Comparative Mapping of Pluto's Sub-Charon Hemisphere: Three Least Squares Models Based on Mutual Event Lightcurves

Abstract

Photometric observations of Pluto-Charon transit events obtained at the University of Texas McDonald Observatory from 1985 through 1990 enable us to construct a normal reflectance map for the Charon-facing hemisphere of Pluto. We apply a least squares procedure within three separate models (spherical harmonic, polynomial, and finite element) to solve for Pluto's surface brightness distribution. A singular valve decomposition (SVD) algorithm is used to implement all of the least squares fits. The SVD method robustly indicates when pairs of fitted parameters are nearly linearly dependent. Through the computation of singular values and the covariance matrix we can thus demonstrate the linear independence of all solutions and rigorously propagate the formal uncertainties associated with the brightness value for each surface element. All three models produce similar mapping solutions showing a large, very bright region over the south pole, a dark band over the mid-southern latitudes, and a bright band over the mid-northern latitudes. Values for the normal reflectances range from 0.2 to 1.0. As the reflectance value of the higher northern latitudes is about the same as Pluto's average, we do not find evidence of a bright cap over the north polar region. We find excellent agreement with the independent map produced by M. W. Buie, D. J. Tholen, and K. Horne (1992, Icarus 97, 211-227), giving us confidence that the deconvolution of mutual event lightcurves yields a reliable resolved view of Pluto. This view allows us to begin making a physical interpretation of the evolution of Pluto's surface.