Heating and Cooling of Early-Accreting Planetesimals and Effects on Dynamo Generation

Increasing evidence exists for partial differentiation in planetesimals in the early solar system, and by extension, for the existence of partially differentiated asteroids today. Remanent magnetism in angrites and in Allende indicates they had core dynamos, requiring internal melting and core formation. Vesta, Ceres, Pallas, and Lutetia have densities and moments of inertia consistent with internal differentiation. Lutetia also has a largely intact chondritic lid overlying a possibly differentiated interior. Though metamorphism and melting of planetesimals through radiogenic heat in the first few million years after CAIs have been modeled by a variety of researchers over the past 30 years, new constraints of a magnetic dynamo are forcing more complex and nuanced models. Our understanding of the physics and chemistry of planetesimal melting and solidification processes, however, are incomplete, such that multiple plausible processes may be posited for the same steps in planetesimal evolution. Here we discuss the processes of melting and solidification with emphasis on the areas of least constraint, and their possible consequences for the physical and chemical structure of the resulting planetesimal, and for its magnetic dynamo generation. Some of the least constrained but most important processes are release and transport of hydrous fluids, rise and eruption of silicate magma, development and cessation of convective heat transport in the interior magma ocean, settling of crystals from fluid flow, growth or erosion of any conductive lid, and patterns of growth of the solids in the iron-nickel core. These processes have implications for planetesimal heat flow, and therefore for the intensity and duration of dynamo activity. The resulting planetesimal radii and bulk densities vary by total mass, time of accretion, bulk radiogenic content, and, of course, the physical processes expected during heating and cooling. Possible radii and densities are presented, with the intention that they will inform our observations of asteroids.