The Dark Side of Pluto

The stack presented in Figure 7 now rotated to place north at the top of the figure. The dashed circle shows the location of Pluto’s limb. The graphical representation presented in Figure 1 is repeated in the upper left and lower right, scaled to match the size of Pluto’s disk in the stack. The destreaked stack in the upper-right panel of Figure 7 is repeated in the upper-right panel of this figure. The low-pass filtered stack in Figure 7 is repeated in the lower-left of this figure, but with a harder stretch to better highlight the faint pattern of Charon-light illuminated terrain. The candidate bright basin is at the center of the disk. The low-albedo south polar region is evident as the dark area right of the Charon-light terminator and at the bottom of the disk.

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.

Tod R. Lauer, John R. Spencer, Tanguy Bertrand, Ross A. Beyer, Kirby D, Runyon, Oliver L, White, Leslie A. Young, Kimberly Ennico, William B. McKinnon, Jeffrey M. Moore, Catherine B. Olkin, S. Alan Stern, Harold A. Weaver

Comments: 24 pages, 10 figures, published in the Planetary Science Journal
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Journal reference: Planet. Sci. J. (2021), 2, 214
Cite as: arXiv:2110.11976 [astro-ph.EP] (or arXiv:2110.11976v1 [astro-ph.EP] for this version)
Submission history
From: Tod R. Lauer
[v1] Fri, 22 Oct 2021 18:00:03 UTC (5,625 KB)

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