Near-Infrared Spectral Monitoring of Pluto's Ices: Spatial Distribution and Secular Evolution

TL;DR

This study uses near-infrared spectral monitoring over a decade to analyze the spatial distribution and long-term changes of ices on Pluto's surface, revealing seasonal and secular variations in methane, carbon monoxide, and nitrogen ices.

Contribution

It provides the first long-term spectral dataset of Pluto's surface ices, showing their spatial distribution and secular evolution over 12 years.

Findings

CH4 absorption bands have deepened over time.

CO and N2 absorptions are declining, indicating distribution shifts.

Diurnal variations in ice absorption are observed and analyzed.

Abstract

We report results from monitoring Pluto's 0.8 to 2.4 {\mu}m reflectance spectrum with IRTF/SpeX on 65 nights over the dozen years from 2001 to 2012. The spectra show vibrational absorption features of simple molecules CH4, CO, and N2 condensed as ices on Pluto's surface. These absorptions are modulated by the planet's 6.39 day rotation period, enabling us to constrain the longitudinal distributions of the three ices. Absorptions of CO and N2 are concentrated on Pluto's anti-Charon hemisphere, unlike absorptions of less volatile CH4 ice that are offset by roughly 90{\deg} from the longitude of maximum CO and N2 absorption. In addition to the diurnal variations, the spectra show longer term trends. On decadal timescales, Pluto's stronger CH4 absorption bands have been getting deeper, while the amplitude of their diurnal variation is diminishing, consistent with additional CH4 absorption at high northern latitudes rotating into view as the sub-Earth latitude moves north (as defined by the system's angular momentum vector). Unlike the CH4 absorptions, Pluto's CO and N2 absorptions appear to be declining over time, suggesting more equatorial or southerly distributions of those species. Comparisons of geometrically-matched pairs of observations favor geometric explanations for the observed secular changes in CO and N2 absorption, although seasonal volatile transport could be at least partly responsible. The case for a volatile transport contribution to the secular evolution looks strongest for CH4 ice, despite it being the least volatile of the three ices.