Measuring temperature and ammonia hydrate ice on Charon in 2015 from Keck/OSIRIS spectra

TL;DR

This study used Keck/OSIRIS spectra from 2015 to analyze the surface ices on Charon, finding a uniform temperature around 45K and detecting ammonia hydrate, suggesting equilibrium processes and diffusion mechanisms.

Contribution

First detailed spectral analysis of Charon's surface ices from 2015 data, revealing temperature stability and ammonia presence without requiring cryovolcanism.

Findings

Mean ice temperature is 45±14 K with no significant variation.

Ammonia hydrate detected across a wide longitude range.

Crystalline ice likely reached equilibrium over 3.5 Gyr ago.

Abstract

In this work we investigated the longitudinal (zonal) variability of H$_2$O and ammonia (NH$_3$) hydrate ices on the surface of Charon through analysis of the 1.65 $\mu$m and 2.21 $\mu$m absorption features, respectively. Near-infrared spectra presented here were obtained between 2015-07-14 and 2015-08-30 UT with the OSIRIS integral field spectrograph on Keck I. Spectra centered on six different sub-observer longitudes were obtained through the Hbb (1.473-1.803 $\mu$m) and Kbb (1.965-2.381 $\mu$m) filters. Gaussian functions were fit to the aforementioned bands to obtain information on band center, band depth, full width at half maximum, and band area. The shift in the band center of the temperature-dependent 1.65 $\mu$m feature was used to calculate the H$_2$O ice temperature. The mean temperature of the ice on the observable portion of Charon's surface is 45$\pm$14 K and we report no statistically significant variations in temperature across the surface. We hypothesize that the crystalline and amorphous phases of water ice reached equilibrium over 3.5 Gyr ago, with thermal recrystallization balancing the effects of irradiation amorphization. We do not believe that cryovolcanism is necessary to explain the presence of crystalline water ice on the surface of Charon. Absorption from ammonia species is detected between 12$^{\circ}$ and 290$^{\circ}$, in agreement with results from New Horizons. Ongoing diffusion of ammonia through the rocky mantle and upper layer of water ice is one possible mechanism for maintaining its presence in Charon's surface ice. Reduced Charon spectra corrected for telluric and solar absorption are available as supplementary online material.