Abstract

In 2018-2019, Pluto reached a viewing geometry that will not be repeated for 161 years: true opposition. This configuration, when the face of the planet and Charon were fully illuminated, enables a unique study of the physical properties of Pluto’s surface and the measurement of its scattering parameters. These latter include the bolometric Bond albedo, which determines heat transfer on the planet. This unique opportunity was captured with the adaptive optics (AO) system on the Palomar Hale telescope, which can separate the disks of Pluto and Charon for separate observation and analysis.

In 2018, the historically small solar phase angle was reached on the side that was not observed closely by the New Horizons spacecraft. Both Pluto and Charon exhibit a staggering 30-35% increase in brightness in the last fraction of a degree – greater than any other body so far observed. Observations of the side of Pluto containing its “heart” were successfully obtained in July 2019. This is the first time a methane-dominated body has been observed through a full range of solar phase angles. The New Horizons spacecraft observed Pluto at large solar phase angles not visible form Earth, but it never attained the unique geometry observed in 2018-2019.

These data reveal the textural properties of Pluto’s surface and can test various models for the reason Pluto, and other planetary surface as well, exhibit such a surge in brightness as they become fully illuminated to an observer, a phenomenon known as the opposition surge.

Copyright 2019. Government funding acknowledged.

Plain Language Summary

Pluto and Charon reached a unique viewing geometry in 2018-2019 that will not be repeated for 161 years: Their faces were completely illuminated when seen from Earth. Like the Moon, Pluto and Charon show a huge surge in brightness, but much larger than that of the well-known brightening of the full Moon.