Titan has long been known as the satellite shrouded by an atmosphere with
at least a trace of methane. Its startling true nature was revealed by the
close flyby of Voyager I, which revealed that the atmosphere was primarily
nitrogen with surface temperatures and pressures of 94 K and 1.5 bars. [e.g.,
Lindal, et al., 1983]. The warmer upper atmosphere is rich in
hydrocarbons and nitriles both in vapor and aerosol form [e.g. Hanel, et
al., 1981; Kunde, et al., 1981 and Samuelson, et al., 1981].
While it is expected that Titan's rotation is tidally locked to its orbital
period of 16 days, nothing has been measured to determine is rotational vector,
although rotational variations of Titan's brightness are consistent with a
tidal lock. The atmosphere is opaque at wavelengths shorter than 1 cm except
for a partial window near 1 micron. With a bulk density of about 1.9
gm/cm
, it is reasonable to assume that Titan is primarily ice with
mixtures of ``rock'' of unknown differentiation and composition. The lightcurve
near 1 micron varies with the orbital period, suggesting that the rotational
period is nearly synchronous as reported by Griffith, Owen and Wagener
[1991], Griffith [1993] and Lemmon, Karkoschka and Tomasko [1993].
Lunine, Stevenson and Yung [1983] presented compelling arguments that
Titan must be covered by a deep ocean of liquid ethane and methane in order to
supply the atmosphere with methane vapor. Muhleman, et al. [1990] were
able to obtain strong radar echoes at the VLA which ruled out a global
hydrocarbon ocean, but left room for lakes and puddles. Radar experiments
over the last few years have shown that over all of the surface observed, the
reflectivity is greater than 10%, consistent with a dirty ice surface like
Callisto's and strongly inconsistent with hydrocarbons, which would have about
a 2 percent reflectivity.
The question of Titan's rotational state remains open as suggested by Muhleman, et al. [1992 and 1993] but should be resolved before the Cassini spacecraft gets there in the next century. Very recently, images made near 1 micron with the Hubble telescope revealed more clearly the strong brightening (reflectivity) on the leading hemisphere of Titan. This region corresponds to the area of enhanced radar reflectivity and could be caused by the ice surface being unusually clean. This work is not published as yet.