  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.
