Mariner 9 polarimetry of Phobos and Deimos

We have used the Mariner 9 A-camera system to measure the polarization (P) of Phobos and Deimos at large phase angles (α). For Deimos, P = +22 ± 4% at α = 74°; for Phobos P = +20.5 ± 4% at α = 77°, and P = +24.5 ± 4% at α = 81°. These measurements refer to orange light at about 0.57 μm. A comparison with laboratory measurements of powdered rock samples indicates that the observations are consistent with the presence of regoliths on the satellites.