Middle Ultraviolet Spectroscopy of Pluto and Charon

Abstract

The first Pluto's spectrum in the range 2000 to 2500 Å has been extracted from the Hubble Space Telescope (HST) archive observations acquired during 24 orbits. Although the spectral intensity is very weak and smaller than that for Titan by a factor of 100, the spectrum has a signal-to-noise ratio of 10–15 at the resolution of 10 Å. The mean geometric albedo is almost constant at 0.24 in this range. Observations at different longitudes reveal variations which correlate with the visible lightcurve at 3300–2500 Å and become more irregular at the shorter wavelengths. Variable quantities of haze, clouds, and rime may explain the higher variations at 2200–2300 Å. A search for gaseous absorptions resulted in upper limits to C 4H 2, C 6H 2, HC 3N, and C 4N 2. The first three limits impose important constraints to photochemical models of Pluto's atmosphere. The Cameron bands cannot be used for detection of CO on Pluto and Triton because of the very low atmospheric pressures. The first ultraviolet spectra of Charon were extracted from the HST archive observation acquired during 16 orbits. The spectra cover the range 2250–3300 Å with a signal-to-noise ratio varying from ≈70 at 3300–2650 Å to ≈8 at 2400–2250 Å. The geometric albedo of Charon is 0.25 and is almost constant in the measured range. The north south ratio is 1.04±0.02 from 2550 to 3300 Å. Comparing the extracted spectra with laboratory spectra of H 2O, CO 2, NH 3, and SO 2 ices, upper limits to concentrations of these species in the ices of Pluto and Charon have been established. However, these limits are high and nonrestrictive.