Pluto in Glory: Discovery of its Huge Opposition Surge

Near-infrared observations of the Pluto Charon system were captured with the Palomar High Angular Resolution Observer (PHARO) adaptive optics system on the 200-inch Hale telescope during the historically small solar phase angles in 2018-2019. Both objects exhibit large opposition surges of ~30-35% in the last half degree of solar phase angle, which is among the largest observed for icy moons and other Kuiper Belt Objects. In addition, Pluto’s surge is exceptionally steep. Pluto’s unusual phase curve may be due to an unusual surface texture caused by seasonal volatile transport and active geologic processes. These observations enable accurate determination of Pluto’s geometric albedo in the JHK filter system, which we find to be 0.86±0.04, 0.59±0.05, and 0.39±0.04 for Pluto, respectively, and 0.68±0.06 for Charon in the J-filter.