The taxonomy of planetesimals: Consequences for planets

Introduction In past decades, planetary differentiation was posited to be a simple three-step process: accretion, core formation, and melting to form a crust. Now we know that differentiation is a far more complex process, that it occurred in very small early bodies long before completion of rocky planet formation, and that it has a variety of partial end-states. Iron meteorites demonstrate the existence of differentiated planetesimals in the first 500 000 years after solids formed in the disk (Scherstén et al., 2006), and (4) Vesta has differentiated into a metal core and silicate mantle (Raymond et al., Chapter 15, this volume). Johansen et al. (2015) suggest the icy asteroids formed 2 to 4 Myr after calcium-aluminum-rich inclusions (CAIs) (Castillo-Rogez and Young, Chapter 5, this volume). The breakthrough of pebble accretion, which shows that pebble-sized objects accrete to larger objects extremely efficiently through gravitational perturbation of their orbits, indicates that accretion timescale could have been as short as a few thousand years for 100-km-diameter objects (Johansen et al., 2014). This extremely short timescale supports the use of simple models that assume nearly instantaneous accretion relative to the timescale of ²⁶Al heating, though pebble accretion would have continued past the point of ²⁶Al activity, and coated the young planetesimals with unmelted rinds (A. Johansen, 2016, personal communication). Processes of Differentiation in Planetesimals All processes of differentiation begin with heating. Heating in planetesimals is the result of decay of ²⁶Al (Urey, 1955); impacts are inefficient at raising temperature globally in such small and porous bodies. Models indicate that heating may have been rapid and extreme (Hevey and Sanders, 2006), but heating is moderated by ²⁶Al content (Krot et al., 2012), ice content (Castillo-Rogez and McCord, 2010), and by the rate of accretion (see Castillo-Rogez and Young, Chapter 5, this volume). Metallic core formation relies on the ability of the separated, small metal blebs in the primitive material to flow together and downward in the small gravity field of the planetesimals. The Fe-FeS eutectic occurs at 31 wt% sulfur (equivalent to 85% FeS and 15% Fe) and at a temperature of 988 °C (Brett and Bell, 1969).