Chaotic Rotation of Hyperion and Modeling the Radar Properties of the Icy Galilean Satellites as a Coherent Backscatter Effect
Abstract
The rotation of Saturn's moon Hyperion is investigated by numerical integration of the full threedimensional Euler equations. During the 1981 Voyager encounter Hyperion was rotating about an axis near its minimum moment of inertia at a rate roughly four times its mean orbital rate. The integrations demonstrate that this and slower states are completely chaotic. In 1981 the motion was dominated by two semiregular periods, one of ~300 days and another of ~7 days which are identified respectively as forced precession in the Saturnian gravity field and wobble due to the nonprincipal axis rotation. The lightcurve based upon this rotation state is in excellent agreement with that derived from low resolution Voyager images. The radar scattering properties of Europa, Ganymede, and Callisto are unlike those of any other object observed with planetary radars. They are strongly backscattering with specific radar cross sections that can exceed unity. Polarization ratios are also high, ~1.5, indicative of multiple scattering, and the echos follow a diffuse scattering law at all incident angles with no indication of quasispecular reflections. Observations made in 1988 and 1990 with the Arecibo radar at 70 cm wavelength are analyzed. The cross sections at this wavelength are much lower than previous results at wavelengths of 3.5 cm and 12.6 cm, although the polarization ratios are consistent. The unusual radar properties of these objects are modeled as a coherent backscatter effect which results from scatterers embedded in the weakly absorbing water ice surfaces of these moons. The model is applied to the observed wavelength variations of the radar properties to derive generalized properties of the scattering layer. This model can reproduce the data with scatterers following fairly steep power law size distributions with exponents of 3.5 to 4.0 and maximum sizes of 0.51.0 m. Efficient scatterers are required, and partially absorbing scatterers such as silicates cannot produce the high cross sections. The model is less sensitive to absorption in the scattering layer or its thickness, but suggests that the radar wave needs to penetrate only several tens of meters for this mechanism to produce the observed scattering properties.
 Publication:

Ph.D. Thesis
 Pub Date:
 1997
 Bibcode:
 1997PhDT........17B
 Keywords:

 MOONS;
 JUPITER;
 SATURN;
 EUROPA;
 GANYMEDE;
 CALLISTO;
 Physics: Astronomy and Astrophysics