Abstract
Abstract We present the first measurements of Charon’s far-ultraviolet (FUV) surface reflectance, obtained by the Alice spectrograph on New Horizons. We find no measurable flux shortward of 1650 Å, and Charon’s geometric albedo is <0.019 (3σ) at 1600 Å. From 1650 to 1725 Å, Charon’s geometric albedo increases to 0.166 ± 0.068 and remains nearly constant until 1850 Å. As this spectral shape is characteristic of H2O ice absorption, Charon is the first Kuiper Belt object with a H2O ice surface to be detected in the FUV. Charon’s geometric albedo is ∼3.7 times lower than Enceladus’s 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.
Highlights
NASA’s New Horizons mission completed its historic flyby of the Pluto system in July 2015 (Stern et al 2015a)
We take the ratio of Charon’s radiance to the radiance of a Lambertian surface normally illuminated by the Sun (i.e., F /π) to derive Charon’s surface reflectance (I/F )
We find that Charon has the upturn in reflectance at 1650 Å that is diagnostic of H2O ice
Summary
NASA’s New Horizons mission completed its historic flyby of the Pluto system in July 2015 (Stern et al 2015a). Steffl et al (2020) found that Pluto has a wavelength-independent surface reflectance (I/F ) of 0.17 between 1400 and 1850 Å. Pluto’s FUV reflectance is in the middle of the range found for other icy bodies. We report the first measurements of Charon’s FUV surface reflectance, measured by New Horizons’ Alice ultraviolet spectrograph (Stern et al 2008). Unlike Pluto, Charon’s surface is composed primarily of H2O ice (Buie et al 1987; Grundy et al 2016), so its FUV reflectance should more closely resemble that of the icy satellites of the giant planets. C Variance-weighted mean I/F of Charon from 1750–1850 Å; the first uncertainty is the random uncertainty and the second is the systematic uncertainty (see Section 3.1)
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