Titan 475
latitudes by reducing centrifugal forces in the poleward
branch. A strong seasonal cycle due to Titan’s obliquity of
26.7◦was also established: During most of the Titan year,
the meridional motion is dominated by a large Hadley
cell extending from the winter to the summer pole, with
the symmetric two-cell configuration typical of equinoxes
occurring only in a limited transition period. In the model,
the jet is located close to 60◦in the winter hemisphere, while
the summer zonal circulation is close to solid body rotation.
The radiative time constant is long in the troposphere,
but the surface has a smaller thermal inertia, so the sur-
face temperature does respond to seasonal forcing, albeit
by only a few Kelvin. This surface temperature variation is
sufficient to reverse the circulation pattern of the Hadley
circulation after the equinox when the Sun moves to the
opposite hemisphere. Also the development of convective
methane clouds is partly ascribed to seasonal surface heat-
ing. The reversal of the Hadley circulation may play an
important role in the methane “hydrological” cycle because
the vertical and horizontal transport of methane would vary
seasonally.
Direct evidence for wave processes in Titan’s atmosphere
remains scarce, despite their importance in the mainte-
nance of superrotation. Because baroclinic processes are
excluded, waves essentiallybarotropicin nature should be
expected as the principal carrier of momentum from high
to low latitudes. Modeling predictswavenumber-2 waves
with an amplitude of the zonal component about 10% of the
mean wind speed, and in principle they can be inferred from
horizontal maps of temperature and trace species exhibiting
strong latitudinal contrasts. The firstCassini/CIRS temper-
ature maps at 1.8 mbar do show spatial inhomogeneity, but
long time series and better spatial coverage are needed to
constrain spatial and temporal variations.
Another relevant nonaxisymmetric phenomenon in
Titan’s troposphere is the gravitational tide exerted by
Saturn. The eccentric orbit of Titan around Saturn gives
rise to a tidal force, resulting in periodical oscillation in the
atmospheric pressure and wind with a period of a Titan day
(16 days), among which the most notable effect is the peri-
odical reversal of the north–south component of the wind.
In the lower atmosphere, the effect of this tide is modest,
with a maximum temperature amplitude about 0.3 K and
winds of 2m s–^1.
Temperature inversions have been detected in both the
HuygensHASI measurements and in stellar occultation
data. Inversion layers were present close to 510 km altitude
in HASI and 2003 occultation data, and at 425 and 455 km
in 1989 occultation light curves. Vertical wavelengths were
on the order of 100 km.
2.4 Haze and Clouds on Titan
It was recognized quite early that another important as-
pect of Titan’s atmosphere was the presence of aerosols.
Pre-Cassinimodels treated the dissociation of methane
molecules by solar actinic radiation, followed by chemical
combination to heavier hydrocarbons that condense into
particles. The cloud physics models with sedimentation and
coagulation predicted a strong increase in haze density with
decreasing altitude.2.4.1 TITAN’S HAZE
The analysis of high-phase Voyager images indicated
aerosol radii between 0.2 and 0.5μm. These “smog” parti-
cles form a layer that enshrouds the entire globe of Titan and
stretches from the surface to an altitude of about 200 km.
A detached haze layer at 340–360 km altitude with large,
compact, irregular dark particles was also found. The small
haze particles required byVoyagermeasurements (radii less
than or equal to 0.1μm) produce a strong increase in optical
depth with decreasing wavelength shortward of 1μm. To
fit the observations in the methane bands, it was necessary
to remove the haze permitted by the cloud physics calcula-
tions at altitudes below about 70–90 km (called cut-off alti-
tude) by invoking condensation of organic gases produced
at high altitudes as they diffused down to colder levels. The
condensation of many organic gases produced by photo-
chemistry at high altitudes on Titan seemed consistent with
this view. The next step in the development of Titan haze
models included the use of fractal aggregate particles com-
posed of several tens of small (0.06μm in radius) monomers
to produce strong linear polarization. Monomers composed
of 45 aggregates with an effective radius of about 0.35μm
matched theVoyagerobservations.
Starting from the upper atmosphere, theCassiniISS
camera showed a faint thin haze layer that encircles the
denser stratospheric haze (Fig. 1b) and could be the equiv-
alent of the “detached haze layer” observed byVoyager
25 years ago, except for the difference in altitudes: The thin
current haze layer is indeed located 150–200 km higher
than the one seen byVoyager. Current models are still un-
able to render the complexity of seasonal phenomena or
circulation patterns on Titan, which could be responsible
for such an upward shift.
Cassiniimages also show a multilayer structure in the
north polar hood region and, in some cases, at lower lat-
itudes. These features could be due to gravity waves that
have been detected on Titan at lower altitudes. Some of
these layers may be related to the two global inversion lay-
ers observed in stellar occultations of Titan above 400 km
in altitude.
The nature of the haze aerosols measured byHuy-
gens/DISR during the descent through Titan’s lower at-
mosphere came as a surprise to scientists recalling the re-
sults fromPioneerandVoyager, as well as predictions by
cloud physics models with sedimentation and coagulation.
The new observations estimate the monomer radius to be
0.05μm, in good agreement with previous values. However,