Future missions to the planet Mars might include microphones to listen for sounds in the tenuous Martian atmosphere. The chemical composition of the atmosphere is well established by previous missions and ground based observations. The dominant constituent is CO2 with a minor amount of N2 and Argon and smaller amounts of H2O. The factors important to compute the absorption of sound in a gas are reasonably well known, the most uncertain being the relaxation time of CO2 at the low temperatures encountered (200–300 K). By extrapolating higher temperature measurements of relaxation times, analytical expressions have been developed for the absorption due to viscosity and thermal conduction (classical absorption), rotational relaxation, and vibrational relaxation. Calculations are presented for a surface level pressure of 6 millibars (600 Pa). The absorption at mid-audio frequencies (500 Hz) is 0.03 (200 K) to 0.1 (300 K) Np/m. This is about 100–500 times greater than for the earth’s atmosphere (depending upon relative humidity).