This paper will describe the identification and characterization of synthetic membranes for the separation and purification of CO, from the Martian atmosphere for in situ resource utilization (ISRU) applications. An ISRU application is the production of propellant (methane) on the surface of Mars from CO2. Candidate materials should have high selectivity for carbon dioxide over nitrogen and argon, and a glass transition temperature of -40 degrees C or less to remain in rubbery state at low temperature for high permeances. Membrane materials we identified include the rubbery polymers poly(dimethyl siloxane) (PDMS) and the copolymer poly(dimethyl, methylphenyl siloxane) (PMPS). Pure and mixed gas permeation experiments with CO2, N-2 and Ar were performed with these membrane materials at below ambient temperatures. In experiments with commercially obtained PDMS membranes, the pure gas CO, permeability increases from 2645 to 2792 Barrers as the temperature decreases from 22 to -20 degrees C and differential pressure of 1033 mbar (15 psi). The CO2N2 ideal separation factor increases from 10.8 to 19.5 over the same temperature range. However, in pure gas experiments at low pressures (50-500 mbar) for the PDMS membrane, the CO2/N-2 ideal separation factor decreases as feed pressure decreases which is primarily due to decrease in CO2 permeability with pressure. Pure gas permeation results with PMPS membranes also showed an increase in CO, permeability from 1450 to 1650 Barrers as the temperature decreases from 21 to -10 degrees C for differential feed pressure of 1378 mbar (20 psi). The CO2/N-2 ideal separation factor increased from 12 to 27 over the same range of temperature. The ideal separation factors measured at lower feed pressures are closely related to the values at higher feed pressures for the PMPS membrane. This article includes transport data for simulated Martian atmosphere temperature and pressure conditions performed at Lockheed Martin, Denver. An interesting behavior in CO2 permeance is observed at low feed pressures for both PDMS and PMPS membranes. The trend in CO, permeance with temperature changes with variation in feed pressure. A hypothesis is presented for this observation. (c) 2005 Elsevier B.V. All rights reserved.