Spatial Distribution and Secular Evolution of Pluto’s Ices as seen from IRTF/SpeX

NASA’s New Horizons mission has revealed the complex cryogenic world of Pluto (Stern et al. 2015, Science 350). The spatial heterogeneity of this large trans-Neptunian object results from a variety of processes, including the seasonal transport of nitrogen (N₂), methane (CH₄), and carbon monoxide (CO) ices. All three ices are indeed mobile at Pluto’s surface temperature of ~40K, with CO and N₂ being much more volatile than CH₄. Over seasonal timescales, varying patterns of insolation are thought to cause sublimation and redistribution of these volatile ices.

While New Horizons returned a detailed, up-close snapshot of Pluto in 2015 (Stern et al. 2015, Science 350), a broad seasonal context is needed to improve our understanding of the processes responsible for Pluto’s evolution. This can only be achieved through analysis of data that monitor Pluto over timescales of years. We present an extension of the analysis of the IRTF/SpeX data spanning the timeframe from 2001 to 2013 (Grundy et al. 2013, Icarus 223; Grundy et al. 2014, Icarus 235) to now include data up through 2021. This study relies on the evaluation of spectral parameters, including band depth and equivalent width. In addition, we have systematically modeled all IRTF/SpeX data with a modern radiative transfer code (Hapke 2012, Icarus 221; Protopapa et al. 2017, Icarus 287). This enables us to interpret the varying spectral profile of Pluto across the full data set in terms of variations in mixing ratio and/or path length of Pluto’s volatile ices. Pluto’s composition derived from the IRTF/SpeX ground-based measurements acquired in 2015 will be compared with the New Horizons composition maps of Pluto (e.g., Protopapa et al. 2017, Icarus 287; Gabasova et al. 2021, Icarus 356), permitting direct linkage between spectral features in disk integrated spectra and geological units revealed by New Horizons. Overall, this work will shed light onto the role played by each volatile ice in shaping the surface composition of Pluto and its temporal evolution over a two decade-long time period.

This work was funded by the NASA Solar System Workings and New Frontiers Data Analysis Program via contracts 80NSSC19K0554 and 80NSSC19K0821 to SwRI (PI: Protopapa).