762 Encyclopedia of the Solar System
FIGURE 24 Arecibo OC and SC
echo spectra obtained for comet
IRAS–Araki–Alcock, truncated at
2% of the maximum OC
amplitude. The narrowband echo
from the nucleus is flanked by
broadband echo from large (= 1
cm) particles in a 1000-km-radius
cloud surrounding the nucleus.
(From Harmon, J. K., Campbell,
D. B., Hine, A. A., Shapiro, I. I.,
and Marsden, B. G., 1989,
Astrophys. J. 338 , 1071–1093.)the scale of the smaller wavelength. Delay–Doppler reso-
lution of ring echoes indicates that the portions of the ring
system that are brightest optically (the A and B rings) also
return most of the radar echoes. The C ring has a very low
radar reflectivity, presumably because of either a low parti-
cle density in that region or compositions or particle sizes
that lead to inefficient scattering.
Recent 13-cm images show a pronounced azimuthal
asymmetry in the reflectivity of the A ring. The analogous
phenomenon at visual wavelengths is ascribed to gravita-
tional “wakes” generated by individual large ring particles
or arising from internal instabilities, which are distorted
by Keplerian shear into elongated structures trailing at an-
gles of 70◦from the radial direction. The strength of the
radar asymmetry may be due to strongly forward-scattering
meter-size ice particles and the resultant sensitivity to opti-
cal depth variations.
3.14.2 TITAN
Titan’s thick, hazy atmosphere poses challenges to visible-
wavelength and near-infrared imaging of its surface.Voy-
agerand ground-based data indicate a surface temperature
and pressure of 94 K and 1.5 bar and show that the atmo-
sphere is mostly N 2 with traces of hydrocarbons and nitriles.
Thermodynamic considerations imply a near-surface reser-
voir of liquid hydrocarbons. Arecibo 13-cm echoes show
most of the power to be diffusely scattered, and the longi-
tude dependence of the radar albedo mimics the depen-
dence of the disk-integrated near-IR albedos, indicating
that whatever properties of the surface—roughness, com-
position, etc.—that are responsible for the variation in the
near-IR albedo are also responsible for the variation in the
radar cross section. A specular component is present for
about 75% of the sub-Earth locations. The most specular
echoes (e.g., Fig 25), which are subradar glints that must
come from extremely smooth surfaces, have properties con-
sistent with those expected for irregularly shaped 50-km or
larger bodies of liquid hydrocarbons.
Titan is the primary target of the Cassini mis-
sion. Cassini’s radar instrument, a 13.8-GHz (2.2-cm)FIGURE 25 Arecibo 13-cm OC radar echo spectrum ot Titan at
1.0-Hz resolution for sub-Earth longitude of 80◦.Afitofa
composite model with Hagfors and Cosine terms gives an rms
slope of 0.2◦and a reflection coefficient of 0.023. Titan’s echo
bandwidth is 325 Hz. (From Campbell, D. B., Black, G. J.,
Carter, L. M., and Ostro, S. J., 2003,Science 302 , 431–434.)