The Diverse Solar Phase Curves of Distant Icy Bodies. II. The Cause of the Opposition Surges and Their Correlations

TL;DR

This study analyzes the opposition surge properties of icy bodies and moons, identifying the dominant scattering mechanisms, and explores correlations with surface and orbital characteristics to understand their surface properties and origins.

Contribution

It introduces criteria to distinguish shadow hiding from coherent backscattering as the surge mechanism and provides a graphical method to determine Hapke parameters, advancing surface property analysis.

Findings

CB dominates the opposition surge in most icy bodies at low phase angles.

KBOs and Centaurs exhibit high surge amplitudes and specific phase curve widths.

Correlations suggest young surfaces have low surge slopes, high albedo, and gray colors.

Abstract

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 data base 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 (SH) 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, 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 B_C0 and h_C. (4) The Kuiper Belt Objects (KBOs) and Centaurs have relatively high surge amplitudes, B_C0 > ~0.5 and widths with h_C ~ 3 deg. (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.