Viscous relaxation of Oort and Edgeworth craters on Pluto: Possible indication of early high heat flows

We use upper envelope, quantile regressions to estimate the depth-diameter relation for pristine complex craters on Pluto, based on the rim-to-floor depths in Robbins et al. (Icarus 356, 2020) derived from the New Horizons stereo DEM (Schenk et al., Icarus 314, 2018). Extrapolating to smaller craters, and assuming a depth/diameter for simple craters on Pluto of 0.2, we estimate a simple-to-complex transition between 7.5 and 11.5 km diameter— a range consistent with the morphological transitions quantified in Robbins et al. Other than the Sputnik and Burney basins, the largest identified craters on the encounter hemisphere are 80-km Elliot, 115-km Oort, and 140-km Edgeworth. Notably, they are all anomalously shallow compared with our derived depth-diameter relations. Elliot is infilled with bright (CH₄- and presumably N₂-rich) ice, which can account for its shallowness. Edgeworth on the other hand is particularly shallow, and its floor appears bowed up above the original ground plane, a classic hallmark of viscous relaxation in a surface whose viscosity decreases rapidly with depth. We estimate a pristine (immediate post-impact) depth for Edgeworth of at least 6.2 km, which when compared with its present depth of ~1 km implies a relaxation fraction (RF) of at least 84%. Oort is intermediate, and if its depth of 2.4 km is due to viscous relaxation, then its RF is at least 57%. The difference in RF (and morphology) between the two is somewhat puzzling, as both craters lie in dark, western Cthulhu, and are similar enough in size, location (only 400 km apart), and apparent age (morphological preservation) that one suspects they resulted from the impact of a (tight) Kuiper belt binary. But we have no explicit evidence that they are in fact coeval. Possibly Oort is somewhat younger and was less affected by an early epoch of high heat flow. Finite element calculations show that this heat flow would have to have been substantial, far above steady-state radiogenic values. Such constraints are always subject to uncertainties in the subsurface conductivity structure, however. We expect fracturing and porosity to accompany complex crater formation and collapse, reducing thermal conductivity, but interstitial subsurface nitrogen (gas and possibly liquid) may compensate.