文档介绍：CLBE001-ESS2E November 7, 2006 17:55
470 Encyclopedia of the Solar System
Thermal Structure Besides the Huygens measurements, few constraints are
available for the temperature structure in Titan’s higher
The first definitive measurement of the atmospheric tem- atmosphere. The V1/UVS experiment recorded a temper-
perature structure was made by Voyager. The V1/RRS ature of 186 ± 20 K at 1265 km during a solar occultation
radio-occultation experiment provided density and tem- for a methane mixing ratio of 8 ± 3% toward 1125 km,
perature profiles in Titan’s atmosphere from refractivity placing the homopause level at around 925 ± 70 km. A
measurements. Titan’s temperature profile was measured value of 183 ± 11 K near 450 km was derived from the
in situ on January, 14, 2005 by the Cassini–Huygens Atmo- July 3, 1989, stellar occultation of Titan. The occultation
spheric Structure Instrument (HASI) at the probe’s land- of star 28 Sgr by Titan was observed from places as widely
ing site (15◦S, 192◦W) from 1400 km in altitude down to dispersed as Israel, the Vatican, and Paris. This rare event
the surface, where ± K were measured for a provided information in the 250–500 km altitude range. A
surface pressure of 1467 ± 1 mbar. As Voyager did before, mean scale height of 48 km at 450 km altitude (∼3 mbar
HASI found Titan’s atmosphere to exhibit the features that level) was inferred. This allowed the mean temperature to
characterize the Earth’s thermal structure: the atmospheric be constrained at that level to between 149 and 178 K.
layers include an exosphere, a mesosphere, a stratosphere From V1 infrared disk-resolved measurements, temper-
and a troposphere, with two major temperature inversions ature latitudinal variations were already demonstrated to
at 40 and 250 km, corresponding to the tropopause and exist in Titan’s stratosphere. At that time, a maximal tem-
stratopause, associated with temperatures of K (min) peratu