Dynamics in Black and White: The Contamination of Iapetus by Phoebe Dust

Details Dynamics in Black and White: The Contamination of Iapetus by Phoebe Dust Dynamics in Black and White: The Contamination of Iapetus by Phoebe Dust

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996dps....28.0428w&link_type=abstract

American Astronomical Society, DPS meeting #28, #04.28; Bulletin of the American Astronomical Society, Vol. 28, p.1076

Astronomy and Astrophysics

Astronomy

Scientific paper

Iapetus, the second-most distant moon of Saturn, is a tidally-locked satellite with a black hemisphere in front (i.e., in the direction of its orbital motion) and a white hemisphere in back. One explanation for this remarkable albedo asymmetry suggests that dust launched from Saturn's dark outermost moon Phoebe impacts the leading face of Iapetus, painting it black (Soter 1974, Burns \etal 1995). Radiation pressure is important for the motions of micron-sized dust around planets; it imparts high eccentricities to dust grain orbits and allows Phoebe dust to coat more than 180(deg) of longitude on Iapetus as is observed. Poynting-Robertson drag (PR-drag), also due to interactions between dust and solar photons, brings dust grains inward from Phoebe to Iapetus. Here we employ new numerical and analytical techniques to combine radiation pressure and drag effects in a realistic way. We start with the orbit-averaged Hamiltonian formulation (Hamilton and Krivov 1996) and study the evolution of 2D dust orbits in phase space, i.e., polar plots of e and phi_sun (where e is eccentricity and phi_sun is the angle between pericenter and the Sun). The Hamiltonian is conserved when radiation pressure acts alone, but not when PR-drag is included. For a weak drag affecting only the orbital semimajor axis, however, we find that an adiabatic invariant exists. Our analytic derivation, which has been confirmed by numerical integrations, shows that the area enclosed by the grain's phase space trajectory - the adiabatic invariant - is an approximate constant of its motion. Use of this invariant allows the trajectory of a dust grain evolving inward to Iapetus to be predicted analytically from its initial launch conditions at Phoebe. PR-drag also also affects orbital eccentricities; this causes the area enclosed by a grain's phase space trajectory to shrink in time, collapsing toward an equilibrium orbit with non-zero eccentricity. Combining radiation pressure with the full representation of PR-drag, we characterize the ensemble of orbits at Iapetus from grains launched all along Phoebe's orbit. While we focus on Phoebe and Iapetus, our methods can be applied to a range of Solar System problems.

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