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B. J. Buratti (Pomona College), J. K. Hillier (Grays Harbor College), A. Heinze (Caltech), M. D. Hicks (JPL)
Pluto is the only planet in the solar system that has not been scrutinized by a spacecraft. With its high eccentricity - Pluto's distance from the sun varies from about 30 to nearly 50 AU - and obliquity of 122 degrees, seasonal changes on the planet should be significant. The vapor pressures of its major atmospheric components (nitrogen, methane, and carbon dioxide) vary by several orders of magnitude during Pluto's transit around the sun (Stern and Trafton, Icarus 57, 1984; Stern et al., Icarus 75, 1988; Hansen and Paige, Icarus 120, 1996). The planet's entire atmosphere may be seasonal, sublimating only in the period around perihelion, and "collapsing" back onto the surface after as little as two decades.
Evidence for the seasonal deposition or sublimation of ice on Pluto's surface should be detectable with Earth-based telescopes. Rotational lightcurves obtained between 1954 and 1993 have increased markedly in amplitude, and concomitant decreases have occurred in the planet's geometric albedo (Marcialis, A. J. 95, 1988; Tholen and Tedesco, Icarus 108, 1995; Buie et al., Icarus 125, 1997). 25 nights of new BVR measurements of Pluto were obtained at Table Mountain Observatory during its apparition in 1999, to provide nearly 5 decades of photometric measurements of the planet. These data were compared with model lightcurves based on the HST albedo map (Stern et al., A. J. 113, 1997) and the epoch-appropriate viewing geometry. We find that changes in Pluto's lightcurve can be explained primarily by the effects of viewing geometry rather than by seasonal volatile transport.
Funded by NASA