PHOTOMETRY OF PLUTO 2008-2014: EVIDENCE OF ONGOING SEASONAL VOLATILE TRANSPORT AND ACTIVITY

The New Horizons. spacecraft will encounter Pluto in 2015 July. As this fast flyby will yield a picture of Pluto frozen in time, ground-based observations are key to understanding this dwarf ice planet, especially with regard to the seasonal transport of surface volatiles. This paper reports on changes in Pluto's rotational light curve as evidence for this transport. Historical observations are consistent with a stable frost pattern, but since 2002, changes began to appear in both light curves and Hubble Space Telescope. maps. Our BVR. observations at Table Mountain Observatory from 2008 to 2014 show evidence for sustained and continued albedo and color changes on Pluto. The B and V albedos are stable, but Pluto is becoming redder in color, particularly on its low-albedo side. This view is consistent with the transport of a bright volatile (nitrogen) with the uncovering of a substrate of red material such as photolyzed methane. As Buie et al. reported a B - V of 0.96 in 2002-2003, and our B - V was higher in 2008-2012, Pluto may have experienced a transient reddening in the 1999-2012 period. We also discovered an opposition supersurge in all three colors at very small solar phase angles (similar to 0.degrees 10). Explosive geysers have been observed on Triton and Mars, the two other celestial bodies with receding polar caps. Because the physical conditions existing on Pluto are similar to those on Triton, we predict that plume deposits and possibly active plumes will be found on its surface.