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In this work we investigated the longitudinal (zonal) variability of H$_2$Oand ammonia (NH$_3$) hydrate ices on the surface of Charon through analysis ofthe 1.65 $\mu$m and 2.21 $\mu$m absorption features, respectively.Near-infrared spectra presented here were obtained between 2015-07-14 and2015-08-30 UT with the OSIRIS integral field spectrograph on Keck I. Spectracentered 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 functionswere fit to the aforementioned bands to obtain information on band center, banddepth, full width at half maximum, and band area. The shift in the band centerof the temperature-dependent 1.65 $\mu$m feature was used to calculate theH$_2$O ice temperature. The mean temperature of the ice on the observableportion of Charon's surface is 45$\pm$14 K and we report no statisticallysignificant variations in temperature across the surface. We hypothesize thatthe crystalline and amorphous phases of water ice reached equilibrium over 3.5Gyr ago, with thermal recrystallization balancing the effects of irradiationamorphization. We do not believe that cryovolcanism is necessary to explain thepresence of crystalline water ice on the surface of Charon. Absorption fromammonia species is detected between 12$^{\circ}$ and 290$^{\circ}$, inagreement with results from New Horizons. Ongoing diffusion of ammonia throughthe rocky mantle and upper layer of water ice is one possible mechanism formaintaining its presence in Charon's surface ice. Reduced Charon spectracorrected for telluric and solar absorption are available as supplementaryonline material.

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