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

TL;DR

This study used near-infrared spectroscopy from 2014 to 2017 to detect surface composition changes on Pluto, revealing increased methane absorption features likely due to nitrogen sublimation before the 2029 summer solstice.

Contribution

It provides the first temporal analysis of Pluto's surface composition over multiple years using matched hemisphere observations, highlighting volatile-driven surface changes.

Findings

Methane band areas increased significantly between 2014 and 2015.

No significant shifts in methane band centers were observed.

Surface changes are consistent with nitrogen sublimation before summer solstice.

Abstract

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 $\sim$14 months apart, thus removing the effects of viewing geometry and rotation phase. Comparison of the CH$_4$ (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 $\mu$m CH$_4$ absorption features exhibited a $>$5-$\sigma$ increase between 2014-06-17 and 2015-08-19, corresponding to a sub-observer hemisphere centered at $\sim$280$^{\circ}$E, with the latter date only 1 month after the New Horizons flyby of Pluto. The majority of matched pairs were obtained of the anti-Charon hemisphere, home to the bright, volatile-rich Sputnik Planitia, and did not present statistically significant changes in CH$_4$ band areas. CH$_4$ band center shifts, which provide information on the mixing state of CH$_4$ and N$_2$ 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 N$_2$ from the northern latitudes of Pluto in the lead-up to northern hemisphere summer solstice in 2029, leading to an increase in CH$_4$ concentration.