Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aj....112.1212t&link_type=abstract
Astronomical Journal v.112, p.1212
Astronomy and Astrophysics
Astronomy
9
Planets And Satellites: Individual: Pluto
Scientific paper
Ultraviolet spectra of Pluto, excluding Charon, have been acquired at seven rotational phases by using the Faint Object Spectrograph on the Hubble Space Telescope (HST). We present the geometric albedo spectra at each rotational phase over the range 2500-4800 Å, and their average over rotational phase. We compare these results with earlier lUE spectra of Pluto and with FOS spectra of Triton obtained with HST. Pluto's geometric albedo continuum, averaged over seven rotational phases, declines in brightness down to 2600 Å, where a minimum may occur. No atmospheric absorption bands are readily apparent over this spectral range. Pluto's rotationally averaged spectrum is remarkably like Triton's spectrum over the range 2500- 3200 Å, apart from a scale factor. Pluto's UV-blue color slope above 2900 Å has only a weak correlation with albedo but nevertheless becomes flattest (least red) near light-curve maximum. The lack of any strong change with wavelength of the reflectance over the 3000-4000 Å regime rules out significant global-surface ice contamination by refractory minerals and sulfur compounds. Below about 3100 Å, the light-curve profiles and albedos are nearly independent of wavelength. The absolute amplitude of Pluto's rotational lightcurve remains approximately constant at 0.11-0.12 in units of geometric albedo from 4700 down through 2600 Å, but the amplitude relative to Pluto's phase-averaged albedo increases 50% over this range. The "shelf" of Pluto's light curve, between east longitudes 260° and 30°, appears to be diminished in the UV relative to the visual light curve. The more significant differences among the various light curves are that the redder profiles become brighter with increasing wavelength while the UV ones do not; and that the redder profiles are blunter, causing an anticorrelation in UV and B albedo near the light-curve maximum. This likely indicates a UV/visible difference in Pluto' s appearance at rotational phases near maximum brightness. We find an upper limit of 0.54 to the haze vertical optical depth at 2600 Å. Extrapolating to visual wavelengths, this leads to a maximum normal haze extinction of 0.26, which is still too high to rule out a haze as the cause of the sudden drop-off in the occultation light curve.
Alan Stern S.
Trafton Laurence M.
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