Investigating the extent of bladed terrain on Pluto via photometric surface roughness

Abstract

NASA's New Horizons spacecraft discovered fields of sub-parallel sets of steep ridges situated in the high-altitude, low-latitude regions in Pluto's encounter hemisphere called 'bladed terrain'. Thought to be formed due to sublimational erosion of methane ice, bladed terrain represents an active response of Pluto's landscape to current and past climates. The observation of a strong methane signature within the low latitudes of Pluto's non-encounter hemisphere points to the possibility that this terrain type is also present there. To test this hypothesis, in the absence of high resolution images of Pluto's non-encounter hemisphere, we employ photometric analysis of the methane rich regions. We specifically focus on determining the macroscopic surface roughness in selected images, whose photometric-effect can be apparent even in low-resolution images. We employ the `crater-roughness' photometric model of Buratti & Veverka (1985), which assumes that the surface is covered with parabolic depressions defined by a depth-to-radius ratio parameter $q$ (higher $q$ values correspond to rougher surfaces). Despite the high uncertainty in the retrieved roughness values from our analysis, we can safely conclude that the hypothesized bladed terrain region on the non-encounter hemisphere of Pluto is very rough ($q = 0.47_{-0.11}^{+0.10}$, 2$\sigma$), with the median roughness more than twice that of other broad regions of Pluto studied in this work, including the encounter-hemisphere bladed terrain region ($q = 0.21_{-0.18}^{+0.08}$, 2$\sigma$).