WUPPE observed Mars on Mar12, 1995, and an HPOL observation was
obtained at PBO about 5 hours later, giving us a wavelength
range about 2000-10,200Å.
The polarization is zero at about 9400Å, and increases to 0.3% at 5700Å, then decreases to zero at 4600Å. At that wavelength the position angle changes by 90°. At shorter wavelengths the polarization increases to a maximum of 5% at 2000Å.
This behavior is interpreted as the polarization at long wavelengths being due to scattering from the surface, and at shorter wavelengths being due to scattering in the atmosphere. At the phase angle of Mars at the time of observation the position angle of polarization is expected to be in the scattering plane (i.e., the plane formed by Sun- Mars-Earth) for light scattered off the surface (from other visible light observations which have found it to be so), while the position angle expected for polarization due to scattering in the atmosphere should be perpendicular to the scattering plane.
WUPPE polarization observations can be used to calculate the atmospheric pressure, which turns out to be about 6 mbar, very close to the values measured by spacecraft on and near Mars.
These results have appeared in the Astronomical Journal.