Models of radar absorption in Europan ice

The detection of a sub-surface present-day ocean on Europa is of considerab le interest. One possible method of detecting an ocean is by an orbiting ra dar sounder. The effects of a range of possible Europan ice chemistries on radar attenuation are investigated, using plausible Europa ice temperature profiles. Ice chemistries are derived from geochemical models of Europa pre dicting a sulfate-dominated ocean, a chloride-dominated ocean scaled from t he Earth, and on experimental data on marine ice formed beneath ice shelves on Earth, on low-salinity sea ice and models of rock and ice mixtures. Chl oride ions are expected to dominate the radar absorption because they are i ncorporated into the ice lattice, though if freezing rates are rapid or sim ilar to sea ice, then brine pockets will dominate losses. In the case of an ocean being present underneath the ice, the range of attenuation found in the models is from about 5 dB/km for rock/ice mixtures up to 80 dB/km for s ea ice models. However, perhaps the best model at present is for ice formed from a plausible sulfate-dominated ocean with the fraction of chloride inc orporated into the ice set to the same as for low accretion rate Ronne Ice Shelf marine ice. This has a radar absorption of 9-16 dB/km for surface tem peratures of 50-100 K. In the case of a convecting isothermal ice layer ben eath a conducting ice lid, absorption in the conducting lid is lower for al l the models than it is over an ocean as the convecting ice is modeled to b e 250-260 K. Absorption in the isothermal layers is very high, but the inte rface between conducting and convecting ice may be marked by a reflection c oefficient that enables it to be imaged. It is concluded that realistic ice -penetrating radars are likely to be able to penetrate some kilometers into the ice, though problems of interpretation caused by scattering are not co nsidered here. (C) 2000 Academic Press.