474 Encyclopedia of the Solar System
changes of the disk-integrated brightness as the combined
action of the high inclination of the rotation axis and the sea-
sonally varying north–south asymmetry. The NSA thatVoy-
ager 1observed in 1980, with a darker northern hemisphere
in visible light, has since been observed to reverse, as Titan’s
season shifted from northern spring to present-day north-
ern winter. When theHubble Space Telescope (HST)first
observed Titan in 1994, a little over a quarter of a Titan year
after theVoyagerencounters, the northern hemisphere was
found to be brighter than the southern hemisphere. The
turnover was later also found to occur gradually, starting at
higher altitudes in the atmosphere.
Modeling with a two-dimensional general circulation
model provided a qualitative description of the seasonal
variations of the haze, where both the gradual inversion
of the asymmetry and the detached haze layer can be ex-
plained by a seasonally varyingHadley circulation. The
meridional wind in the upper branch of the Hadley cell is
stronger close to the production zone (at 450 km) than be-
low, and particles there are more rapidly transported toward
the pole, where they sink. The asymmetry thus reverses first
at higher altitudes. But this is not the only effect. As the sea-
son changes, shortly after equinox, the circulation reverses
and an ascending motion sets in where the particles were
previously descending. At the time of the transition, the
polar haze, which was previously descending, is then re-
distributed about a scale height below the production zone,
becoming physically separated from the freshly created par-
ticles aloft.
Meridional variations were also established for the gases
in Titan’s stratosphere, and these are also tightly coupled
with the circulation. The molecular abundances found by
Cassiniat this era indicate an enhancement for some species
in the stratosphere at high latitudes, albeit not as dramatic
as at the time of theVoyagerencounter (Fig. 4). The differ-
ence in magnitude between theVoyager 1and theCassini
eras may be due to the difference in seasons, and it will
be exciting to await the arrival of northern spring equinox
toward the end of theCassinimission and to measure the
meridional variations then to see if we return to the IRIS
inferences.
In the meantime, such latitudinal contrasts observed in
the chemical trace species may be explained by invoking
photochemical and dynamical reasons. The UV radiation
from the Sun acts on methane and nitrogen to form radi-
cals that combine into nitriles and the higher hydrocarbons.
This production occurs in the mesosphere at high altitudes
(above 300 km or 0.1 mbar). Eddy mixing transports these
molecules into the lower stratosphere and troposphere
where most of them condense. Photodissociation by UV
radiation occurs on timescales ranging from days to thou-
sands of years. The combination of these processes leads
to a vertical variation in the mixing ratio, which usually in-
creases with height towards the production zone. Three-
dimensional computation ofactinic fluxessuggests that
this mechanism alone cannot explain the latitudinal con-
trasts and that circulation must intervene. Simulations cou-
pling photochemistry and atmospheric dynamics provide a
consistent view: Competition between rapid sinking of air
from the upper stratosphere in the winter polar vortex and
latitudinal mixing controls the vertical distribution profiles
of most species. The magnitude of the polar enrichment
is controlled by downwelling over the winter pole, which
brings enriched air from the production zone to the strato-
sphere, and by the level of condensation. Short-lived species
are more sensitive to the downwelling due to steeper verti-
cal composition gradients and exhibit higher contrasts.
In the stratosphere, the calculated radiative relaxation
time is longer than the Titan season, so the temperature con-
trasts should be symmetric about the equator. That they are
not indicates that the Hadley circulation must be connected
with the lower atmosphere, where the time constant is much
longer. This is consistent with the small thermal contrasts
of 2–3 K in the troposphere, which suggest an efficient heat
redistribution. Since Titan’s slow rotation and small radius
rule out nonaxisymmetric processes, such asbarocliniced-
dies, as a preferred mechanism for heat transport, consid-
erable meridional motions must be inferred. Latitudinal
contrasts would be much larger if heat were not being trans-
ported poleward by Hadley advection.
Another phenomenon was first reported in 2001 from
adaptive optics data taken in 1998. A diurnal change
was found, manifested in an east–west asymmetry, with a
brighter morning limb observed on Titan on several occa-
sions. This dawn haze enhancement could be due to an ac-
cumulation of condensates during the Titan night (8 Earth
days, though the superrotation of Titan’s atmosphere would
lead to shorter nights for stratospheric clouds).2.3.3 A THREE-DIMENSIONAL VIEW AND WAVES
Meridional contrasts are apparent in Titan’s atmospheric
distributions of composition, haze, and temperature, and
their seasonal variability is proof for a strong coupling with
an underlying meridional circulation that has never been
directly detected.
The superrotation observed in the stratosphere, a
dynamical state in which the averaged angular momentum
is much greater than that corresponding to corotation
with the surface, is difficult to explain and has defied our
understanding in the much better documented Venus case,
the paradigm of a slowly rotating body with an atmosphere
in rapid rotation. In recent studies, such a process has been
identified under the form of planetary waves, forced by
instabilities in the equatorward flank of the high-latitude
jet. Two factors play a key role in facilitating the acceler-
ation process. On the one hand, high altitude absorption
processes decouple upper atmosphere dynamics from
dissipation occurring at the surface layer, while on the other
hand the slow rotation allows the Hadley cell to reach high