The Ultraviolet Albedo of Titan

Abstract

Ultraviolet spectra of Titan obtained with the Faint Object Spectrograph 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 Å. These results are in excellent agreement with previous UV measurements from 2200-3300 Å spanning two decades, but have a significantly higher signal to noise ratio and spectral resolution. We add ∼400 Å of new spectral information from 1800-2200 Å, a wavelength region in which Titan has not been detected previously. The albedo above 2200 Å is a factor of 1.18 higher in August 1992 than October 1991, which is most likely due to the 4.3° difference in Titan's phase angle between the two dates. Below 2200 Å the albedo decreases only modestly with decreasing wavelength and does not show unambiguous evidence for discrete spectral features characteristic of several of the known hydrocarbons, particularly acetylene (C₂H₂). Comparison of the albedo with best-fit models provides constraints on the optical properties and distribution of the small particle component of Titan's haze and on the C₂H₂mole fraction. The small particle haze seems to be darker in the UV than the "Titan" tholins analog produced in the laboratory, and the vertical extent limited to altitudes above ∼120-150 km. The acetylene distribution is consistent with the Voyager IRIS determination in the 130- to 180-km altitude range, where the mole fraction is of order 2 × 10^-6.