478 Encyclopedia of the Solar System
variations indicative of a brighter leading hemisphere and a
darker trailing one. The leading side corresponds to Titan’s
Greatest Eastern Elongation (GEE) at about 90◦Longitude
of the Central Meridian (LCM—as opposed to geograph-
ical longitude, which is about 210◦), when Titan rotates
synchronously with Saturn; the trailing side is near 270◦
LCM or Greatest Western Elongation (GWE). The longi-
tude at which this “bright” behavior is found was also subse-
quently identified in Titan images as a bright large area near
the equator (see hereafter). At conjunctions (i.e., on the
hemispheres facing Saturn and its opposite), the albedo
was similar, of intermediate values between the maximum
appearing near 120◦LCM and the minimum near 230◦
LCM. As a consequence, Titan’s surface had then to be
heterogeneous and rather “dry” with the hydrocarbon ocean
stored in the porous, uppermost few kilometers ofmethane
clathrateor water ice, “bed rock.”
The Titan surface spectrum seemed to indicate the pres-
ence of two lower-albedo regions near 1.6 and 2μm (with
respect to the continuum near 1μm). These are also found
in Hyperion and Callisto data where they are due to the
water ice bands. The existence of a second (or more) sur-
face component(s) was advocated by the orbital variations.
It could be spectrally neutral or not and mixed with water
ice zonally or intimately. Complex organics (tholins) show
a neutral and fairly bright spectrum in the near-infrared, in
agreement with high absolute albedos, but should be dis-
tributed uniformly with longitude. Hydrocarbon lakes or
ices, silicate components, and other dark material are pos-
sible. Another possibility would be that the orbital variations
may be due to longitudinal differences in the ice morphol-
ogy (fresh or old, big or small particles, etc.).
Another technique, high-resolution imaging with the
possibility to resolve Titan’s disk, offered further constraints
on the Titan surface problem. Starting in 1994, two sets of
data taken independently and with different methods were
conclusively analyzed and presented to the public. The im-
ages showed clearly extensive quasi-permanent features,
which were furthermore too bright to be hydrocarbon liq-
uid. The heterogeneity of Titan’s surface, indicated in the
near-infrared and with radar lightcurves, was graphically re-
vealed by observations of Titan’s surface using theHubble
Space Telescopeand adaptive optics technique.
On Titan images obtained with theHubble Space Tele-
scope,features were made discernible on Titan’s surface.
Maps were produced of the surface in the 940 nm and
1070 nm windows, showing in more detail the bright lead-
ing and dark trailing sides, with notably a large (2500× 4000
km) bright region, at 114◦E and 10◦S (nowadays known as
Xanadu, this region has also a peculiar spectral behavior in
that it appears bright at all investigated wavelengths (0.9,
1.1, 1.3, 1.6 and 2.0μm), which may be indicative of an ice-
covered mountain or something equivalent), as well as at a
number of less bright regions. SubsequentHSTdata have
confirmed the initial findings with more extensive mapping
at 1.6 and 2.0μm and allowed identification of spectrally
distinct surface units, which may indicate regions of differ-
ent composition.
At the same time, images taken using the adaptive optics
system at the 3.6-m European Southern Observatory (ESO)
Telescope at Chile, showed the same bright region at the
equator and near 120◦orbital longitude but also revealed a
north–south hemispheric asymmetry apparent on Titan’s
darker side. Adaptive optics is now a generally adopted
method, and such systems exist in almost all the large
Earth-based telescopes. Prior to theCassiniencounter, the
adaptive optics system at the Canadian French Hawaiian
Telescope on top of Mauna Kea and its twin at the Very
Large Telescope (VLT) in Chile, as well as the Keck tele-
scope, were applied to Titan and returned some of the
most interesting and ground-breaking images of the satel-
lite (Fig. 7). The contrast on the adaptive optics images can
achieve 50% under good observing conditions.FIGURE 7 Three maps of Titan’s surface taken with the
Cassini/ISS at 0.94μm (upper panel); the adaptive optics system
NAOS at the VLT at 1.28μm (middle panel) and theHST
NICMOS at 1.6μm (lower panel). The surface features are
coherent from one data set to the other. The bright areas
dominate Titan’s leading hemisphere, while the darker ones
prevail on the other side. Xanadu Regio is observed near
110 ◦LCM. TheHuygenslanding site is marked with an “X” near
192 ◦LCM and 10◦S. (Porco et al., 2005,Nature 434 , 159–168;
Coustenis et al., 2005; Icarus 177 , 89–105; Meier et al., 2000,
Icarus 145 , 462–473).