Polarimetry of Pluto.

Pluto has been observed at a solar phase of 0.8 deg and found to have a degree of polarization of 0.27 percent (probable error 0.02 percent) and a position angle of polarization of 156 deg (probable error 2 deg). The observations indicate that Pluto has a polarization curve with a deep negative branch and, therefore, that its surface is likely to be microscopically rough. The observed polarization is compatible with the small geometric albedo of Pluto. No convincing evidence of variable polarization associated with Pluto's rotation has been found.