A model for planetesimal meltdown by ²⁶Al and its implications for meteorite parent bodies

Abstract

Abstract-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 calciumaluminum-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.

Differentiation and core formation in accreting planetesimals

“…The existence of an unmelted upper layer further suggests that the maximal possible core radius, which depends on the iron fraction and can only be obtained for an entirely molten planetesimal, is never reached. In contrast to the assumption of Hevey & Sanders (2006) that the parent bodies of differentiated meteorites formed prior to the accretion of the most chondritic parent bodies, our results indicate that both chondritic meteorites and differentiated meteorites can originate from one and the same parent body. This finding is consistent to the results of Weiss et al (2010) and Elkins-Tanton et al (2011), where a differentiated interior is covered with undifferentiated crust to explain the remanent magnetisation of the CV meteorite Allende.…”

“…Hevey & Sanders 2006;Moskovitz & Gaidos 2011;Henke et al 2012). The later the onset time of accretion the larger must be the planetesimals for melting to occur.…”

“…Miyamoto et al 1981;Grimm & McSween Jr. 1993;Sahijpal et al 1995;Bennett & McSween Jr. 1996;Ghosh & McSween Jr. 1998Merk et al 2002;Ghosh et al 2003;Yoshino et al 2003;Sahijpal & Soni 2005;Bizzarro et al 2005;Hevey & Sanders 2006;Sahijpal et al 2007;Moskovitz & Gaidos 2011). The aim of these works is the study of the thermal metamorphism and/or melting of asteroids and planetesimals.…”

“…In a more recent work, Hevey & Sanders (2006) also incorporated convection in their thermal evolution models of planetesimals assuming that mantle regions where the degree of partial melting exceeded 50%, i.e. a magma ocean (Taylor et al 1993) would be convecting.…”

“…Ghosh & McSween Jr. (1998), Merk et al (2002), Moskovitz & Gaidos (2011), Sramek et al (2012 do not consider sintering of an initially porous planetesimal. Similarly, accretion is not modelled in Ghosh & McSween Jr. (1998), Hevey & Sanders (2006) and Moskovitz & Gaidos (2011). Velocities of melt migration are not calculated self-consistently but are either assumed to be constant (Sahijpal et al 2007) or do not depend on radius, i.e.…”