about t = 2.5 Myr, the crust thickens rapidly as the power of 26 Al fades. Planetesimals probably melt in this manner when they accrete before t = 1.3 Myr and are large enough to insulate themselves (R >20 km for accretion at t = 0, rising to >80 km at t = 1.3 Myr). Melting behavior will also be affected by the level of 60 Fe in nebular dust, by the extent of devolatilization reactions and basalt segregation during heating, and by gradual accretion. The model suggests that a) the parent bodies of differentiated meteorites had accreted before about t = 1.5 to 2 Myr and before most chondritic parent bodies had formed, and b) that molten planetesimals may be a source for chondrule melt droplets. |
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