Spectrophotometry and organic matter on Iapetus .2. Models of interhemispheric asymmetry

The albedo asymmetry of Iapetus is unique in the Solar System. Many models have been proposed to explain why the leading hemisphere has a reflectance 10-20 times lower than the trailing hemisphere and the poles. Compositional and observational constraints appear to rule out many of these models. Little attempt has been made to explain why, of all the moons in the Solar System, only Iapetus displays such properties. The photometric differences between Phoebe and the dark material on Iapetus, and the expectation that accreted dust from Phoebe would darken the poles more than the antapex of orbital motion on Iapetus, make the Phoebe dust model inconsistent with observations. Other models that instead have Iapetus accreting circumsaturnian material coming from either Hyperion or Iapetus itself are also beset by an inability to explain the bright poles. The endogenous extrusion of dark organics, by itself, is ruled out, since it is extremely unlikely that this model would produce elliptical albedo contours centered on the apex of orbital motion. The modern impact flux does not deliver enough kinetic energy to Iapetus's surface to vaporize significant quantities of water ice and produce a lag deposit. Also, impact vaporization is expected to form a similar lag deposit on the trailing hemisphere. Our most optimistic estimate of the amount of organic material that accumulates on the surface from in situ radiation synthesis of solid organics from methane clathrate is more than sufficient to explain the albedo and spectrum. This process, however, is more effective on the trailing than leading hemisphere, so the reverse asymmetry is expected. Iapetus's asymmetry is best explained by a thick primordial low-albedo subsurface layer of organics exhumed by impact erosion. The layer may be the consequence of ancient thermochemistry in Iapetus's interior, processing methane and/or HCN into dark organic matter which was then extruded over the entire surface. The model requires a thin layer of nearly pure water ice, either exogenous or endogenous, deposited over the dark organics. Subsequent impacts would have preferentially eroded the ice on the leading hemisphere, revealing the underlying dark material. The uniqueness of Iapetus's albedo asymmetry can be understood by its impact-induced erosion rate, size, and formation distance from the Sun and Saturn-a combination of parameters duplicated nowhere else in the Solar System. Tests of these conclusions can be made by Cassini. (C) 1996 Academic Press, Inc.