Sputnik Planitia (SP), also known as the western portion of Pluto’s heart, is a high albedo depression on Pluto composed of nitrogen, methane and carbon monoxide ices (Grundy et al. 2016; Protopapa et al. 2017). It is thought that Pluto’s atmosphere is the source of these volatile ices, which condensed out of the atmosphere and were deposited on the surface over time as a high albedo unit. Adjacent to SP are several regions with lower albedos, including Cthulhu Macula, Viking Terra and Venera Terra. Within SP there are numerous pits of varying sizes, hypothesized to have formed through sublimation, that have a base unit composed of a lower albedo material (Stern et al. 2015; Keeney et al. 2020). We investigated the compositional characteristics of the base of three large pits within SP in order to constrain the origin of the material and to test the hypothesis that the base of the pits is composed of the same material as Cthulhu Macula or another surrounding dark maculae.
We investigated the composition of the base of the pits using spectral data from the New Horizons Ralph instrument (Reuter et al. 2008), which consists of a visible multicolor/ panchromatic mapper, the Multispectral Visible Imaging Camera (MVIC) and a mapping IR composition spectrometer, the Linear Etalon Imaging Spectral Array (LEISA). LEISA spectra from the base of the pits have spectral signatures indicating the presence of methane and nitrogen but a lack of spectral signatures of water ice. Furthermore, the LEISA spectra of the base of the three large pits we studied are distinctly different from spectra of Cthulhu Macula and Viking Terra. Observed spectral signatures may indicate that the composition of the base of these pits is similar to that of Sputnik Planitia. Alternatively, the observed spectra may be optically contaminated by Sputnik Planitia and we may not be capturing the true composition of the base of the pits. Further tests with LEISA data will help discern whether or not this spectral contamination is occurring. The dark albedo and red color of the base of the pits (Keeney et al. 2020) may indicate that they are partially composed of tholins, similar to Cthulhu Macula or another surrounding dark maculae. Future work will incorporate lower wavelength MVIC data to compare the composition of the base of the pits to the spectral signature of tholins.