[Previous] | [Session 66] | [Next]
W.B. McKinnon (Dept. EPSc, Washington Univ.)
The proposed radar sounding of the Europan shell is based on the superb transparency of cold water ice at radar frequencies. As modeled by Chyba et al. (1998, Icarus 134), it can be defeated by contamination by higher conductivity components (such as silicates or salts) or substantial amounts of hot (T >250 K) ice in a convecting sublayer. The point of this talk is to show how convection within the Europan shell may actually aid the detection of any ocean below. It is based on numerical calculations of convective planforms in model ice shells by the author and M. Gurnis (1999, LPSC XXX). When convection is weak (conditions near the threshold for convective instability), the planform consists of well-spaced steady-state diapiric upwellings and broad regions of cool, sluggish downflows (conditions that may be amplified when tidal heating of the ice is explicitly modeled). The average temperature of the ice in the convecting sublayer is indeed high, as theory suggests (McKinnon 1999, GRL 26), but as long as the convection is weak the temperature variation across the sublayer is strong. Most importantly, temperature profiles in downwelling regions are cooler than the initial conductive profile. Hence, using the attenuation model for ice in Chyba et al., we find assuming a threshold two-way attenuation of ~50 dB and a rock contamination fraction of 1% that the ice/ocean interface can be imaged to a depth of greater than ~20 km at Europa's equator (more than a factor of two increase over the conductive case). Greater depths are potentially soundable at the poles, but convection is less likely there (the viscosity contrast and tidal heating are greater, resulting in a thinner, stiffer shell). Even for vigorously convecting sublayers, the overlying stagnant lid is thicker than previously modeled, resulting in comparatively greater sounding depths. These increases in soundability may prove important in the face of mounting evidence for substantial sulfate salts in Europa's ice.