Polarimetry of Mars.

A summary of results of observations of the polarization of Mars shows that surface pressure determinations from these data have not yielded satisfactory results in spite of the extensive number of observations available. It is suggested that the difficulty lies mainly in the neglect of radiative components resulting from a combination of diffuse transmission and surface reflection, the effects of an unknown and variable aerosol component of the atmosphere, and the concentration of the observations in the longer visible wavelengths corresponding to very small atmospheric optical thicknesses. Computations of atmospheric effects to be expected from various Rayleigh and aerosol models of the atmosphere show that the polarizing effects of realistic aerosol models can vary widely, depending on particle parameters, and that polarization due to Rayleigh scattering by representative models of the Martian atmosphere can only serve to shift the position of the neutral point to smaller phase angles and to shift the polarization curve in the positive direction from its position for only the surface-reflected radiation. The extent to which the surface of Mars is composed of hydrated iron oxides is still an open question. Polarization measurements show that limonite is one, but by no means unique, possibility for a major constituent of the surface. Guidelines for polarization measurements from space probes are that emphasis be on uv wavelengths for atmospheric determinations, that intensity and polarization gradients be measured on center-to-limb scans of the planet, and that measurements of the position and magnitude of the maximum of the polarizationcurve, expected to be at phase angles of 90 degrees to 120 degrees , be given a high priority.