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Evidence for longitudinal variability of ethane ice on the surface of Pluto

We present the results of an investigation using near-infrared spectra of Pluto taken on 72 separate nights using SpeX/IRTF. These data were obtained between 2001 and 2013 at various sub-observer longitudes. The aim of this work was to confirm the presence of ethane ice and to determine any longitudinal trends on the surface of Pluto. We computed models of the continuum near the 2.405 μm band using Hapke theory and calculated an equivalent width of the ethane absorption feature for six evenly-spaced longitude bins and a grand average spectrum. The 2.405 μm band on Pluto was detected at the 7.5-σ level from the grand average spectrum. Additionally, the band was found to vary longitudinally with the highest absorption occurring in the N$_2$-rich region and the lowest absorption occurring in the visibly dark region. The longitudinal variability of $^{12}$CO does not match that of the 2.405 μm band, suggesting a minimal contribution to the band by $^{13}$CO. We argue for ethane production in the atmosphere and present a theory of volatile transport to explain the observed longitudinal trend.