THE ULTRAVIOLET ALBEDO OF TITAN

Ultraviolet spectra of Titan obtained with the Faint Object Spectrogra ph of the Hubble Space Telescope in October 1991 and August 1992 have yielded a disk-average geometric albedo of 0.02-0.044 from 1800-3300 A ngstrom. These results are in excellent agreement with previous UV mea surements from 2200-3300 Angstrom spanning two decades, but have a sig nificantly higher signal to noise ratio and spectral resolution. We ad d similar to 400 Angstrom of new spectral information from 1800-2200 A ngstrom, a wavelength region in which Titan has not been detected prev iously. The albedo above 2200 Angstrom is a factor of 1.18 higher in A ugust 1992 than October 1991, which is most likely due to the 4.3 degr ees difference in Titan's phase angle between the two dates. Below 220 0 Angstrom the albedo decreases only modestly with decreasing waveleng th and does not show unambiguous evidence for discrete spectral featur es characteristic of several of the known hydrocarbons, particularly a cetylene (C2H2). Comparison of the albedo with best-fit models provide s constraints on the optical properties and distribution of the small particle component of Titan's haze and on the C2H2 mole fraction, The small particle haze seems to be darker in the UV than the ''Titan'' th olins analog produced in the laboratory, and the vertical extent limit ed to altitudes above similar to 120-150 km. The acetylene distributio n is consistent with the Voyager IRIS determination in the 130- to 180 -km altitude range, where the mole fraction is of order 2 x 10(-6). (C ) 1998 Academic Press.