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L. M. Carter, D. B. Campbell (Cornell University), J. L. Margot (Caltech), B. A. Campbell (Smithsonian Institution)
The Magellan spacecraft returned hundreds of images of parabolic craters, radar-dark halos, and wind features on Venus. It is clear that dust layers are associated with some of these features, and thin layers that are not visible on the Magellan imagery may be present for others. Such layers can be detected from multi-polarization radar observations, something that the Magellan radar was not equipped for. If a radar wave is incident on a surface at angles significantly different from normal, penetrates a deposit with depths of up to a few wavelengths, and is reflected by embedded scatterers or by an underlying structure, then the returned radar echo will have a linearly polarized component. The linear polarization is due to differing transmission coefficients in, and perpendicular to, the plane of incidence and reflection. Measurement of the amount of linear polarization can be used to model some of the physical properties of the deposit.
In March 2001 we used the upgraded Arecibo radar system to make 12.6 cm wavelength radar observations of Venus. A circularly polarized wave was transmitted, the echo was received in both circular polarizations, and delay-Doppler images were formed in the four Stokes' polarization parameters. From the Stokes' parameters, images of the degree and angle of any linearly polarized component can be obtained. Distinct linearly polarized echoes were found from several regions in the northern hemisphere and from two regions in the southern hemisphere. Comparing the linear polarization data to Magellan imagery reveals that linear polarization is associated with several types of surface morphologies including crater deposits, areas with wind streaks, and small clusters of volcanic domes. The linearly polarized surface return is usually associated with diffuse, radar-bright areas. Simple surface models can help to determine the nature of these deposits, such as their dielectric constant. The polarization data will be presented along with a discussion of preliminary results of surface models.