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

1339 average optical geometrical albedos, plotted vs. distance from Saturn. A very pronounced radar-
1340 optical correlation means that variations in optical and radar albedos involve the same or related
1341 contaminant(s) and/or surface processes. Absolutely pure water ice at satellite surface temperatures
1342 is extremely transparent to radar waves. Contamination of water ice with no more than trace
1343 concentrations of virtually any other substance (including earth rocks, chondritic meteorites, lunar
1344 soil, ammonia, metallic iron, iron oxides and tholins) can decrease its radar transparency (and hence
1345 the regolith's radar albedo) by one to two orders of magnitude. Phenomena that depend on the
1346 distance from Saturn include primordial composition, meteoroid flux and the influx of dark material
1347 (perhaps from several sources), the balance between the influx of ammonia-containing particles and
1348 the removal of ammonia by a combination of ion erosion and micrometeoroid gardening, variation
1349 in E-ring fluxes, and the systematics of thermal volatiles redistribution.
1350 Modeling of icy Galilean satellite echoes (Black et al., 2001b) suggests that penetration to about
1351 half a meter should be adequate to produce the range of measured values of SL-2.
1352 The explanation by Clark et al. (2008b, 2009) for dark Saturnian system material, which suggests
1353 that it consists of sub-micron iron particles from meteorites which oxidized and became mixed with
1354 ice, would also suffice to explain, at least qualitatively, the overall radar-optical albedo correlation
1355 in Fig. 18 if the contamination of the ice extends some one to several decimeters below the optically
1356 visible surface, as expected by Clark et al. (2008b, 2009). The tentative detection of ammonia in
1357 VIMS spectra by Clark et al. (2008a) opens up the possibility that contamination by this material
1358 may play an important role in inter- and intra-satellite radar albedo variations. If so, this
1359
1360
contamination would affect the radar but not the optical albedo.
Almost all Cassini radar data on the icy satellites consist of real-aperture scatterometry, which
1361 means that the beam size determines the resolution. Cassini has no real-aperture resolution of
1362 Mimas, Enceladus, Hyperion or Phoebe, but does have abundant measurements for Tethys, Dione,
1363 Rhea and Iapetus with beams smaller than, and in many cases less than half, the size of the disk
1364 (Ostro et al., 2006).
1365 The SL-2 distributions of the individual satellites can be summarized briefly as follows. As regards
1366 Mimas, Herschel may or may not contribute to the highest value of the disc-integrated albedo.
1367 Enceladus shows no dramatic leading/trailing asymmetry and no noteworthy geographical
1368 variations except for a low albedo in an observation of the Saturn-facing side. Tethys' leading side
1369 is brighter both optically and in SL-2. Dione's highest-absolute-latitude views give the highest radar
1370 albedos. Rhea's leading side has a lower SL-2 in the north, where there is much cratering, and
1371 higher in the south, where there are optically bright rays. Its trailing and Saturn-facing sides, which
1372 look younger, show a relatively high SL-2. Iapetus displays an extremely marked correlation
1373 between optical brightness and SL-2.
1374 In general, a first-order understanding of the SL-2 distribution is that higher SL-2 means cleaner
1375 near-surface ice, but the nature of the contaminant(s) and the depth of the contamination are open
1376 questions. The latter can be constrained by measuring radar albedos at more than one wavelength.
1377 Fortunately, there are Arecibo 13-cm observations of Enceladus, Tethys, Dione, Rhea (Black et al.,
1378 2004) and Iapetus (Black et al., 2007).
1379 Also fortunately, the wavelength dependence observed for Europa, Ganymede and Callisto (EGC)
1380 gives us a context for discussing the wavelength dependence observed for Saturn's satellites.
1381
1382 Wavelength Dependence
1383 EGC total-power radar albedos and circular polarization ratios at 3.5cm and 13cm are
1384 indistinguishable. This means that their regolith structure (distribution of density variations) and ice
1385 cleanliness look identical at the two wavelengths, for each object. That is, near-surface structures
1386 are 'self-similar' at 3.5cm and 13cm scales; there are no depth-dependent phenomena, like
1387 contamination levels or structural character, perceptible in EGC's 3.5cm and 13cm radar signatures.
1388 At 70cm, moreover, EGC SC/OC ratios show little wavelength dependence, so subsurface multiple
1389 scattering remains the primary source of the echo even on this longer scale. However, the radar
1390 albedos (TP-70) plummet for all three objects. The implication is that at 70cm, scattering
1391 heterogeneities are relatively sparse, and the self-similarity of the regolith structure disappears on
1392 the larger scale. On Europa at least, with the most striking albedo drop from TP-13 to TP-70, the
1393 regolith may simply be too young for it to be heterogeneous at 70cm.
1394 Within the framework of the Black et al. (2001b) model, this implies that at 70cm 'the scattering
1395 layer is disappearing in the sense that there are too few scatterer at this scale and they are confined
1396 to a layer too shallow to permit numerous multiple scatterings. At even longer wavelengths, of the
1397
order of several meters, the reflectivity from the surfaces of these moons, and Europa in particular,
26