Middle ultraviolet spectroscopy of Pluto and Charon

The first Pluto's spectrum in the range 2000 to 2500 Angstrom has been extr acted from the Hubble Space Telescope (HST) archive observations acquired d uring 24 orbits. Although the spectral intensity is very weak and smaller t han that for Titan by a factor of 100, the spectrum has a signal-to-noise r atio of 10-15 at the resolution of 10 Angstrom. The mean geometric albedo i s almost constant at 0.24 in this range. Observations at different longitud es reveal variations which correlate with the visible lightcurve at 3300-25 00 Angstrom and become more irregular at the shorter wavelengths. Variable quantities of haze, clouds, and rime may explain the higher variations at 2 200-2300 Angstrom. A search for gaseous absorptions resulted in tipper limi ts to C4H2, C6H2, HC3N, and C4N2. The first three limits impose important c onstraints 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 2250-33 00 Angstrom with a signal-to-noise ratio varying from approximate to 70 at 3300-2650 Angstrom to approximate to8 at 2400-2250 Angstrom. 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 Angstrom. Comparing th e extracted spectra with laboratory spectra of H2O, CO2, NH3, and SO2 ices, upper limits to concentrations of these species in the ices of Pluto and C haron have been established. However, these limits are high and nonrestrict ive. (C) 2001 Academic Press.