Spatial Distribution and Secular Evolution of Pluto's Ices as Seen from the Ground and Space

An up-close, detailed snapshot of the complex cryogenic world of Pluto was revealed by NASA's New Horizons mission (Stern et al. 2015, Science 350). The striking surface heterogeneity of this large trans-Neptunian object is due in part to the seasonal transport of three volatile ices: nitrogen (N2), methane (CH4), and carbon monoxide (CO). In order to better understand the processes responsible for the sublimation and redistribution of these volatile ices on Pluto, a broad seasonal context is necessary. This can only be achieved through analysis of data monitoring Pluto over timescales of years.

With the goal of deriving information on the role that each volatile ice plays in sculpting the variable surface composition of Pluto, we performed an analysis of IRTF/SpeX near-infrared data covering the timeframe from 2001 to 2021 (Grundy et al. 2013, Icarus 223; Grundy et al. 2014, Icarus 235) and New Horizons spectral maps acquired in 2015. Our analysis relies on the evaluation of spectral parameters, including band depth and equivalent width and spectral modeling adopting a modern radiative transfer code (Hapke 2012, Icarus 221; Protopapa et al. 2017, Icarus 287). We interpret the varying spectral profile of Pluto in terms of variations in mixing ratio and/or path length of Pluto's volatile ices. Overall, these results (1) enable us to interpret the temporal evolution and spatial distribution of N2, CH4, and CO over two decades and (2) permit a direct linkage between spectral features in disk integrated spectra acquired in 2015 and geological units revealed by New Horizons compositional maps (e.g., Protopapa et al. 2017, Icarus 287; Gabasova et al. 2021, Icarus 356).

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).