Icy Galilean Satellites: Radar Echoes Due to a Coherent Backscatter Effect
G. J. Black, D. B. Campbell, P. D. Nicholson (Cornell University)
We have modeled the radar scattering properties of Europa, Ganymede, and Callisto using a vector formulation of the coherent backscatter effect [1]. At wavelengths of 3.5 cm and 12.6 cm, the specific radar cross sections [2] of the icy Galilean satellites are not only an order of magnitude larger than those of typical inner Solar System targets, but also greater than unity, indicating that these icy surfaces preferentially backscatter. Even more unusual is that the icy surface reflection mechanism largely preserves the incident polarization. At 70 cm wavelength the cross sections of Europa, Ganymede, and Callisto are lower than those measured at the shorter wavelengths by factors of 4-10, while the polarization ratios appear to remain at high values [3].
In order to explain the radar data several possible mechanisms have been proposed by various workers, all of which are based on the fact that the surfaces and upper layers of these moons are mainly water ice. Water ice at the low temperatures found on these objects is a poor absorber of radio-wavelength radiation, permitting the radar signal to penetrate more efficiently into the surface than is possible with silicate compositions. We have adapted a model of the coherent backscatter effect [1] to explain the observed wavelength dependence of the radar cross sections. Since the subsurface attenuation coefficient can be quite small, the presence of embedded wavelength-scale scatterers may result in this particular multiple-scattering process, thus enhancing the echo near exact backscatter as well as preserving the incident polarization. By assuming Mie scatterers that are distributed in size as a power law, we fit the observed wavelength variations in cross section and polarization properties. Model parameters that best reproduce the data indicate that the scattering layers are on the order of ten meters thick. The scatterer size distributions follow fairly steep power laws, with the maximum scatterer sizes falling between the two longest wavelengths. The absorption lengths in the media are not well constrained but must be longer than the depth of the respective layer.
[1] Peters, K. (1992). Phys. Rev. B 46, 801. [2] Ostro, S. J., et al. (1992). JGR 97, 18227. [3] Black, G. J., et al. (1996). LPSC XXVII 1, 143.