Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.6109g&link_type=abstract
American Astronomical Society, DPS meeting #38, #61.09; Bulletin of the American Astronomical Society, Vol. 38, p.604
Astronomy and Astrophysics
Astronomy
Scientific paper
Between August and December 2005, we conducted 76 oblique-incidence scattering experiments using the SRI 46-m antenna in the Stanford foothills to illuminate Mars for 20 min. periods with an unmodulated 75 cm-λ, circularly polarized wave. The direct signal and a Martian surface echo, which are separated by Doppler frequency, were received simultaneously by the one-bit receiver on board the Mars Odyssey spacecraft. Out of 45 experiments with high signal-to-noise ratios, 27 were in the northern hemisphere, while 18 were in the southern hemisphere, where preliminary data analysis is available.
The surface echoes are characterized by both fluctuating amplitude and varying spectral width, which correspond roughly to the surface reflectivity and roughness, respectively. Analysis of the data is based on quasi-specular scattering theory, but interpretation of the echoes is complicated by Odyssey's reception of only the right-circular polarized (RCP) wave component, and by the high incidence angles involved (f > 60 deg.), for which the scattering theory is not well developed.
Our analysis of the echoes makes use of MOLA topographic maps at a resolution of 128 points per deg. of longitude and latitude, to model the scattering surface in three dimensions along the specular track. We can account for most of the echo amplitude fluctuations by the variation in number of surface-model facets tilted to produce a specular reflection towards Odyssey, indicating that MOLA scale topography is sufficient to capture an important scattering mechanism at this wavelength. With this we have accomplished a first step in differentiating between changes in echo signal strength due to surface reflectivity and surface shape. At the same time, we obtain a measure of the small scale surface roughness by finding the maximum tilt angle away from a perfectly mirroring surface facet which contributes significantly to the echo at each time step.
Callas John L.
Cousins M. D.
Gunnarsdottir Hrefna M.
Linscott Ivan R.
Tyler Leonard G.
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