The Solar System at Radio Wavelengths 703
planet’s rings is dominated by Saturn’s thermal radiation re-
flected off the ring particles. Only a small fraction of the ra-
diation at centimeter wavelengths is thermal emission from
the rings themselves.
Like on Jupiter, radio spectra of the atmospheric emis-
sion can be interpreted in terms of its ammonia abundance
and local variations therein with altitude and latitude. The
ammonia and hydrogen sulfide abundances on Saturn are
likely∼3 times more enhanced than on Jupiter. The lat-
itudinal structure on Saturn’s disk, presumably caused by
latitudinal variations in microwave opacity, changes consid-
erably over time.
The classical A, B and C rings, with the Cassini Division,
are clearly visible on Fig. 8. The inner B ring is brightest,
with a brightness temperature of∼10 K. At 1–3 mm the
temperature rises to∼20–25 K. In front of the planet, the
rings block out part of Saturn’s radio emission, resulting in
an absorption feature. From this feature one can determine
the optical depth of the rings, which is approximately 1 in
the B ring. The West (right) ring ansa is usually somewhat
brighter than the East side, which has been attributed to the
presence of gravitational ‘wakes’, which are 10–100 m sized
density enhancements behind large ring particles which,
because of Keplerian shear, travel at an angle to the big
particle’s orbit. Similar asymmetries have been seen in the
A ring in front of the planet.
A combination of radio and radar data show that the ring
particles have sizes from∼1cmupto∼5–10 m, where
the number of particles,N, at a given size,R, varies ap-
proximately asN∼R−^3. Such a particle size distribution
would be expected from a collisionally evolved population
of particles.
2.5.4 URANUS AND NEPTUNE
Radio spectra of Uranus and Neptune (Fig. 6b) suggest
an overall depletion of ammonia gas in their upper atmo-
spheres, by roughly 2 orders of magnitude compared to
the solar nitrogen value. This apparent depletion is likely
caused by a nearly complete removal of NH 3 gas in the
upper atmosphere through the formation of NH 4 SH. This
is possible if H 2 S is considerably (factor of>5) enhanced
above solar S. Radio models predict enhancements by a fac-
tor of∼10 on Uranus and∼30 on Neptune. Good fits to
Uranus’ spectrum are obtained if NH 3 is close to the solar
N abundance in Uranus’ deep atmosphere. However, am-
monia gas must be depleted in Neptune’s atmosphere to
match radio spectra. Nitrogen on Neptune may therefore
be present in the form of both N 2 and NH 3 , rather than
only in the form of ammonia gas. An alternative idea that is
advocated by some researchers is based on a large uptake
of ammonia in the icy giant’s ionic oceans, deep in their
interiors.
Uranus is unique among the planets in having its rotation
axis closely aligned with the plane in which the planet or-
bits the Sun. With its orbital period of 84 years, the seasons
on Uranus last 21 years. During theVoyagerencounter,
in 1986, Uranus’ south pole was facing the Sun (and us).
Since that time, this pole is slowly moving out of sight, while
the north pole is coming into view. Uranus brightness tem-
perature has been monitored since 1966. A pronounced
increase in brightness temperature was noticed when the
south pole came into view, followed by a decrease when the
pole moved away again (Fig. 6b). These measurements sug-
gest that Uranus’ south pole is considerably warmer than
the equatorial region, a theory later confirmed by radio im-
ages from the VLA. Figure 9 on the following page shows
one such image taken in the summer of 2003, along with
an image at near-infrared wavelengths (1.6μm) taken with
the adaptive optics system on the Keck telescope. The VLA
image shows that the south pole is brightest. It also shows
enhanced brightness in the far-north (to the right on the
image). At near-infrared wavelengths, Uranus is visible in
reflected sunlight. The bright regions are clouds at high
(upper troposphere) altitudes. The bright band around the
south pole is at the lower edge of the VLA-bright south
polar region. Air in this band may rise up, with condens-
ables forming clouds, and descend over the pole. At radio
wavelengths, this dry air allows us to probe deeper warmer
layers in Uranus’ atmosphere.
On Neptune we also see the poles (at least the visible
south pole) to be the hottest region on the planet, indicative
of a similarity in atmospheric dynamics between the two ice
giants.2.6 Major Satellites and Small Bodies
2.6.1 GALILEAN SATELLITES
Radio spectra of the Galilean satellites are diverse. The
brightness temperature at infrared wavelengths can be re-
lated directly to the satellite’s albedo, and hence Callisto,
with its relatively low albedo (A= 0 .13) is warmer than
Io and Europa. The brightness temperature at radio wave-
lengths is determined by the physical temperature and ra-
dio emissivity,e, of the subsurface,e= 1 −a, withathe
radar geometric albedo. The observed brightness tempera-
tures for Ganymede and, in particular, Europa are well be-
low the physical temperature of the subsurface layers. This
measurement is consistent with the high radar albedo for
these objects:a= 0 .33 for Ganymede and 0.65 for Europa.
These high albedos and consequently low emissivities and
radio brightness temperatures are likely caused by coherent
backscattering in fractured ice.
Since the detection of an ionosphere around Io by the
Pioneer 10spacecraft in 1973, this satellite is known to
posses a tenuous atmosphere. The first detection of a global
atmosphere was obtained in 1990, where a rotational line
of sulfur dioxide (SO 2 ) gas was measured at 222 GHz. Io
is the only object with an atmosphere dominated by SO 2
gas, the origin of which can ultimately be attributed to the
satellite’s volcanism.