Determining the Elastic Thickness of Sputnik Planitia on Pluto and its Surrounding Using Topography and Inverse Theory

The 2014 New Horizons flyby of Pluto revealed an unexpected feature, Sputnik Planitia, a 1,300 km by 500 km elliptical basin with a topographic deflection of about 3 km. The question of the origin of Sputnik Planitia arose as soon as the basin was discovered, and thus far, two major hypotheses have been proposed. The first hypothesis is that an impactor created the basin and subsequent deposition of ice proceeded (Moore et al., 2016). The second hypothesis was that nitrogen ice accumulated at the surface of Pluto and a feedback between Pluto's orientation and nitrogen deposition produces a single ice cap that is large enough to deform the crust and form a basin (Hamilton et al., 2016). Much of that nitrogen ice must be gone today. The ice cap hypothesis implies that the lithosphere of Pluto would have been flexed over large distance. Therefore, we evaluate here whether the current topography of Sputnik Planitia and its surroundings shows evidence for an bulge that would have formed in the context elastic flexure induced by a large nitrogen ice load inside Sputnik Planitia. First, an inversion model based on the superposition of numerous loads in a distributed area was tested using a synthetic model of elastic flexure and was determined to be able to extract flexural parameters that were used to find elastic thickness. Then, a set of 19 one-dimensional topographic profiles were extracted across the rim of Sputnik Planitia to evaluate best-fitting flexural parameter, which is then converted to elastic thickness estimate. Complications arise from defining the initial elevation of the surrounding plains and also the presence of crater and other degradation of the plains. Some profiles do not show evidence for a flexural signal. They can be flat right up to Sputnik Planitia, or have an essentially regular slope over several hundred km. However, 13 out of the 19 profiles show a clear flexural bulge implying elastic thicknesses of 11 to 40 km. Most of these profiles are located across the eastern or western edge of Sputnik Planitia.