Pluto’s Dark Heart? Albedos of Large Pits in Sputnik Planitia

The New Horizons spacecraft’s Long-Range Reconnaissance Imager (LORRI; Cheng et al. 2008) and Multispectral Visible Imaging Camera (Ralph/MVIC; Reuter et al. 2008) revealed that the bright nitrogen glaciers in the southern and western regions of Sputnik Planitia are littered with large pits with relatively dark bottoms (Stern et al. 2015). We have cataloged 317 pits in the highest resolution (76-234 m/pix) LORRI images with lengths >1 km and widths >350 m. Most of these pits are oriented roughly north-south and have length/width ratios of 2-4. Accounting for local lighting conditions and assuming a pit depth of 250 m (P. Schenk, pers. comm.), at least 245 (77%) of the cataloged pits have unshadowed substrate visible on their bottoms.

We compare the properties of the unshadowed pit bottoms with those of the relatively dark regions on crater rims and floors in Burney Basin, which is located ~1000 km northwest of Sputnik Planitia. We find that the Sputnik pits and the dark regions in Burney have similar colors in MVIC images (655 m/pix), and are intermediate in color between the glaciers of Sputnik and the dark maculae (i.e., Cthulhu Macula).

Employing a simplified approach with respect to Hapke modeling of Pluto, we are able to reproduce the results of Protopapa et al. (2020), and confirm that single scattering albedo and spectral slope are highly correlated. We find that Sputnik pit bottoms (0.76 ± 0.07 at 607 nm) and dark regions in Burney (0.72 ± 0.09 at 607 nm) have very similar single scattering albedos that are, perhaps surprisingly, closer to that of Sputnik (0.99 ± 0.01 at 624 nm) than Cthulhu (0.32 ± 0.11 at 624 nm). We will discuss similarities and differences in terms of albedo and colors between the Sputnik pit bottoms and other dark regions across Pluto’s surface (e.g., Burney and Cthulhu) with the goal of shedding light on the processes that gave rise to these geologic structures.