The Dark Side of Pluto

TL;DR

This paper details the challenging process of recovering faint Charon-reflected light in images of Pluto's dark hemisphere taken by New Horizons, revealing high-albedo regions possibly due to ice deposits.

Contribution

It introduces a novel image reconstruction method using PCA to correct overexposed and scattered light in spacecraft images of Pluto's night side.

Findings

Identification of a high-albedo region in Pluto's southern hemisphere.

Detection of lower albedo in Pluto's south polar region compared to the north.

Evidence suggesting N2 or CH4 ice deposits and seasonal sublimation or deposition processes.

Abstract

During its departure from Pluto, New Horizons used its LORRI camera to image a portion of Pluto's southern hemisphere that was in a decades-long seasonal winter darkness, but still very faintly illuminated by sunlight reflected by Charon. Recovery of this faint signal was technically challenging. The bright ring of sunlight forward-scattered by haze in the Plutonian atmosphere encircling the nightside hemisphere was severely overexposed, defeating the standard smeared-charge removal required for LORRI images. Reconstruction of the overexposed portions of the raw images, however, allowed adequate corrections to be accomplished. The small solar elongation of Pluto during the departure phase also generated a complex scattered-sunlight background in the images that was three orders of magnitude stronger than the estimated Charon-light flux (the Charon-light flux is similar to the flux of moonlight on Earth a few days before first quarter). A model background image was constructed for each Pluto image based on principal component analysis (PCA) applied to an ensemble of scattered-sunlight images taken at identical Sun-spacecraft geometry to the Pluto images. The recovered Charon-light image revealed a high-albedo region in the southern hemisphere. We argue that this may be a regional deposit of N_2 or CH_4 ice. The Charon-light image also shows that the south polar region currently has markedly lower albedo than the north polar region of Pluto, which may reflect the sublimation of N_2 ice or the deposition of haze particulates during the recent southern summer.