Towards a Comprehensive Explanation of Iapetus' Albedo Asymmetry

Author: Charles A. Peterson
November 8, 2010
Journal Article
Bulletin of the American Astronomical Society

Peterson, C.A. (2010) Towards a Comprehensive Explanation of Iapetus’ Albedo Asymmetry, BAAS 42, 956.

Saturn's satellite Iapetus, which is 6 times brighter on its trailing hemisphere than on its leading hemisphere, has an albedo asymmetry that is unique in our Solar System. From the time of its discovery by Cassini in 1671 until the present, no explanation for this extreme appearance has gained general acceptance. An updated hypothesis, originally proposed in 1975 (Icarus 24, 499-503) and revisited in light of recent observations, is presented with the central assumption that Iapetus started out with a dark surface that continues to be coated asymmetrically with bright reflective water ice. This ice is stable for long periods of time in the region of the outer Solar System that includes Saturn and, therefore, can be conveyed to the satellite's surface by three distinct populations of exogenic impactors whose sources are all exterior to Iapetus’ orbit. Evidence for the existence of these populations is provided by the recent discovery by the Spitzer Observatory of a very large dust ring orbiting Saturn at the distance expected for ejecta from its 38 known irregular satellites including Phoebe. Although just one particular impactor population could account for a hemispherical albedo asymmetry, the combined effect of all three populations acting together enhances and characterizes this asymmetry even further. Relevant aspects of impact dynamics involving the collision of these ice-bearing particles with Iapetus’ surface will be summarized. Proving this revised hypothesis will require the extensive modeling of ejecta modification and transport on the surface of Iapetus. Such an effort should contribute significantly towards not only resolving this 300-year old problem in planetary science, but also providing a better understanding of the much less extreme hemispheric asymmetries observed for most other regular satellites in the Solar System.