NOTE: Prelimineary Measurements of the Cryogenic Dielectric Properties of Water-Ammonia Ices: Implications for Radar Observations of Icy Satelites

considered as a pure rock–ice mixture (see Lorenz and Lunine 1997 for a discussion). It is possible that organic compounds (both liquid and I report preliminary measurements of the complex permittivsolid) are present on Titan’s surface—these may allow it to appear optiity of frozen aqueous ammonia solutions at liquid nitrogen cally bright yet defeat the volume scattering effects that would make an temperatures, representative of those in the saturnian system. icy surface radar-bright. It has not yet been demonstrated which (or, The real part of the dielectric constant of 30% ammonia ice is indeed, whether any) combination of rock, ice, and organics can reproduce around 4.5 at near-DC frequencies and at p1 MHz, compared the radar and optical reflectivities simultaneously and in a geologically plausible manner. with around 3.1 for pure water ice. The loss tangents of ammoAnother possibility is that Titan’s surface has a significant non-water nia-rich ices seem somewhat (p50%) higher than those for ice component. The possibility that Titan may have incorporated both water ice, for which the few low-temperature experiments to methane clathrate ice and ammonia hydrate was suggested at an early date indicate values comparable with predictions by Thompson stage (Lewis 1973). Unlike the jovian subnebula, the saturnian nebula and Squyres (1990, Icarus 86, 336–354) and Maetzler (1998, would have been cool enough to incorporate these compounds (e.g., in Solar System Ices (B. Schmitt, C. DeBergh, and M. Festou, Pollack et al. 1976). The fact that methane is present on Titan is consistent Eds.), pp. 241–257, Kluwer Academic, Dordrecht), but considwith the fact that methane clathrate is incorporated into Titan: since erably higher than models by Chyba et al. (1998, Icarus, in ammonia hydrate has a higher condensation temperature, it, too, should press). Ammonia-rich ice may reconcile the radar and optical have been included in the growing Titan (see, e.g., Lewis 1973, Consolappearance of Titan’s surface: the detectability of water– magno and Lewis 1978). According to this scenario, after accretion Titan would have had a ammonia ice on Titan by the Cassini mission and the implicawater–ammonia ocean, with an ammonia-rich atmosphere above. The tions for Titan’s origin and evolution are discussed.  1998 Acainitial ammonia atmosphere would have been processed by photolysis demic Press (Atreya et al. 1978) and/or shock processing (Jones and Lewis 1987) to generate the nitrogen atmosphere we see today: this is a favored model for the formation of Titan’s atmosphere (Lunine 1989). As the ocean cooled, ammonia-free ice I would have formed above it (see, e.g., Lunine Introduction. The optical and microwave properties of water ice close and Stevenson 1987). to its melting point, with a variety of solutes, have received much attention This leaves an ammonia-rich mantle, which could persist as liquid to this owing to their importance in terrestrial remote sensing. The remarkable day, (e.g., Grasset and Sotin 1997). The 176 K water–ammonia melting transparency of cold (,260 K) water ice to microwave radiation has been isotherm could be only a few tens of kilometers below the surface, and noted in returns from both terrestrial and astronomical targets, leading could erupt through cracks in the ice crust which would propagate faster to both anomalously high backscatter and polarization ratio, although than the ammonia-rich cryomagma in them would freeze (Lorenz 1996). few laboratory measurements of ice dielectric properties at low ( ,200 K) Such events would superimpose ammonia-rich flows over the ice I crust. temperatures have been made. A key question is whether the substantial Thus, it is entirely plausible that part of Titan’s surface may be covered presence of ammonia, likely in the saturnian system, significantly modifies by ammonia-rich ice. This possibility forms the key motivation for this these properties. study. In this paper I describe some crude preliminary experiments with Figure 1 shows disk-averaged optical (1 em) and radar reflectivities water–ammonia ice of various compositions, to determine how the presof the Galilean satellites, Titan’s brighter hemisphere, and the Moon. Of ence of ammonia affects the real and imaginary parts of the complex course, all of the bodies discussed are heterogenous at a variety of scales, dielectric constant (hereafter referred to for brevity as the real and imagibut spatially resolved spectral data on Titan are only now becoming nary dielectric constant). I compare the measurements with the few literaavailable, so these disk-averaged data must serve as the present basis for ture values for pure ice, and explore the planetary implications. comparison. It can be seen that the Galileans and the Moon fall on roughly a mixing line—with the position along the line indicating a crude Sample preparation and observations. Analysis grade ammonia solution (33%) was used in the experiments, together with ordinary tap water. rock : ice ratio. Titan does not fall on the line—evidently it cannot be