704 Encyclopedia of the Solar System
(a) (b)FIGURE 9 (a) VLA image of Uranus at 2 cm wavelength taken in the summer of 2003. Note the hot (red) poles. (M. D. Hofstadter and
B. J. Butler, 2003, Seasonal change in the deep atmosphere of Uranus.Icarus 165 , 168–180.) (b) Infrared (1.6μm) image of Uranus
taken with the Keck adaptive optics system in October 2003. The polar collar around the south pole (left in figure) lines up with the edge
of the hot pole seen at radio wavelengths. Several cloud features are visible in the infrared image, and the thin line near the right is
Uranus’ ring system. Hammel, H. B., I. de Pater, S. Gibband, G. W. Lockwood, and K. Rages, 2005. Uranus in 2003: Zonal winds,
banded structures, and discrete features.Icarus, 175 , 534–545.Part of the gas is of direct volcanic origin, and part is
driven by subliming SO 2 frost, which itself is a product of
volcanic eruptions. Several SO 2 , as well as SO, lines have
now been observed, which have been used to derive Io’s
atmospheric structure. The surface pressure is of the order
of a few, perhaps up to 40 nbar, covering 5–20% of the
surface, and the atmosphere may be relatively hot, 500–
600 K at 40 km altitude on the trailing, and 250–300 K on
the leading hemisphere.
2.6.2 TITAN
Of all solar system bodies, Titan’s atmosphere is most sim-
ilar to that of Earth, being dominated by nitrogen gas and
with a surface pressure 1.5 times that on Earth. Methane
gas, with an abundance of a few percent, has a profound
effect on the atmosphere. [SeeTitan.] Photolysis and sub-
sequent chemical reactions lead to the formation of hydro-
carbons and nitriles. Because CO and the nitriles HCN,
HC 3 N (cyanoacetylene), and CH 3 CN (acetonitrile) have
several transitions at (sub)millimeter wavelengths (Fig. 10),
radio observations can be used to constrain the vertical dis-
tributions of these species. As expected from photochemi-
cal models, their abundances increase with altitude and are
highest in the stratosphere.
Disk-resolved spectra, such as obtained with the Sub-
millimeter Array (SMA) and the IRAM Plateau de Bure
Interferometer, also contain information on the zonal wind
profile. Although 12μm spectroscopic measurements had
already suggested the winds to be prograde at∼100–300 km
altitude, the radio data confirmed the direction of the winds
and reported more precise values for the wind speeds in
the upper stratosphere (160±60 m/s at∼200–400 km
altitude), and lower mesosphere (60±20 m/s at∼350–
550 km). At lower altitudes, the winds were determined
via the Doppler Wind Experiment on theHuygensprobe,
when it went down through Titan’s atmosphere. The radio
signal from the probe (communication to theCassinior-
biter) was recorded by the very long baseline interferometry
(VLBI) network. Winds in Titan’s atmosphere affected the
horizontal velocity of the probe during its descent, which
was measured by ground-based radio telescopes through a
shift in the probe’s transmitted frequency (Doppler shift).
These measurements revealed weak prograde winds near
the surface, rising to∼100 m/s at 100–150 km altitude,
with a substantial drop (down to a few m/s at most) near
60–80 km altitude.
The isotopic carbon and nitrogen ratios were first de-
termined from ground-based radio data, and subsequently
confirmed/improved by instruments on board theCassini
spacecraft andHuygensprobe. The^12 C/^13 C isotope ratio