Reprint - Earth & Planetary Sciences - University of California, Santa ...

1221 The Cassini flyby in June 2004 prior to Saturn orbit insertion afforded views at large phase angles –
1222 important for modeling roughness - and over a full excursion in geographical longitudes. Phoebe
1223 exhibits a substantial forward-scattering component to its single particle phase function. This result
1224 is consistent with a substantial amount of fine-grained dust on the surface of the satellite generated
1225 by particle infall, as suggested by Clark et al. (2005, 2008a). As observed in Cassini images (Porco
1226 et al., 2005a), Phoebe also shows extensive macroscopic roughness, hinting at the violent collisional
1227 history predicted by Nesvorny et al. (2003).
1228 ISS images of the low-albedo hemisphere of Iapetus were fitted to the crater roughness model
1229 (Buratti and Veverka, 1985), yielding a markedly smooth value for the degree of macroscopic
1230 roughness on this side with a mean slope angle of only a few degrees (Lee et al., 2009). This result
1231 suggests that small-scale rough features on the dark side have been filled in (features which
1232 probably dominate the photometric effects; see Helfenstein and Shepard, 1998), which is consistent
1233 with the idea of Spencer and Denk (2009) that thermal migration of water ice leaves behind a fluffy
1234 residue of dark material. The low-albedo side of Iapetus shows a roughness similar to Enceladus,
1235 which is coated with micron-sized particles from its plume and the E-ring (Verbiscer and Veverka,
1236
1237
1994; see Table 21.4.1). Model-fits for the low albedo hemisphere are shown in Fig. 14.
1238 Fig. 16 A macroscopic roughness model for the low-albedo hemisphere of Iapetus, based on the
1239 crater roughness model by Buratti and Veverka (1985). The best-fit roughness function corresponds
1240 to a depth-to-radius of 0.14, or a mean slope angle of 11º, which is much lower than the ~30º typical
1241
1242
1243
of other icy satellites (see Tab. 4). For these fits, a surface phase function f(α) of 0.023 was derived.
From Lee et al., 2009
1244 Finally, observations at very large solar phase angles (155º-165º) have been used to search for
1245 cryovolcanic activity on satellites. By comparing the brightness of Rhea to Enceladus in this phase
1246 angle range at 2.02µm, Pitman et al. (2008) were able to place an upper limit on water vapor
column density based on a possible plume of 1.52 x 10 14 to 1.91 x 10 15 cm -2 1247 , two orders of
1248
1249
magnitude below the observed plume density of Enceladus (Fig. 17).
1250 Fig. 17 Cassini VIMS disk-integrated brightness as a function of solar phase angle at 2.23µ.
1251 Symbols: Normalized Rhea and Enceladus VIMS brightness (every fifth data point plotted). Solid
1252 line: Third-order polynomial fit to Rhea data. Dashed lines: (polynomial fit to Rhea data).
1253 Enceladus's plume can be seen as a peak at a solar phase angle of 159°. Adapted from Pitman et al.
1254
1255
(2008)
1256 The microphysical structure of the E-ring grain coatings of the inner icy moons embedded in the E-
1257 ring can be probed by investigating the opposition surge. Verbiscer et al. (2007) found that the
1258 amplitude and angular width of the opposition effect on the inner icy moons are correlated with the
1259
1260
moons’ position relative to the E-ring.
1261 21.5 Constraints on the Top-Meter Structure and Composition by Radar
1262 The wavelength of Cassini's 13.8GHz RADAR instrument, 2.2cm, is about six times shorter than
1263
1264
the only ground-based radar wavelength available to study the satellites (13cm, at Arecibo) and 22
times longer than the millimeter wavelengths at the limit of the CIRS.
1265 The echoes result from volume scattering, and their strength is sensitive to ice purity. Therefore,
1266 they provide unique information about near-surface structural complexity as well as about
1267 contamination with non-ice material. This section summarizes the most basic aspects of Cassini-
1268 and ground-based radar observations of the satellites, the theoretical context for interpreting the
1269 echoes, and the inferences drawn about subsurface structure and composition.
1270 Cassini measures echoes in the same linear polarization as transmitted, whereas most ground-based
1271 radar astronomy consists of Arecibo 13cm or 70cm observations and Goldstone 3.5cm observations
1272 that almost always use transmission of a circularly polarized signal and simultaneous reception of
1273 echoes in same-circular (SC) and opposite-circular (OC) polarizations. A radar target's radar albedo
1274 is defined as its radar cross section divided by its projected area. The radar cross section is the
1275 projected area of a perfectly reflective isotropic scatterer which, if observed at the target's distance
1276 from the radar and using the same transmitted and received polarizations, would return the observed
1277 echo power. The total-power radar albedo is the sum of the albedos in two orthogonal polarizations,
1278 TP = SL + OL = SC + OC. Here, we will use 'SL-2' to denote the 2cm radar albedo in the SL
1279
polarization, 'TP-13' to denote the 13cm total-power albedo, etc.
24