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

NASA’s New Horizons spacecraft discovered fields of sub-parallel sets of steep ridges 20 situated in the high-altitude, low-latitude regions in Pluto’s encounter hemisphere called 21 ‘bladed terrain’. Thought to be formed due to sublimational erosion of methane ice, bladed 22 terrain represents an active response of Pluto’s landscape to current and past climates. 23 The observation of a strong methane signature within the low latitudes of Pluto’s non24 encounter hemisphere points to the possibility that this terrain type is also present there. 25 To test this hypothesis, in the absence of high resolution images of Pluto’s non-encounter 26 hemisphere, we employ photometric analysis of the methane rich regions. We specifically 27 focus on determining the macroscopic surface roughness in selected images, whose photometric28 effect can be apparent even in low-resolution images. We employ the ‘crater-roughness’ 29 photometric model of Buratti and Veverka (1985), which assumes that the surface is cov30 ered with parabolic depressions defined by a depth-to-radius ratio parameter q (higher 31 q values correspond to rougher surfaces). Despite the high uncertainty in the retrieved 32 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+0.10 −0.11 33 , 34 2σ), 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+0.08 −0.18 35 , 36 2σ).