Retrieval of Pluto’s Spectral Surface Albedo from New Horizons Measurements

The New Horizons flyby of Pluto has revolutionized our knowledge of the geology and atmosphere of this dwarf planet. The temperature of Pluto’s upper atmosphere is now understood to be much cooler than pre-flyby model predictions. Molecular nitrogen is the primary atmospheric constituent, while methane and carbon monoxide are also important trace species. Images of forward-scattered sunlight reveal numerous haze layers extending up to 200 km above the surface, possibly formed by the photolysis of these gaseous constituents.

We constrain the spectral variation of the surface albedo of Pluto from Multispectral Visible Imaging Camera (MVIC) images in the 860-910 nm region. This analysis employs the fast and accurate two-stream-exact-single-scattering (2S-ESS) radiative transfer model, which is particularly applicable to Pluto because of its low atmospheric opacity. A Markov-Chain Monte Carlo (MCMC) algorithm runs the forward radiative transfer model with different values of surface reflectance to find the solution with the maximum posterior probability, and also estimates the distribution of the posterior probability, thereby providing an assessment of the retrieval uncertainty. The near-infrared spectral region has minimal gaseous absorption, simplifying the analysis. The haze opacities and single scattering properties are obtained from a morphology retrieval using observations from multiple instruments onboard New Horizons. We will show results from a Lambertian approximation for the surface reflection and compare them with those obtained using a Hapke model. The successful retrieval of the true albedo of Pluto’s surface will be a major step forward for understanding the chemical nature of the surface material, which, coupled to a subsurface ocean on Pluto, may provide a habitable environment.