Pluto in Glory: Discovery of a Huge Opposition Surge

The July 2015 encounter of the New Horizons spacecraft with Pluto brought a large Kuiper Belt system into sharp focus for the first time. One key observation that was not observed by New Horizons and that can only be observed in 2018-2021 - the next opportunity does not occur for another 161 years - is the Pluto-Charon system at true opposition: the lowest solar phase angles possible. The purpose of this measurement is to model the surface microphysical properties of Pluto and Charon, including their surface compaction states and photometric parameters such as the geometric albedo, which is important for understanding thermal evolution and volatile transport. The information obtained from these measurements has implications for the geophysical evolution of Pluto and Charon - including whether the poorly imaged side of Pluto is geologically active.

Using the Palomar JHK Adaptive Optics setup attached to the Hale Telescope on Palomar Mountain, we successfully obtained measurements in 2018 and 2019, corresponding to the two apparitions in which Pluto’s two opposite hemispheres entered their smallest solar phase angles. These observations allowed for the clean separation of Pluto and Charon signals during the time of this rare phase angle minimum. In the J filter, Pluto exhibits an increase in brightness of about 30% in the last half degree of solar phase angle for both hemispheres, while Charon’s increase is nearly 40%. These increases are far greater than those observed on icy moons, where comparable measurements show an increase of about half this amount (Buratti et al., 2009 LPSC #40 ID#1738). Our data show reasonable agreement with the predictions of Hapke’s coherent backscatter model.

Government funding acknowledged.