Introduction

In the radiative-transport equations for the reflectance and emittance of a particulate medium developed in Chapter 7 it was assumed that polarization can be neglected. For irregular particles that are large compared with the wavelength of the observation, this assumption is justified on the grounds that the light scattered by such particles is relatively weakly polarized (e.g., Figure 6.7b). However, even though it may be small, the polarization of the light scattered by a medium does contain information about the medium and, thus, is potentially a useful tool for remote sensing. One of the advantages of using polarization is that it does not require absolute calibration of the detector, but only a measurement of the ratio of two radiances.

The discovery that sunlight scattered from a planetary regolith was polarized was made as early as 1811 by Arago, who noticed that moonlight was partially linearly polarized and that the dark lunar maria were more strongly polarized than the lighter highlands. Subsequent observations of planetary polarization were made by several persons, including Lord Rosse in Ireland. However, the quantitative measurement of polarization from bodies of the solar system was placed on a firm foundation in the 1920s by the classical studies of Lyot (1929). This work was later continued by Dollfus (1956, 1998) and his colleagues. For a more detailed historical account, the reviews by Dollfus (1961, 1962) and Gehrels (1974) should be consulted.