Encyclopedia of the Solar System 2nd ed

470 Encyclopedia of the Solar System

2.1 Thermal Structure

The first definitive measurement of the atmospheric tem-
perature structure was made byVoyager. TheV1/RRS
radio-occultation experiment provided density and tem-
perature profiles in Titan’s atmosphere from refractivity
measurements. Titan’s temperature profile was measured
in situ on January, 14, 2005 by theCassini–HuygensAtmo-
spheric Structure Instrument (HASI) at the probe’s land-
ing site (15◦S, 192◦W) from 1400 km in altitude down to
the surface, where 93.65±0.25 K were measured for a
surface pressure of 1467±1 mbar. AsVoyagerdid before,
HASI found Titan’s atmosphere to exhibit the features that
characterize the Earth’s thermal structure: the atmospheric
layers include an exosphere, a mesosphere, a stratosphere
and a troposphere, with two major temperature inversions
at 40 and 250 km, corresponding to the tropopause and
stratopause, associated with temperatures of 70.43 K (min)
and 186 K (max), respectively (Fig. 3). At the same time,
the Composite Infrared Radiometer Spectrometer (CIRS)
on the orbiter took spectra that confirmed the presence of a
stratopause around 310 km of altitude for a maximum tem-
perature of 186 K. Another inversion region, less contrasted
than the previous ones and corresponding to the mesopause
can be found at 490 km (for 152 K).
The HASI data furthermore yield more precise and new
information on the upper part of the Titan atmosphere, the
thermosphere, where several temperature fluctuations are
observed due to dynamical (gravity and tidal) phenomena.
Indeed, gravity waves signatures of 10–20 K in amplitude
were recorded above 500 km around an average tempera-
ture of 170 K. HASI moreover found a lower ionospheric
layer between 140 and 40 km, with electrical conductivity
peaking near 60 km. A tentative detection of lightning is
being investigated.

Besides theHuygensmeasurements, few constraints are

available for the temperature structure in Titan’s higher

atmosphere. TheV1/UVS experiment recorded a temper-

ature of 186±20 K at 1265 km during a solar occultation

for a methane mixing ratio of 8±3% toward 1125 km,

placing the homopause level at around 925±70 km. A

value of 183±11 K near 450 km was derived from the

July 3, 1989, stellar occultation of Titan. The occultation

of star 28 Sgr by Titan was observed from places as widely

dispersed as Israel, the Vatican, and Paris. This rare event

provided information in the 250–500 km altitude range. A

meanscale heightof 48 km at 450 km altitude (∼3 mbar

level) was inferred. This allowed the mean temperature to

be constrained at that level to between 149 and 178 K.

FromV1infrared disk-resolved measurements, temper-

ature latitudinal variations were already demonstrated to

exist in Titan’s stratosphere. At that time, a maximal tem-

perature decrease of 17 K at the 0.4-mbar level (225 km

in altitude) was observed between 5◦S (the warmest region

in theVoyagerdata) and 70◦N, whereas the temperature

dropped by only 3 K from 5◦Sto53◦S. The coldest tem-

peratures, found at high northern latitudes, were associated

with enhanced gas concentration and haze opacity (as this

may be caused by more efficient cooling) or/anddynami-

cal inertia. CIRS mapped stratospheric temperatures over

much of Titan in the latter half of 2004, when it was early

southern summer on Titan (solstice was in October 2002).

The warmest temperatures are near the equator. Temper-

atures are moderately colder at high southern latitudes, by

4–5 K near 1 mbar, but they are coldest at high latitudes in

the north, where it is winter.

Titan was also found to have a quite extended iono-

sphere, due to the lack of a strong intrinsic global magnetic

field. Charged particles in the rarified upper atmosphere

are then exposed to bombardment by the solar wind and by

FIGURE 3 Titan’s temperature profile as derived

from Huygens/HASI measurements. The several

large inversion layers in the upper atmosphere

correspond to gravity waves. The inversion at

around 40 km marks the tropopause, whereas the

one at around 250 km is due to the stratopause.

(Adapted from Fulchignoni et al., 2005; Nature

438, 8 Dec. 2005, 785–791).