New Investigations of Dark-floored Pits In the Volatile Ice of Sputnik Planitia on Pluto

Sputnik Planitia, Pluto's gigantic, volatile ice glacier, hosts numerous scientific mysteries, including the presence of thousands of elongated pit structures. We examine various attributes of these pit structures in New Horizons data sets, revealing their length, aspect ratio, and orientation properties; we also study their reflectivities, colors, and compositions, and compare these attributes to some other relevant regions on Pluto. We then comment on origin mechanisms of the pits and also the fate of the missing volatiles represented by the pits on Sputnik Planitia. From a sample of 317 pits, we find typical length/width ratios of 2-4, with their major axis preferentially oriented approximately north-south. We also find that the floors of large pits in our sample have similar single-scattering albedos and colors to dark material on crater rims and floors (i.e., possible subsurface windows) in Burney basin. We also find that the base of the three pits in our sample, large enough to study with LEISA IR spectroscopy, display both CH4 and N-2 absorption features, as do the dark regions in crater windows in Burney basin. Evidence for a sublimation erosion origin for the pits is supported over both the explosion/ejecta venting and structural collapse alternatives. Finally, we find that the mass lost by the pits on Sputnik Planitia most likely lies condensed elsewhere, on Pluto's surface, relocated there by volatile transport as opposed to removal by escape to space or photochemical conversion.