On Charon's Far-Ultraviolet Surface Reflectance

TL;DR

This study presents the first measurements of Charon's far-ultraviolet surface reflectance, revealing water ice absorption features and unique phase behavior, which enhances understanding of its surface composition and comparison with other icy bodies.

Contribution

First measurement of Charon's far-ultraviolet reflectance, identifying water ice features and comparing its surface properties with other icy moons.

Findings

Charon's albedo is less than 0.019 at 1600 A.

Water ice absorption is evident in the spectral shape.

Charon's phase behavior differs from Saturnian satellites.

Abstract

We present the first measurements of Charon's far-ultraviolet surface reflectance, obtained by the Alice spectrograph on New Horizons. We find no measurable flux shortward of 1650 A, and Charon's geometric albedo is $<0.019$ ($3\sigma$) at 1600 A. From 1650--1725 A Charon's geometric albedo increases to $0.166\pm0.068$, and remains nearly constant until 1850 A. As this spectral shape is characteristic of H$_2$O ice absorption, Charon is the first Kuiper belt object with a H$_2$O ice surface to be detected in the far-ultraviolet. Charon's geometric albedo is $\sim3.7$ times lower than Enceladus' at these wavelengths, but has a very similar spectral shape. We attribute this to similarities in their surface compositions, and the difference in absolute reflectivity to a high concentration or more-absorbing contaminants on Charon's surface. Finally, we find that Charon has different solar phase behavior in the FUV than Enceladus, Mimas, Tethys, and Dione, with a stronger opposition surge than Enceladus and a shallower decline at intermediate solar phase angles than any of these Saturnian satellites.