Titan 471
particles precipitated from Saturn’s magnetosphere (creat-
ing an ionospheric layer between 700 and 2700 through
whichCassiniflew during some of its lower flybys of Titan),
as well as by cosmic rays from outer space (producing a sec-
ond layer between 40 and 140 km).Cassinifound that more
than 10% of the ionosphere is made up of ionized hydro-
carbon molecules chemically similar to compounds such as
ethylene, propyne, and diacetylene and that this population
is lost to space at important rates.
2.2 Chemical Composition
Indeed, the nature of Titan’s atmosphere finally emerged as
a combination of the two pre-Voyagermodels. Molecular
nitrogen (N 2 , detected by the UV spectrometer) is by far
the major component of the atmosphere (average of∼95%).
The presence of methane (the next most abundant molecule
with abundances ranging from 0.5 to 3.4% in the strato-
sphere and from 4 to8%atthesurface), traces of hydro-
gen, and a host of organic gases were inferred from emission
bands observed in the infrared interferometer spectrometer
(IRIS) spectra, which cover the 200–1500 cm−^1 spectral re-
gion with a spectral resolution of 4.3 cm−^1 , and later con-
firmed in the Infrared Space Observatory (ISO) and CIRS
observations, that afforded higher spectral resolution. In
1997 ISO Short Wavelength Spectrometer (SWS) spectra
provided a good determination of the chemical abundance
on Titan and also the first detection of water vapor in Ti-
tan’s atmosphere from 2 emission lines around 40μm, for
an associated mole fraction derived at 400 km of altitude
of about 10−^8. ISO also found the first hint of the presence
of benzene (C 6 H 6 ) at 674 cm−^1 for a mole fraction on the
order of a few 10−^10. Since then, the benzene detection has
been confirmed byCassini/CIRS. The water vapor abun-
dance, although seemingly small, implies a water influx on
Titan significantly superior to what might be expected based
on local and interplanetary sources alone (rather in favor of
Saturn). By including the laboratory spectra of these gases
in radiative transfer calculations, the abundances of all of
these constituents can be estimated (Table 2). The spatialTABLE 2 Chemical Composition of Titan’s Atmosphere Today from
Cassini-HuygensResults Unless Otherwise Indicated
Mole Fraction
Constituent (atm. altitude level)Major
Molecular nitrogen, N 2 0.98
Methane, CH 4 4.9× 10 −^2 (surface)
1.4–1.6× 10 −^2 (stratosphere)
Monodeuterated methane, CH 3 D6× 10 −^6 (in CH 3 D, in stratosphere.)
Argon,^36 Ar 2.8× 10 −^7(^40) Ar 4.3× 10 − 5
Minor
Hydrogen, H 2 ∼0.0011
Ethane, C 2 H 6 1.5× 10 −^5 (around 130 km)
Propane, C 3 H 8 5 × 10 −^7 (around 125 km)
Acetylene, C 2 H 2 4 × 10 −^6 (around 140 km)
Ethylene, C 2 H 4 1.5× 10 −^7 (around 130 km)
Methylacetylene,CH 3 C 2 H 6.5× 10 −^9 (around 110 km)a
Diacetylene, C 4 H 2 1.3× 10 −^9 (around 110 km)a
Cyanogen, C 2 N 2 5.5× 10 −^9 (around 120 km)a
Hydrogen cyanide, HCN 1.0× 10 −^7 (around 120 km)a
5 × 10 −^7 (around 200 km)b
5 × 10 −^6 (around 500 km)b
Cyanoacetylene, HC 3 N1× 10 −^9 (around 120 km)a
1 × 10 −^7 (around 500 km)b
Acetonitrile, CH 3 CN 1 × 10 −^8 (around 200 km)c
1 × 10 −^7 (around 500 km)
Water, H 2 O8× 10 −^9 (at 400 km)d
Carbon monoxide, CO 4 × 10 −^5 (uniform profile)e
Carbon dioxide, CO 2 1.5× 10 −^8 (around 120 km)
aIncreasing in the North.
bFrom ground-based heterodyne microwave observations.
cOnly observed from the ground.
dFrom ISO observations.
eFrom Cassini and ground-based data.