THE DIVERSE SOLAR PHASE CURVES OF DISTANT ICY BODIES II. THE CAUSE OF THE OPPOSITION SURGES AND THEIR CORRELATIONS

We collect well-measured opposition surge properties for many icy bodies orbiting the Sun (mostly from our own observations) plus for many icy moons, resulting in a database of surface and orbital properties for 52 icy bodies. (1) We put forward four criteria for determining whether the surge is being dominated by shadow hiding or coherent backscattering (CB) based on readily measured quantities. The CB surge mechanism dominates if the surge is color dependent, the phase curve is steeper than 0.04 mag deg−1, the phase curve shape matches the CB model of Hapke, or if the albedo is higher than roughly 40%. (2) We find that virtually all of our sample have their phase curves dominated by CB at low phase angles. (3) We present a graphical method to determine the Hapke surge parameters BC0 and hC. (4) The Kuiper Belt Objects (KBOs) and Centaurs have relatively high surge amplitudes, BC0 ≳ 0.5 and widths with hC ∼ 3°. (5) We find highly significant but loose correlations between surge properties and the colors, albedos, and inclinations. We interpret this as young surfaces tending to have low surge slopes, high albedo, and gray colors. (6) Nereid has its surface properties similar to other icy moons and greatly different from KBOs and Centaurs, so we conclude that Nereid is likely a nearly ejected inner Neptunian moon rather than a captured KBO.