PHOTOMETRIC VARIABILITY OF CHARON AT 2.2 MU-M

Several dozen Pluto-Charon images were obtained on each of 4 nights with the ProtoCAM on the IRTF, mostly at K (2.2 μm) but also at J (1.2 μm) and H (1.7 μm). The seeing at FWHM ranged from 0.45 to 1.06 arcsec. A two-source image model was fit to the blended images of Pluto and Charon, with the position of Charon and the ratio of its signal to that of Pluto as free parameters. At K, we find Charon to be fainter than Pluto by 1.80±0.09, 2.39±0.05, and 2.09±0.05 mag at lightcurve phases 0.06, 0.42, and 0.95. Combining these magnitudes with combined photometry of the Pluto-Charon system we find apparent K magnitudes for Charon of 15.01 ± 0.08 at lightcurve phase 0.06 and 15.46±0.05 at lightcurve phase 0.42. We conclude that Charon is variable in this filter bandpass. The variation is most likely due to changes in its geometric albedo as a function of longitude. © 1992.