How ozone, clouds, and airborne dust affect CO2 ice formation and sublimation is examined to determine if they help explain the asymmetry in the residual polar caps. The radiative flux striking the surface is calculated for 57 solar and 10 IR wavelength intervals from 0 to 100 microns including the scatering, absorption, and emission by O3, CO2, clouds, and dust. UV absorption by H2O, O2, HO2, and H2O2 is also included, as is Rayleigh scattering. The discrete ordinate method of Stamnes et al. (1988) treats the emission, scattering, and absorption of monochromatic radiation.