Using a special low-temperature cell, thick layers of solid CO2 are grown on an infrared transparent window. The transmission spectra of solid CO2 is measured from 4000 to 360 cm-1 (2.5--27.8 μm). The complex index of refraction is derived for samples of different thicknesses. Scattering in the samples is the major source of error. Away from the major absorption bands the imaginary index is very low. The absorption coefficient is of the order of 1 cm-1. The emissivity in the Viking IRTM 20-μm band of a solid surface of CO2 is estimated at 0.72+0.17-0.10. Snowlike deposits have similar or lower emissivity. The results of this paper relieve the need for exceptional meteorology to explain Martian brightness temperatures below the CO2 frost point.