Revisiting Iapetus following recent Cassini observations

Abstract

The recent observations of the Saturnian satellite Iapetus by the Cassini spacecraft support the theory proposed by us, over three decades ago, explaining the large brightness asymmetry between the leading and trailing faces of this synchronous satellite. Here we will revisit the question of the observed brightness variations, with orbital phase, not only of the outermost large Saturnian satellite Iapetus but also of the large inner ones (Tethys, Dione, Rhea, and Titan). While Titan (which has a significant atmosphere) shows no brightness variation with orbital phase, the remaining inner satellites show small but definite brightness variations with orbital phase, which, curiously, are in the sense opposite to that of Iapetus. In other words, while the leading face of Iapetus is much darker than its trailing face, the leading faces of the large inner satellites (with the exception of Titan) are slightly brighter. Here we provide an explanation of the latter effect too by invoking the impact on the trailing faces of these inner satellites by a class of negatively charged dust grains in the E-ring, moving in near-circular orbits, with super Kepler speeds.