Planetary Radar 753
FIGURE 14 Radar backscatter and digital elevation models of
regions on the Moon from Goldstone 3.5-cm, OC, tristatic and
quadristatic interferometry. (a) The south polar region. The
radar results establish that the interiors of many of the Moon’s
polar craters are in permanent shadow from solar illumination.
(Reprinted with permission from J. M. Margot, D. B. Campbell,
R. F. Jurgens, and M. A. Slade, 1999,Science 284 , 1658–1660,
copyright 1999 American Association for the Advancement of
Science.) (b) The crater Tycho. (J. L. Margot, D. B. Campbell, R.
F. Jurgens, and M. A. Slade, 1999,J. Geophys. Res. 104 , E5,
11875–11882.)
FIGURE 15 Arecibo 13-cm OC delay–Doppler image of Venus.
In the middle of the right half of the image is the bright,
1200-km-wide Alpha Regio, a complex of intersecting ridges.
Just south of Alpha is the 300-km-diameter circular feature Eve.
The three prominent craters in the middle of the left half of the
image are seen close-up in Fig. 17d. Courtesy of D. B. Campbell.behavior. For example, subsurface scattering of an incident
circularly polarized signal results in a linearly polarized
component in the radar echo due to the differing trans-
mission coefficients at a smooth surface boundary for the
horizontally and vertically polarized components of the inci-
dent wave. A linearly polarized component in 70-cm echoes
from certain topographic features on Venus has been at-
tributed to subsurface echoes from a mantled substrate or
from buried rocks.3.9 Radar Evidence for Ice Deposits
at Mercury’s Poles
The first full-disk (Goldstone–VLA) radar portraits of
Mercury surprisingly showed bright polar features withμC
>1, and subsequent delay–Doppler imagery from Arecibo
established that the anomalous echoes originate from inte-
riors of craters that are perpetually shaded from sunlight
because of Mercury’s near-zero obliquity (see Fig. 16). The
angle between the orbital planes of Mercury and Earth is 7◦,
so portions of the permanently shadowed regions are visible
to Earth-based radars. At each pole, bright radar features
correlate exactly with craters seen inMariner 10images;
numerous features also lie in the hemisphere not imaged
by that spacecraft.
Similarities between the radar scattering properties of
the Mars and Mercury polar anomalies and those of the icy
Galilean satellites (see Section 3.7) support the inference
that the radar anomalies are deposits of water ice. Temper-
atures below 120 K in the permanent shadows are expected
and are low enough for ice to be stable against sublimation
for billions of years. Temperatures several tens of kelvins
lower may exist inside high-latitude craters and perhaps