Spectrophotometry of Pluto-Charon Mutual Events: Individual Spectra of Pluto and Charon

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Abstract
Time-resolved spectra of the 3 March and 4 April 1987 mutual events of Pluto and its satellite Charon were obtained with spectral coverage from 5,500 to 10,000 angstroms with 25 angstrom spectral resolution. Since both events were total occultations of Charon by Pluto, spectra were obtained of the anti-Charon—facing hemisphere of Pluto, with no contribution from Charon during totality. On 4 April, a combined spectrum of Pluto and Charon immediately before first contact was also obtained. The spectrum of the Pluto-facing hemisphere of Charon was extracted by differencing the pre-event and totality spectra. The spectra were reduced to reflectances by ratioing them to spectra of solar analog stars. Charon has a featureless reflectance spectrum, with no evidence of methane absorption. Charon's reflectance appears neutral in color and corresponds to a geometric albedo of ∼0.37 at 6000 angstroms. The Pluto reflectance spectrum displays methane absorption bands at 7300, 7900, 8400, 8600, and 8900 angstroms and is red in color, with a geometric albedo of ∼0.56 at 6000 angstroms. The signal-to-noise ratios of the eclipse spectra were not high enough to unambiguously identify the weaker methane band at 6200 angstroms.