Planetary Radar 763
FIGURE 26 CassiniRADAR (2.2-cm, SL) images of Titan. The bright, rough region on the left side of the image
seems to be topographically high terrain cut by channels and bays. The boundary of the bright (rough) region and the
dark (smooth) region appears to be a shoreline. The patterns in the dark area indicate that it may once have been
flooded. The image is 175×330 km and is centered at 66 S, 356 W. (NASA/JPL.)synthetic-aperture radar (SAR) imager, altimeter, scat-
terometer, and radiometer, will operate during around half
of the 44 Titan flybys, covering about a fifth of the surface
with imaging resolution of 2 km or finer. Scatterometry ob-
servations will cover most of the surface, albeit at resolution
no finer than tens of kilometers, and a limited number of
short altimetry tracks will give regional topographic infor-
mation.
At this writing,Cassinihas completed its first 6 Titan SAR
flybys (Fig. 26), revealing a surface with low relief and an
Earth-like variety of surface features providing evidence for
fluvial/pluvial, cryovolcanic, Aeolian, impact, and probably
tectonic modification processes. Diverse styles of channels
are seen; some suggest that precipitated liquids are col-
lected and transported hundreds of kilometers, others indi-
cate a cryovolcanic origin, and some may be spring-fed. Cir-
cular features apparently include cryovolcanic vents as well
as a surprisingly small number of impact craters. Regions
of dune-like forms that run for hundreds of kilometers es-
tablish that particulate matter is available and that there are
winds that can transport them. The radar-bright, continent-
sized landform Xanadu is revealed in the radar images to be
a landmass of Appalachian-sized mountains and valleys cut
by channels and marked with craters and dark patches. Such
patches, seen in numerous images, are tentatively identified
as hydrocarbon lakes or organic sludge.
3.14.3 ICY SATELLITES
Cassiniradar and radiometric observations of Saturn’s icy
satellites yield properties that apparently are dominated by
subsurface volume scattering and are similar to those of the
icy Galilean satellites. Average radar albedos decrease in
the order Enceladus/Tethys, Rhea, Dione, Hyperion, Iape-
tus, and Phoebe. This sequence most likely corresponds to
increasing contamination of near-surface water ice, which
is intrinsically very transparent at radio wavelengths. Plausi-
ble candidates for contaminants include ammonia, silicates,
metallic oxides, and polar organics. There is correlation of
our targets’ radar and optical albedos, probably due to vari-
ations in the concentration of optically dark contaminants in
near-surface water ice and the resulting variable attenuation
of the high-order multiple scattering responsible for high
radar albedos. Iapetus’ 2.2-cm radar albedo is dramatically
higher on the optically bright trailing side than the opti-
cally dark leading side, whereas 13-cm results show hardly
any hemispheric asymmetry and give a mean radar reflec-
tivity several times lower than the reflectivity measured at
2.2 cm. These Iapetus results are understandable if ammo-
nia is much less abundant on both sides within the upper one
to several decimeters than at greater depths, and if the lead-
ing side’s optically dark contaminant is present to depths of
at least one to several decimeters. A combination of ion
erosion and micrometeoroid gardening may have depleted
ammonia from the surfaces of Saturn’s icy satellites. Given
the hypersensitivity of water ice’s absorption length to am-
monia concentration, an increase in ammonia with depth
could allow efficient 2.2-cm scattering from within the top
one to several decimeters while attenuating 13-cm echoes,
which would require a 6-fold thicker scattering layer.4. Prospects for Planetary Radar
There is growing interest in the possibility of a subsurface
ocean on Europa and in the feasibility of using an orbiting