Albedo asymmetry of Iapetus

Abstract

Voyager images of Saturn's moon, Iapetus^1,2, confirm deductions made from Earth-based observations dating back to 1671 of a very dark leading hemisphere and a very bright trailing hemisphere^3–5. Figure 1 displays contours of surface albedo from three Voyager images. The darkest area is at the apex of orbital motion, with a pronounced (∼10×) increase in albedo towards the antapex, constituting the greatest interhemispheric albedo contrast known in the Solar System. The poles are brighter still. Figure 1 also shows that the albedo distribution resembles the calculated areal variation of the trans-saturnian impact flux⁶ remarkably closely. Dark areas correspond to regions with the highest calculated flux. We propose here that the dark areas contain organic chromophores produced in situ by UV irradiation of CH₄-rich ice, and that the albedo pattern results from ballistic redistribution of surface material in response to the impact flux gradient. Where the impact flux is high, net ablation will cause exposure of CH₄-rich darkenable ice, creating a dark surface. Where the flux is low, net accumulation of non-darkenable icy regolith that has lost CH₄ through repeated impact volatilization and evaporation, will create a bright surface.