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L. M. Carter, D. B. Campbell (Cornell University), M. C. Nolan (Arecibo Obs./NAIC)
Most asteroids, except for small ones (~10 km) with low escape velocities, are expected to have regoliths generated primarily by impact events (McKay et al. 1989, Asteroids II). For example, high resolution (1~m/pixel) images of 433 Eros show infilling of craters, slumps, smooth surfaces, and ejecta blocks which indicate substantial covering by impact deposited material (Veverka et al. 2000, Science, 292, 485). Such regoliths are probably present on other asteroids, but may be difficult to detect at the relatively low spatial resolutions obtained by ground based imaging.
Surface layers with depths of centimeters to meters can be detected with multi-polarization radar observations. If a circularly polarized radar wave refracts into a surface that is smooth at wavelength scales 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 occurs because the horizontal and vertical components of the wave have different transmission coefficients into and out of the surface. This technique has been used by Stacy (1993, Ph.D. Thesis, Cornell University) to study the lunar regolith and by Carter et al. (2003, LPSC XXXIV, 1809) to study surface deposits on Venus.
The small (~7 km across) asteroid 1999 JM8 was observed with both the Goldstone and Arecibo radars during its close approach to Earth (Benner et al. 2002, Meteoritics and Planetary Science, 37, 779). The delay-Doppler images from these radar runs have resolutions as small as 15~m/pixel, which gives thousands of pixels on the asteroid. The high resolution data provide a good opportunity to use the linear polarization technique to search for a possible regolith on the asteroid. From the two received circular polarizations we generate images in each of the four Stokes polarization parameters, and we use these to create images showing the degree of linear polarization across the asteroid. We will present the results of this analysis for multiple dates and asteroid viewing geometries.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.