The situation is dramatically different for sea ice, which in general is a mixture of ice, brine, air, and precipitated salts. Brine pockets within the ice are strong scatterers and attenuators that greatly reduce the depth of penetration into ice even at radio frequencies. There can be strong vertical variations in the dielectric constant as well. These change seasonally as thermodynamic forcings cause brine to migrate within the icy matrix. Consequently, the inversion problem is considerably more difficult,6 even with radar systems sufficiently powerful to penetrate sea ice, which is typically only several meters thick.

Radar technology offers the unique potential for detailed and direct mapping of Europa's ice shell and its internal structure. Information on the phase and amplitude of radar echoes from the bottom of the shell may also reveal something about the interface, for example, if the ice rests on water. Nevertheless, the potential of radar has to be tempered against the possibly complicated, three-dimensional variations of the dielectric constant in Europa's ice shell. For example, radar absorption through materials that might compose portions of the shell ranges from 10-5 dB/m for pure ice at 200 K to 1 dB/m for briny ice (Figure 3.1). Recent calculations suggest that available constraints on the properties of Europa's ice place a limit of about 10 km on the depth to which an ocean might be detectable by an orbiting radar system.7 Of course, the actual performance may be better or worse depending on the true temperature and chemical composition of the icy shell.

CHARACTERIZATION OF DEEP INTERIOR STRUCTURE AND DYNAMICAL PROCESSES

Measurements of the topography, gravity field, and magnetic field of Europa will aid efforts to characterize Europa's deep internal structure and dynamics. It is important to determine if Europa has a magnetic field, which will indicate whether convection and dynamo action are occurring in a liquid part of Europa's core. Although Galileo's magnetometer has been able to detect Ganymede's magnetic field,8 it has not been able to detect an intrinsic europan magnetic field.9 A small field, however, could exist but remain undetectable in the presence of larger magnetic-field perturbations due to induction effects in Europa and plasma effects around Europa.

FIGURE 3.1 The attenuation of 50-MHz (or 6-m-wavelength) radar in ice as a function of temperature and impurity content. The solid line shows the attenuation in pure ice, and the dashed curves show the effects of contaminating the ice with varying percentages (by volume) of lunar soil. The attenuation caused by briny ice is even more extreme. Illustration adapted from C.F. Chyba, S.J. Ostro, and B.C. Edwards, "Radar Detectability of a Subsurface Ocean on Europa," Icarus 134: 292, 1998.



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