468 Encyclopedia of the Solar System
gases in the infrared spectrum of Titan; and a “thick nitro-
gen” atmosphere model, which was based on the assump-
tion that ammonia dissociation should produce molecular
nitrogen (transparent in the visible and infrared spectrum)
in large quantities and held that the surface temperature
and pressure could be quite high (200 K for 20 bars).
Independent of these two models, an explanation of the
high observed ground temperatures was advanced: a pro-
nounced greenhouse effect, resulting essentially from H 2 –
H 2 pressure-inducedopacityatwavelengthshigher than
15 μm. This opacity blocks the thermal emission reflected
by the surface, thus creating a heat-up of the lower part of
the atmosphere, as found on Earth.
1.2 Titan’s Exploration
Titan has since then been extensively studied from the
ground and from space. In the latter case, Titan was
“blessed” by several space mission encounters in the course
of the planetary exploration in our solar system. [See also
PlanetaryExplorationMissions.]
Although thePioneer 11spacecraft was the first to take a
close look at the giant planets Jupiter and Saturn, it flew by
Titan at a considerable distance of 363,000 km on Septem-
ber 2, 1979. TheVoyagermissions that followed were also
dedicated to an extended study of the outer solar system.
TheVoyager 1(V1) spacecraft (launched in 1977) arrived
in the Saturnian system and made its closest approach of
Titan on November 12, 1980, at a distance of only 6969 km
(4394 miles) to the satellite’s center.Voyager 2flew by Ti-
tan 9 months later but at a distance a hundred times greater
(663,385 km) so that theVoyager 1encounter was the clos-
est a man-made machine ever came to Titan until 2004.
Titan’s visible appearance at the time was unexciting—
an orange ball, completely covered by thick haze, which al-
lowed no visibility of the surface (Fig. 1a). The most obvious
feature seen byVoyagerwas a difference in the brightness
of the two hemispheres. This difference is of the order of
25% at blue wavelengths and falls to a few percent in the
ultraviolet and at red wavelengths. This so-called north–
south asymmetry (NSA) is probably related to circulation
in the atmosphere pushing haze from one hemisphere to
the other. The altitude of unity vertical optical depth is
of the order of 100 km. Also noticeable was a dark ring
above the north (winter) pole. This feature, termed the po-
lar hood, extending from 70◦to 90◦north latitude, was most
prominent at blue and violet wavelengths, and it has since
then been suggested that it may be associated with lack of
illumination in the polar regions during the winter (since
the subsolar latitude goes up to 26.4◦) and/or subsidence in
global circulation.
Besides the images, theVoyagerinstrument also allowed
for the determination of the chemical composition and tem-
perature structure. The latter and other basic parameters
for Titan (Table 1) were provided by theradio-occultation
FIGURE 1 Titan observed in 1980 with the cameras ofVoyager
1 in the visible and in 2004 with theCassiniISS camera (Team
Leader: C. C. Porco) at 0.94μm. In the first case, the bland
appearance of the satellite belies a complex world. The only
features apparent in the images taken byVoyagerwere the
detached haze layers, the dark polar hood and generally a
difference in brightness between the two hemispheres. In
contrast one of the most recent images byCassinishows Titan’s
surface features. (Image Credit: NASA/JPL.)Voyager experiment obtained by the Radio Science Subsys-
tem (RSS). Titan’s surface radius was found to be 2575±
2 km, with a surface temperature of 94±2 K and a pressure
of about 1.44 bar.
AfterVoyager, scientists had to wait for about 25 years
before getting another close look at Titan.Cassini–Huygens
is a very ambitious mission, planned in the 1980s already. It
is an extremely successful collaboration between ESA and
NASA (with contribution from 17 countries), composed of
an orbiter and a probe (Huygens). Although the mission’s
objectives span the entire Saturnian system, Titan is a priv-
ileged target forCassini(as forVoyagerbefore it), and the
mission is designed to address our principal questions about
this satellite during its 6-year duration from 2004 onwards.
The spacecraft is equipped with 18 science instruments
(12 on the orbiter and 6 carried by the probe), gather-
ing both remote sensing and in situ data. It communicates
through one high-gain and two low-gain antennas. Power isTABLE 1 Titan’s Orbital and Body Parameters, and
Atmospheric PropertiesSurface radius 2575 km
Mass 1.35× 1023 kg (=0.022×Earth)
Mean density 1880 kg−^3
Distance from Saturn 1.23× 109 m(=20 Saturn radii)
Distance from Sun 9.546 AU
Orbital period 15.95 days
around Sun 29.5 years
Obliquity 26.7◦
Surface temperature 93.6 K
Surface pressure 1.467 bar