Titan 473
photolysis of the early atmosphere. The measured^14 N/^15 N
implies that a substantial part of N 2 , from 2 to 10 times the
mass of the early atmosphere, escaped over 4.5 billions of
years. The D/H ratio is very important for Titan cosmogo-
nical models. A lower value for D/H in methane (∼1.2×
10 −^4 ) was found from the analysis ofCassiniobservations
of theν 6 monodeuterated methane (CH 3 D) band at 8.6
μm, confirming a value found fromVoyagerdata analyses.
Both D/H values tend to suggest a deuterium enrichment
in Titan’s atmosphere with respect to the proto-solar value
as well as in that of the giant planets (D/H∼2–3.4× 10 −^5 ).
The interpretation of this enrichment is related to the evo-
lution of CH 4 in the atmosphere. The key point is that CH 4
is continuously photodissociated so that, in the absence of
a substantial reservoir, it would entirely vanish from the
atmosphere in 10–50 Myr. Imaging, infrared, and visible
observations from the orbiter rule out the presence of a
global ocean containing a large amount of CH 4 on the sur-
face of Titan. It is thus likely that methane outgasses from
time to time from the interior of the satellite. Two scenarios
for the origin of the internal CH 4 have been proposed. One
scenario advocates that CH 4 was chemically produced from
H 2 O and CO 2 trapped in the planetesimals that formed Ti-
tan and which easily condensed in the solar nebula. How-
ever, this does not explain the detection of^36 Ar in the atmo-
sphere in an amount higher than that which could possibly
have been trapped in the silicated core. A more plausible
scenario argues that CH 4 and^36 Ar were present as ices or
clathratesin the cool solar nebula and were incorporated
in Titan planetesimals. This is consistent with the assump-
tion that CH 4 was enriched in deuterium by ion-molecules
reactions in the presolar cloud, the resulting D/H in CH 4
then being at least partly preserved in icy grains falling onto
the solar nebula and—since no deuterium fractionation can
occur in the interior—reflecting the value observed in the
atmosphere of the satellite today.
2.3 Dynamical Processes
2.3.1 ZONAL CIRCULATION
At the time of theVoyagerencounter, Titan’s northern
hemisphere was coming out of winter. During theCassini
observations in 2006, Titan’s northern hemisphere was
halfway into winter.
The general faintly banded appearance of Titan’s haze
suggests rapid zonal motions (i.e., winds parallel to the
equator). This impression is reinforced by the infrared tem-
perature maps, which show very small contrasts in the lon-
gitudinal direction and rather large ones (of around 20 K)
between the equator and the winter pole. The mean zonal
winds inferred from this temperature field are weakest at
high southern latitudes and increase toward the north, with
maximum values at and mid-northern latitudes (20–40N)
of about 160 m s−^1. On Titan, pressure gradients are in
cyclostrophic balancewith centrifugal forces.
Stellar occultations are another indirect means to obtain
the zonal winds. The atmospheric oblateness due to the
zonal winds can be constrained from the analysis of the
central flash, the increase of the signal at the center of
the shadow (when the star is behind Titan) due to the fo-
cusing of the atmospheric rays at the limb. On July 3, 1989,
Titan occulted the bright K-type star 28 Sgr, and fast zonal
winds were derived close to 180 ms−^1 at high southern lati-
tudes and close to 100 ms−^1 at low latitudes. Other occulta-
tions occurred on December, 20, 2001, and November, 14,- They seem to suggest a seasonal variation with respect
to 1989. In 2001, a strong 220 ms−^1 jet was located at 60◦N,
with lower winds extending between 20◦S and 60◦S, and a
much slower motion at midlatitudes. The CIRS data suggest
that the strongest northern winds have migrated closer to
the equator with respect to previous measurements, while
the southern winds have weakened.
Space and occultation wind measurements could not
provide the wind direction, a crucial factor for theHuy-
gensprobe mission, so different teams of ground-based
observers tried to measure the zonal winds directly using
alternative methods. The first measurement of prograde
winds (in the sense of the rotation of the surface) was per-
formed using infrared heterodyne spectroscopy of Doppler-
shifted ethane emission lines. The measured winds were
on the order of 210±150 ms−^1 between 7 and 0.1 mbar,
a result that has since been refined. Other Doppler stud-
ies probing somewhat different levels also found prograde
winds, using millimeter-wavelength interferometry of ni-
trile lines or high-resolution spectroscopy of Fraunhofer
solar absorption lines in the visible. The recent advances in
adaptive opticsalso allowed for the first detections of tro-
pospheric clouds from the ground, mainly at circumpolar
southern latitudes, but so far Titan winds remain poorly con-
strained due to the sparse data set of cloud positions. Better
spatially resolvedCassini/International Space Station (ISS)
observations only indicate slow eastward motions, which,
extrapolated to the equator under the assumption of solid-
body rotation, yield 19±15 ms−^1 at around 25 km altitude.
Finally, in 2005, theHuygensprobe provided ground-truth
measurements of the wind magnitude and direction in the
lower stratosphere and troposphere. The Doppler wind ex-
periment shows a marked decrease of winds with decreas-
ing altitude, from 100 ms−^1 at 140 km down to about nil
at 80 km, then an increase up to 40 ms−^1 at 60 km before
decreasing again to null zonal velocity at the surface.
2.3.2 LATITUDINAL AND TEMPORAL VARIATIONS IN THE
ATMOSPHERE OF TITAN AS EVIDENCE OF MERIDIONAL
CIRCULATION
Periodic change of Titan’s disk-integrated brightness has
been monitored from Earth-based observations since the
1970s. Spatially resolved observations, starting withVoy-
ager, have provided an interpretation of the periodic