Radar Detection of Iapetus

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Abstract
Saturn's satellite Iapetus has the largest albedo asymmetry of any natural satellite, with the optical albedo of its trailing hemisphere as much as 10 times that of its leading hemisphere ([ 1 ][1]). The sharp contrast on this synchronously rotating satellite suggests that exogenic materials, such as dust from small saturnian satellites ([ 2 ][2], [ 3 ][3]) or atmospheric photochemical products from Titan ([ 4 ][4]), have been preferentially deposited on the leading hemisphere. However, an endogenic origin ([ 5 ][5]), involving either eruption of dark or clean material, selective sputtering, deposition, or a combination thereof, has not been conclusively ruled out. Spectral features of the dark material have not been definitively correlated with any material ([ 4 ][4]). The brighter hemisphere is covered mainly by water ice, which, given the moon's bulk density of 1.2 g cm–3 ([ 6 ][6]), is also the most likely dominant component of the entire body. The brighter side has a visual albedo and water-ice infrared absorption-band depths ([ 7 ][7]) similar to those of Ganymede. We used the Arecibo Observatory's 12.6-cm (2.38-GHz) radar system ([ 8 ][8]) to observe Iapetus' optically bright, trailing hemisphere on 8, 9, and 10 January 2002, and the optically dark, leading hemisphere on 9 and 10 January 2003. Weighted sums of echo spectra ([Fig. 1][9]) yield radar albedos for Iapetus ([Table 1][10]) that do not mirror the marked hemispheric asymmetry seen in its optical albedos. The globally low radar reflectivity indicates that the upper few meters of Iapetus' regolith on both hemispheres contains an electrically absorbing contaminant that attenuates the radar signal. Two candidates for the contaminant are ammonia and the material darkening the leading hemisphere. ![ Fig. 1. ][11] Fig. 1. Radar echo spectra of the trailing and leading hemispheres of Iapetus. Solid lines show echoes in the circular polarization sense opposite to that transmitted (OC); dashed lines show the same circular sense (SC). The frequency resolution is 5 Hz, barely resolving the echo, which is Doppler-broadened by Iapetus' rotation to ∼20 Hz. A 1-SD bar is shown. Above the spectra are airbrushed maps of Iapetus at the observing epochs, based on Voyager imagery. | Satellite | σ | σtot | μC | |:----------- | --------- | --------- | --------- | | lapetus (T) | 0.12±0.02 | 0.17±0.04 | 0.46±0.10 | | lapetus (L) | 0.10±0.02 | 0.13±0.04 | 0.33±0.07 | | Callisto | 0.32 | 0.69 | 1.17 | | Moon | 0.07 | 0.08 | 0.1 | Table 1. The radar properties of both hemispheres of Iapetus at 12.6-cm wavelength, compared to those of Callisto ([ 12 ][12]) and the Moon ([ 13 ][13]). These integrated albedos are normalized by the target's projected area, with a mean radius of 732 km for lapetus ([ 14 ][14]). The uncertainties for lapetus are based on 1 standard deviation of the noise statistics and not on possible systematics, which could be as much as 25%. T, trailing; L, leading; σ, albedo in the OC polarization; σtot, total albedo in both polarizations; μC, ratio of SC albedo to the OC albedo. The addition of 10 to 30% by weight of ammonia to water ice can increase its microwave absorption ([ 9 ][15]). Ammonia may have been an abundant constituent in the saturnian protonebula that would have been incorporated into its satellites ([ 10 ][16]). The absence of spectral evidence for ammonia and ammonia products on the surface may be the result of selective depletion by ion sputtering ([ 11 ][17]), leaving an ammonia-poor crust over an ammonia-rich ice that would affect the radar reflectivity yet remain undetected at optical and infrared wavelengths. A less likely absorber candidate is the dark material that covers the leading hemisphere. Although it appears to have a minor presence on the brighter, trailing hemisphere, an admixture of material in the ice below the surface could still attenuate the radar signal. To match the optical and infrared surface, this scenario would require a mechanism to deposit clean ice over the dark material on the trailing side. That the radar sees little hemispheric asymmetry suggests that the leading side's dark material is either electrically nonabsorbing and/or thin enough to be essentially transparent to the radar, i.e., only a few centimeters. The dark material's composition is unknown, but plausible materials include dust or organics from nearby moons. If it is thin, then its deposition must occur at a rate rapid enough to avoid impact exposure of the underlying ice. Although uncertain, Iapetus' radar polarization ratios appear to fall between the low ratio for the Moon and the high ratios of the icy Galilean satellites. 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