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We have obtained the first ultraviolet linear spectropolarimetry of Mars using the Wisconsin Ultraviolet PhotoPolarimetry Experiment (WUPPE), which flew on the Astro-2 shuttle mission. Mars was observed on March 12, 1995 at a phase angle of 21 degrees. A ground-based support observation was taken within 24 hours at the Pine Bluff Observatory (PBO), providing a total spectral coverage of 2000 - 10,000 \AA.
The ultraviolet polarzation increases almost linearly with decreasing wavelength. The polarization reaches a maximum of 5\% at 2000 \AA. An empirical fit to the UV data predict zero polarization at about 4600 \AA. This is clearly seen in the PBO data, as is a position angle rotation of 90 degrees -- as would be expected at this phase angle if the dominant polarization source were switching from atmospheric to surface-based.
We will present an analysis of the WUPPE+PBO polarimetry, examining several possible mechanisms. Our efforts will involve the characterizing the state of the Martian atmosphere at the time of the WUPPE observations using recent WFPC2 and FOS data (courtesy of Phil James, University of Toledo). WUPPE is supported by NASA contract NAS5-26777
