FOR several years the most widely accepted value of 90 mbar for the surface pressure on Mars had been based on visible photometric and polarimetric work, particularly by Dollfus1 (cf. de Vaucouleurs2 for a detailed discussion). Recently, however, a value of 25 +/- 15 mbar has been derived by Kaplan, Münch, and Spinrad3 from a curve-of-growth analysis of pressure-broadened carbon-dioxide vibration-rotation lines in near infra-red Martian spectra. A possible explanation of the discrepancy is that aerosol particles in the Martian atmosphere contribute an appreciable component to the observed brightness and polarization of Mars. Dollfus assumed that Rayleigh scattering by molecules dominates the atmospheric brightness and polarization at λ 6100 Å and that the aerosol contribution is insignificant. If this is not the case, then certain important deductions may have to be changed. The purpose of this communication is to illustrate what these changes may be.