472 Encyclopedia of the Solar System
FIGURE 4 Titan’s chemical composition and variations in almost one Titan year (30 Earth years) fromVoyager 1
IRIS observations in 1980 toCassiniCIRS (2004) measurements in 2004. Note that the enhancement at the
North Pole is not as pronounced now as it was during theVoyagerencounter, because winter is only beginning at
the present in Titan’s North Pole.distribution (latitudinal and vertical) of these constituents
was also retrieved (Fig. 4). The vertical distributions gen-
erally increase with altitude, confirming the prediction of
photochemical models that these species form in the upper
atmosphere and then diffuse downward in the stratosphere.
Below the condensation level of each gas, the distributions
are assumed to decrease following the respective vapor sat-
uration law.
Ground-based high-resolution heterodyne millimeter
observations of Titan offered the opportunity to determine
vertical profiles and partial mapping in some cases of HCN,
CO, HC 3 N, and CH 3 CN, which showed that the nitrile
abundances increase with altitude. Subsidence causes the
abundance of these species to decrease in the lower atmo-
sphere.
Curiously, the bulk composition of Titan was more
difficult to determine than the abundances of the trace
constituents.Cassini–Huygensfinally allowed firm deter-
minations for the major components:HuygensGas Chro-
matograph Mass Spectrometer (GCMS) found a methane
mole fraction of 1.41× 10 −^2 in the stratosphere, increasing
below the tropopause and reaching 4.9 5 10−^2 near the sur-
face, in good agreement with the stratospheric CH 4 value
inferred by CIRS on theCassiniorbiter (1.6±0.5× 10 −^2 )
and the surface estimate given by theHuygensDescent Im-
ages Spectral Radiometer (DISR) spectra (also 5%). The
GCMS also saw a rapid increase of the methane signal after
landing, which suggests that liquid methane exists on the
surface, together with other trace organic species, including
cyanogen, benzene, ethane, and carbon dioxide. The only
noble gas detected to date is argon, found in the form of
primordial^36 Ar (2.8× 10 −^7 ) and its radiogenic isotope^40 Ar
(4.32× 10 −^5 ) by GCMS. The low abundance of primordial
noble gases on Titan implies that nitrogen was originally
captured as NH 3 rather than N 2. Subsequent photolysis
may have created the N 2 atmosphere we see today.
Isotopic ratios were determined fromCassiniandHuy-
gensinstruments:^12 C/^13 C (82.3±1),^14 N/^15 N (measured
in situ in N 2 , 183±5, which is 1.5 times less than on Earth)
and D/H (measured in situ in H 2 , 2.3±0.5× 10 −^4 , from
the GCMS, and in CH 4 , 1.2× 10 −^4 , from remote sensing
of infrared spectra recorded aboard the Cassini orbiter with
CIRS). It is believed that nitrogen was initially brought in
Titan in the form of NH 3 and converted into N 2 by