Evaluation of short-term temporal evolution of Pluto's surface composition from 2014-2017 with APO/TripleSpec

In this work we present the results of a spectral observing campaign of Pluto to search for temporal changes in surface composition on 1- to 3-year timescales. Near-infrared spectra of Pluto were obtained from June 2014 to August 2017 with the TripleSpec cross-dispersed spectrograph at the Apache Point Observatory's 3.5-meter Astrophysical Research Consortium (ARC) telescope. Observations were requested in order to obtain spectra of approximately the same sub-observer hemisphere ∼14 months apart, thus removing the effects of viewing geometry and rotation phase. Comparison of the CH₄ (methane) band areas and band center shifts between each component of these "matched pairs" revealed a surface in transition. Band areas for the 1.66 and 1 . 72 μm CH₄ absorption features exhibited a >5- σ increase between 2014-06-17 and 2015-08-19, corresponding to a sub-observer hemisphere centered at ∼280°E, with the latter date only 1 month after the New Horizons flyby of Pluto. The majority of matched pairs were obtained of the anti-Charon hemisphere, home to the bright, volatile-rich Sputnik Planitia, and did not present statistically significant changes in CH₄ band areas. CH₄ band center shifts, which provide information on the mixing state of CH₄ and N₂ in solid solution, were calculated between components of each matched pair, with no significant band shifts detected. The favored explanation for these combined results is the sublimation of more-volatile N₂ from the northern latitudes of Pluto in the lead-up to northern hemisphere summer solstice in 2029, leading to an increase in CH₄ concentration.