Titan tholin like materials across the surface of Pluto

Pluto presents in enhanced visible color images acquired with the New Horizons' Multi-spectral Visible Imaging Camera (MVIC, Reuter et al. 2008) a wide range of colors from vivid red, brown, to yellow colors, highly correlated with Pluto's varied underlying geological structures (Stern et al. 2015; Olkin et al. 2017). The color contrast is less obvious in natural-color images. Tholins, which are the refractory residues obtained from the irradiation of gases and ices containing hydrocarbons (Cruikshank 2005), are thought to be present on the surface of Pluto, serving as coloring agents (e.g., Stern et al. 2018). However, the number of distinct types of tholins on the surface of Pluto, and the processes responsible for their formation and distribution remains subject of investigation. We investigate this problem by means of 1) a multi-wavelength, regionally dependent photometric analysis of Pluto's encounter hemisphere using the color images collected by the Ralph/MVIC instrument on board of New Horizons at four visible wavelengths from 400 nm to 910 nm and 2) analysis, using a multiple-scattering radiative transfer model (Hapke, 2012), of combined MVIC and LEISA (a mapping infrared composition spectrometer covering the wavelength range 1.25-2.50 µm) spectra of eastern Cthulhu and Lowell Regio. Cthulhu and Krun Maculae are significantly darker and redder than the rest of the surface. Regions dominated by volatile ices such as the yellow material across Pluto's north pole observed in enhanced color images present single scattering albedos of ∼ 0.98 or higher, and almost neutral across the visible wavelength range. This result indicates a very limited contribution of tholin materials on the optically active surfaces in these regions. We use a tholin material with optical constants very similar to that of Titan tholin by Khare et al. (1984) to reproduce the spectral properties of these two regions with such diverse coloration, compositions, morphologies, and ages. Because a single pigment can be used to account for all of Pluto's colors and this is consistent with a Titan tholin like material, we concur with the idea suggested first by Grundy et al. (2018) that Pluto's coloration is the result of photochemical products mostly produced in the atmosphere. Although cosmic rays and ultraviolet photons at wavelengths longer than 145 nm do reach Pluto's surface, and can be expected to drive chemical processing there, the observations of diverse colors do not require different chemical products to be responsible for the colors in different environments.