Disk-Integrated Polarimetry of Mercury in 2000–2002

Abstract

The orbital and rotation characteristics of Mercury lead to distinctly different solar irradiation intensities (photons, solar wind and cosmic rays) and radiative heating of the Mercury hemispheres with central meridians 0°, 180° on the one hand and 90°, 270° on the other. These differences allow us to hypothesize that the intensity of Mercury regolith maturation may depend noticeably on the planetocentric longitude, which would result in a corresponding variation of optical properties over the surface. The purpose of this study is to explore the variation of polarization over the Mercury surface. Polarimetric observations of Mercury were carried out during its three apparitions in 2000–2002 with a 70-cm reflector (Cassegrain configuration, f/16) and a single-channel photoelectric polarimeter. The polarization phase curve of Mercury obtained in the phase-angle interval of 39.1° → 135.5° shows measurements that approximate a curve with a maximal polarization difference ΔP = 1%, which is about one order of magnitude larger than the measurement accuracy. The dependence of differences between measured and approximated polarization ΔP on the planetocentric longitude of the center of the illuminated part of the visible disk revealed polarization variations with amplitude of approximately 1.5% over the range of observed planetocentric longitudes 265° → 330°. In order to obtain such variations in the whole interval of longitudes and to clarify their nature, the authors intend to carry out new observations of Mercury during its nearest apparitions.