TITAN - 1-5-MU-M PHOTOMETRY AND SPECTROPHOTOMETRY AND A SEARCH FOR VARIABILITY

We report photometric and spectroscopic observations of Titan made between 1 and 5 μm in 1990 and 1991. The 4.8-μm albedo we measured is more than a factor of three lower than those found in observations in the early 1970s. Long wavelength filter leaks could have contaminated earlier measurements of Titan's 4.8-μm albedo, although we cannot rule out a change in Titan itself. Titan's 1.25-μm albedo that we measured in 1991 is 14 ± 3% greater than that measured in 1979; the 1.65- and 2.20-μm albedos are the same. There was no evidence for variations with orbital phase at any wavelength over the eastern half of Titan's orbit in 1990 and 1991. We also made new measurements of Uranus and Neptune at 4.8 μm that agree with previous observations. We acquired low-resolution (R = 50) spectra of Titan between 3.1 and 5.1 μm. The spectra contain evidence for CO and CH3D absorptions. The measured band depths require a reflecting layer located in the troposphere (near 200 mbar) or below. Spectra of Callisto's, Ganymede's and Europa's leading hemispheres in the 4.5-5.1 μm spectral region are featureless and have albedos of 0.094 ± 0.004, 0.044 ± 0.006, and 0.020 ± 0.002, respectively. If Titan's atmosphere is transparent near 5.0 μm, its surface albedo there is similar to Callisto's. © 1993 by Academic Press, Inc.