It is now well established that Titan's atmosphere is at least partially
transparent in several near-infrared windows. Resolved images made with
the Hubble Space Telescope show clearly defined albedo features including
a prominent bright feature on Titan's leading hemisphere. Lightcurves
measured at 1.3, 1.6, and 2.0 micron show a clearly defined peak near
eastern elongation. We have recently shown that one more window exists in
Titan's spectrum from 4.9 to 5.2 micron, but we have been unable to measure
a lightcurve. Lightcurves of the icy Galilean satellites at 5 micron are
anti-correlated with lightcurves in the near infrared, indicative of the
presence of silicate minerals in the dark, non-ice component of the
surface. We propose to use ISO to measure the 5 micron lightcurve of Titan
to search for clues to the composition of its surface.