Preliminary measurements of the cryogenic dielectric properties of water-ammonia ices: Implications for radar observations of icy satellites

I report preliminary measurements of the complex permittivity of frozen aqueous ammonia solutions at liquid nitrogen temperatures, representative of those in the saturnian system. The real part of the dielectric constant of 30% ammonia ice is around 4.5 at near-DC frequencies and at similar to 1 MHz, compared with around 3.1 for pure water ice. The loss tangents of ammonia-rich ices seem somewhat (similar to 50%) higher than those for water ice, for which the few low-temperature experiments to date indicate values comparable with predictions by Thompson and Squyres (1990, Icarus 86, 336-354) and Maetzler (1998, in Solar System Ices (B. Schmitt, C. DeBergh, and Rtl. Festou, Eds,), pp, 241-257, Kluffer Academic, Dordrecht), but considerably higher than models by Chyba et al, (1998, Icarus, in press). Ammonia-rich ice may reconcile the radar and optical appearance of Titan's surface: the detectability of water-ammonia ice on Titan by: the Cassini mission and the implications for Titan's origin and evolution are discussed. (C) 1998 Academic Press.