Abstract
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 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 N2 or CH4 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 N2 ice or the deposition of haze particulates during the recent southern summer.
Highlights
The timing of the New Horizons encounter was phased for optimal viewing of Sputnik Planitia,12 which is opposite Charon at longitude 180°
12 The feature designated as Sputnik Planitia was evident as a large highalbedo feature in the Buie et al (2010) HST high-resolution maps prepared in advance of the encounter, and it was known to correspond to the planet’s light-curve maximum and a region of anomalously high CO abundance
Our solution was to characterize the ensemble of background images with a principal component analysis (PCA) approach, which generated a set of orthogonal “eigenimages” that could be used to represent the domain over which the background varied
Summary
As NASA’s New Horizons spacecraft departed Pluto following its 2015 July 14 flyby encounter (summarized in Stern et al 2015), the planet presented almost its entire nightside hemisphere for observation. In the case of the P_DEEPIM sequence, this meant that the standard LORRI pipeline smearing correction was incomplete, leaving smeared light from the bright haze ring behind in the dark nightside disk of Pluto interior to the ring This artifact is evident in panel (e) of Figure 5. A crucial part of this operation was to first multiply the reference by the CCD flat field appropriate for the location of the overexposed ring in the given P_DEEPIM image This step was required, as the charge smearing depends on the true pixel sensitivity at any location in the LORRI field, not the uniform sensitivity established by flat-field calibration. The new algorithm provides both more efficient and significantly better charge-smear correction
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