How Iapetus is Painted by Saturn's Irregular Satellites

Details How Iapetus is Painted by Saturn's Irregular Satellites How Iapetus is Painted by Saturn's Irregular Satellites

Oct 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.0105t&link_type=abstract

American Astronomical Society, DPS meeting #42, #1.05; Bulletin of the American Astronomical Society, Vol. 42, p.942

Mathematics

Probability

Scientific paper

The leading hemisphere of Iapetus, Saturn's outermost regular satellite, is ten times darker than its trailing side. To explain this unique albedo distribution, Soter (1974) proposed that collisionally generated dust from the dark outer irregular satellite Phoebe has evolved inward due to radiation forces and coated Iapetus’ leading side. The recent discovery (Verbiscer et al. 2009) of the colossal Phoebe ring between the orbits of Iapetus and Phoebe indicates that Soter's mechanism is active at some level.
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To calibrate the effectiveness of this source, we follow the histories of Phoebe-ring dust under the relevant perturbations and thereby evaluate the particles’ probability of striking Iapetus, as well as the expected spatial distribution on the Iapetan surface. We find that, of the long-lived particles (those larger than 4 µm), those larger than 10 µm are virtually certain to strike Iapetus. Their calculated distribution across the surface matches well the measured albedo pattern in longitude. To explain the observed bright polar caps, our computed polar-dust-deposition rates must be overwhelmed by sublimation products from equatorial regions as proposed in the thermal runaway model of Spencer & Denk (2010); we thus constrain the latter model. We also track the dust originating from all the other known irregular moons, finding that a substantial fraction of the material from retrograde moons would eventually coat Iapetus--perhaps explaining why the spectrum of Iapetus’ dark material differs somewhat from Phoebe's (Buratti et al. 2005). We find that dust from lower-eccentricity moons with inclinations nearer 180° is more likely to strike Iapetus.
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Finally, of those dust particles that do not strike Iapetus, we find that most land on Titan, with a smaller fraction hitting Hyperion. As has been previously conjectured (Burns et al. 1996), such exogenous dust, coupled with Hyperion's chaotic rotation could explain Hyperion's roughly uniform, moderate-albedo surface.

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