Title:

Early evolution of outer solar system planetesimals impact on satellite evolution

Authors:

T. V. Johnson (1), J. C. Castillo-Rogez (1), D. L. Matson (1), J. I. Lunine (2), M. Choukroun (1)

Abstract:

Different models have been proposed for the growth of planetesimals in the transneptunian region and asteroid belt. Growth times range from 0.1 to 10 My for a planetesimal about 100 km in radius. The formation timescale has consequences for the amount of physical and chemical alteration the planetesimals experience prior to their accretion into larger, objects, such as satellites, asteroids, transneptunian objects, and dwarf planets. It has been suggested that planetesimals as large as 100 km could have been accreted in satellites and asteroids (cf. Mosqueira and Estrada 2003, 2005; Bottke et al. 2009).

We present a detailed chemical, thermal and structural model of planetesimals in the size range 10-100 km. The end-member model is compared against the properties of Saturn’s irregular satellite Phoebe. We consider different initial conditions, especially the time of formation with respect to the production of calcium-aluminum inclusions and the nature of the ice (amorphous vs. crystalline with clathrate hydrates). From this model we infer constraints on the structure, temperature, and chemistry of the planetesimals at the time of their accretion into larger objects, whether these planetesimals preserved their integrity or resulted from the disruption of larger specimens. In this framework, icy satellites most probably accreted a mixture of water ice and second-phase volatile impurities, salt and clathrate hydrates, hydrated and dry silicates, organics, etc.

Forming Titan in this context leads to a model that is differentiated in a core dominated by hydrated silicate and an icy shell enriched in low-eutectic impurities. The loss of long-lived radioisotopes from the core helps to keep it cool over the long-term, and leads to an interior model fully consistent with gravity data yielded by the Cassini Orbiter (Castillo-Rogez and Lunine, GRL, in press).

Acknowledgement: This work has been conduction at the Jet Propulsion Laboratory, California Institute of Technology under a contract with the National Aeronautics and Space Administration. Government sponsorship acknowledged.