476 Encyclopedia of the Solar System
contrary to previous assumptions, the DISR data seem to
show that the size of the aggregate particles is several times
as large as previously supposed.
In addition, measurements by the DISR violet photome-
ter extend the optical measurements of the haze to wave-
lengths as short as the band from 350 to 480 nm, also helping
to constrain the size of the haze particles. The number den-
sity of the haze particles does not increase with depth nearly
as dramatically as predicted by the older cloud physics mod-
els. In fact, the number density increases by only a factor
of a few over the altitude range from 150 km to the sur-
face. This implies that vertical mixing is much less than had
been assumed in the older models where the particles were
distributed approximately as the gas is with altitude. In any
event, the clear space at low altitudes, which was suggested
earlier, was not observed.
The methane mole fraction of 1.4–1.6% measured in
the stratosphere by the CIRS and the GCMS is consistent
with the DISR spectral measurements. At very low altitudes
(20 m), DISR and the GCMS measured 5±1% for the
methane mole fraction.
2.5 Clouds
Cassini–Huygenshas provided new information on the role
of methane and the methane cycle in Titan’s atmosphere.
The relative humidity of methane (about 50%) at the sur-
face found by DISR and the evaporation witnessed by the
GCMS show that fluid flows have existed and will prob-
ably again exist on the surface, implying precipitation of
methane through the atmosphere.
Although some discussion took place as to whether Ti-
tan’s lower atmosphere could support convection and as
to whether methane was supersaturated, there is clear ev-
idence today that clouds exist in Titan’s troposphere, al-
though in general they tend to appear higher than expected
and are mostly restricted to high southern latitudes.
Methane clouds in Titan’s troposphere were first sus-
pected from variability in the methane spectrum observed
from the ground. Direct imaging of clouds on Titan has been
achieved from Earth-based observatories since the turning
of the century. Most of the currently detected clouds are lo-
cated in Titan’s southern hemisphere, as expected given the
season on Titan (summer in the south), which means that so-
lar heating is concentrated there as are rising motions. Other
than the large, bright South Pole system observed for the
past 5 years or so, discrete clouds detected at midlatitudes
are infrequent, small and short-lived (CassiniVisual and
Infrared Mapping Spectrometer (VIMS) observations tend
to indicate that they rise quickly to the upper troposphere
and dissipate through rain within an hour). Keck and Gem-
ini data indicate that they tend to cluster near 350◦W and
40 ◦S. They may be related to some surface–atmosphere ex-
change (such as geysering orcryovolcanism) because they
don’t seem to be easily explained by a shift in global circula-
tion. A dozen or so large-scale zonal streaks have also been
FIGURE 5 Titan’s meteorology observed withCassini/ISS. (a–d)
A sequence of four methane continuum (IRP0-IR3, 928 nm)
images showing the temporal evolution over the period
05:05–09:38 of the Titan south polar cloud field on 2 July 2004.
(e–g) Three examples of discrete midlatitude clouds (arrows) for
which motions have been tracked in CB3 images. Image e: 38◦S,
81 ◦W (29 May 2004); this image was also viewed through an
infrared polarizing filter. Image f: 43◦S, 67◦W (23 October
2004). Image g: 65◦S, 110◦W (25 October 2004). (From Porco
et al., 2005,Nature 434 , 159–168. Image Credit: NASA/JPL.)observed byCassinipreferentially at low southern latitudes
and mostly between 50 and 200◦W.
The large south polar system has been visible consis-
tently essentially in the near-infrared (at 2.12μm for in-
stance) since 1999, while no previous indication of it was
ever reported. It was extremely bright in 2001–2002, and
recentCassiniimages have shown that it is disappearing
(indeed it was visible only during the few first Titan flybys
and not afterwards, see Fig. 5). Its shape is irregular and
changing with time, recently resembling more a cluster of
smaller-scale clouds than a large compact field. Should it
prove that this system’s life was indeed on the order of
5–6 years (fairly close to a Titan season), stringent con-
straints can be retrieved on seasonal and circulation pat-
terns on Titan. The cloud made a reappearance in 2006.
Note that DISR reported no definite detection of clouds
during its descent through Titan’s atmosphere. However,
the data are compatible with a thin haze layer at an altitude
of 21 km, which could be due to methane condensation.3. The Surface of Titan
To the eyes of the public and many scientists, the most
important features revealed by theCassini–Huygensmis-
sion were those found on Titan’s surface, finally observed
in close-up by the orbiter since 2004 and even in situ con-
ditions by the Huygens probe instruments on January 14,- The spaceship has offered detailed views of Titan’s
surface in the visible and the near-infrared with its cam-
era, the mapping spectrometer, and radar. Descending
through the atmosphere, theHuygensprobe returned fan-
tastic images of a first-seen domain, the farthest location
a human-made vessel has ever landed upon. Although we