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M.W. Buie, W.M. Grundy (Lowell Obs.), E.F. Young, L.A. Young, S.A. Stern (SwRI, Boulder)
Imaging the surface of Pluto remains a challenge even with the best of current technology. Owing to Pluto's orbital eccentricity and obliquity, the surface insolation changes dramatically over its 240 year orbital period. The presence of active volatiles on the surface has led to widespread predictions that at least some areas of the surface would change. To date there have been two distinct epochs of global albedo maps. The first, in the mid- to late-1980's, used mutual events and rotational lightcurve data to extract a variable resolution map. The second epoch, in 1994, used the Faint Object Camera (FOC) of HST. While the resolution is not sufficient to show any geologic detail, the data are useful for constraining large-scale albedo provinces that give rise to the long-known rotational lightcurve.
We present results from data collected during 12 orbits of HST time in Cycle 12 with the Advanced Camera for Surveys (ACS). This instrument differs from the FOC in several important ways. The ACS has a coarser pixel scale than FOC but collects photons and images much faster than FOC. Our data samples Pluto with 30 degree longitude resolution in two filters F439W (B) and F555W (V). The full dataset is comprised of 192 dithered images per filter. To extract the full-resolution map of Pluto's surface we used a forward-modeling technique. The model calculation is extremely time-consuming owing to the need to compute the geometric distortion of the camera. To reach an answer in a finite time, we have implemented a grid-computing methodology to permit parallel processing of the calculations.
Our map is a 20x10 grid of pixels using a cylindrical transformation onto the surface with 144 free parameters. The map contains values of the single-scattering albedo for using the Hapke scattering equations. The other functional values need by the theory, phase function, backscatter parameter, compaction parameters, and mean surface slope, were all set to global values that were forced to be consistent with the global phase coefficient of Pluto (0.015 mag/degree). This work is supported by a grant from the Space Telescope Science Institute.
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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.