Abstract
We present new maps of Mercury, Ganymede, and Callisto derived from Earth-based radar telescope observations. Standard range-Doppler imaging of solar system objects suffers from two significant shortcomings: first, fold-over caused by the inability to separate echoes from the northern and southern hemispheres, and second, aliasing due to high rotation rates relative to the physical size of the objects. Our experiments were designed to minimize these error sources. In the case of Mercury, aliasing was prevented by using a random code as the range modulation waveform. For Ganymede and Callisto, we employed a single transmitting antenna and an array of receiving telescopes to synthesize an aperture and avoid north-south ambiguity in the images.

Our experiments resulted in 6 km resolution maps of the northern and southern polar regions of Mercury from the Goldstone Solar System Radar, and 360 km resolution maps of Ganymede and Callisto from the combined Goldstone - Very Large Array instrument. These observations are the first high resolution images of Mercury's polar region at the 3.5 cm wavelength, and the first unambiguous centimeter wavelength radar images of Ganymede and Callisto, respectively. Previous radar observations of Mercury's polar regions noted enhanced scattering from deposits on the floors of craters at high latitudes. This same bright scattering occurs when observing the largest moons of Jupiter, which are known to be covered with an icy regolith. This implies that cold-trapped volatiles, such as water ice, cause the enhanced scattering from Mercury's polar craters. High resolution imaging allows for the identification of terrains which exhibit the enhanced scattering, and can lead to improved models of the dynamics of resurfacing and volatile transport on these planetary bodies.

The study of planets and their moons has benefited greatly from the ability to launch a small optical telescope or radar instrument into orbit a few hundred miles above another planet in the solar system. However, the price tags of such missions, ranging from a few hundred million to several billion dollars, lead to long delays between successive opportunities and spectacular financial and political disasters in the case of mission failure. Improved Earth-based observations of planets, while often not as detailed as spacecraft-based measurements, offer a cost effective alternative to the big interplanetary missions.