Titan 479
It was also then essentially demonstrated that Titan’s sur-
face was much more complex than initially thought and that
the “dark” hemisphere was—fortunately (because it was
soon found out that theHuygensprobe was not going to
land where initially scheduled, close to the bright region,
but rather on the trailing side)—not all that dark, showing
some fine structure with bright areas.
3.2 The View from the Orbiter
The ISS and VIMS cameras confirmed these results and
showed that the borders of these regions were linear but
not smooth and that dramatic changes in surface albedo
could be noted in the maps produced by these measure-
ments (Fig. 7). It is notable how well the distribution of
bright and dark areas agrees among these three maps. The
best resolution achieved by ISS was of a few kilometers on
Titan’s surface. The large bright area around the equator
first observed by theHSTand the adaptive optics in 1994
was resolved and finely observed byCassiniinstruments. It
is centered at 10◦S and 100◦W and officially named Xanadu
Regio. The midlatitude regions around the equator on Ti-
tan were found to be rather uniformly bright, while the
southern pole is relatively dark. What exactly is causing the
albedo variations is still uncertain. A plausible candidate
for the darker regions could be accumulations of hydrocar-
bons (in liquid or solid form), precipitating down from the
atmosphere.
These variations are more readily attributed to the pres-
ence on the surface of constituents with different albedos
rather than topography, although contribution from the lat-
ter is also expected. The reason is that theCassinicamera
observing at 0.94μm cannot see shadows and also Titan’s
icy bulk does not plead for high topographic structures on
the surface (mountains should not exceed 3 km or so).
For the brighter regions, the task of interpreting the data
is more difficult. It has been hypothesized that they could
be associated with some topography and more exposed ice
content, and this tends to be in agreement with findings by
theHuygens/DISR instrument whose stereoscopic imaging
revealed that the brighter terrain was also more elevated
than the darker, smoother, and lower ice regions. The exact
ice constituent that can satisfy the constraints imposed by all
the observations is not easy to determine, hydrocarbon ice
has been invoked on the basis of Xanadu appearing bright
at all the near-infrared wavelengths observed to date.
A bright circular structure (about 30 km in diameter)
found in the VIMS hyperspectral images is interpreted as a
cryovolcanic dome in an area dominated by extension. The
VIMS team hypothesized that the dry channels observed
on Titan are related to upwelling “hot ice” and contami-
nated by hydrocarbons that vaporize as they get close to
the surface (to account for the methane gas in the atmo-
sphere), which are similar to those mechanisms operating
for silicate volcanism on Earth (using tidal heating as an
energy source) and which may lead to flows of non-H 2 O
ices on Titan’s surface. Following such eruptions, methane
rain could produce the dendritic dark structures seen by
Cassini–Huygens. If these structures are indeed channels,
they could have dried out due to the short timescale for
methane dissociation in the atmosphere. Studying volcan-
ism on Titan (if Cassini definitely yields evidence for it)
is important to understand not only the thermal history of
Titan (which must surely have evolved differently because
it differs in its incorporation of volatiles from the Galilean
satellites) but also how volatiles—in particular, methane—
were delivered to the surface.
Titan’s present environment is very placid—tidal cur-
rents are weak; rainfall, if it occurs, is soft; and the diurnal
temperature contrasts are small (and therefore winds are
gentle). The solubility of ice in hydrocarbons is smaller than
that of most rocks in water. Thus, except where the surface
is more susceptible to erosion, due to organic deposits or
perhaps water–ammonia ice, Titan’s topography should not
be significantly modified by erosion.
TheCassiniinstruments have found no obvious evi-
dence for a heavy craterization on the bright or the dark
areas of Titan so far. A few features interpreted as impact
craters have been announced to date:Cassini’s RADAR and
VIMS saw a 440-km diameter impact crater on Titan dur-
ing two separate flybys in early 2005. The coloring of the
feature indicates that its terrain is rough, with different ma-
terial for the crater floor and the ejecta and tilted toward
the radar during the observations. The multiringed impact
basin was named Circus Maximus by the science team. A
smaller crater of about 40 km was also observed, exhibiting
a parabola-shaped ejecta blanket. In spite of the detection
of a third crater-like feature, such formations, identified by
the RADAR, VIMS, or the ISS are rare. This may mean
that the surface of Titan is young (less than a billion years)
or highly eroded/modified.
Other features observed by theCassiniorbiter include
areas covered with analogs to terrestrial dunes in a set of
linear dark features visible across a large part of the RADAR
swath to the west of the large crater. These formations are
aligned west to east covering hundreds of kilometers and
rising to about 100 m. They are expected to have formed
by a process similar to that on Earth, but the nature of
this “sand” is quite different, consisting of fine grains of ice
or organic material, rather than of silicates. The winds re-
sponsible for these structures (about 0.5 m/s on the surface)
should primarily be attributable to the influence of Saturn,
through tidal forces 400 times greater than on Earth and
could easily move the Titanian “sand” in this world of low
gravity.
Additionally, the RADAR onboardCassinihas discov-
ered lakes sprinkled over the high northern altitudes of
Titan (Fig. 8). In the images recorded, a variety of dark
patches is observed, some of which extended outward (or
inward) by means of channels, seemingly carved by liquid.