The specialized polymers that make up these membranes, some of which were developed at the Idaho National Environmental and Engineering Laboratory, are engineered to increase carbon dioxide solubility. "We add in groups of molecules that are polar - they carry an electric charge," says Way. Because carbon dioxide molecules are also polar, they're attracted to charged groups in the membrane.
The membranes are tested in a special chamber that simulates the Martian environment, explains Larry Mason. The device, which is about a metre high, is divided into two compartments. One contains a Mars-like atmosphere, and the other side contains a vacuum. They’re separated by a membrane that's about one square inch in surface area. A mass spectrometre measures how easily each gas moves into the vacuum side.
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"In the best [membrane] material we've found," says Way, "at Martian conditions, CO2 was transferred across the membrane about 50 times faster than nitrogen."
"Right now," adds Mason, “we’re screening different candidate materials to find the ones that permeate CO2 the best. Once we find that, we can concentrate on getting enough through in an appropriate amount of time, by changing the amount of area, packaging it, and so on."
The researchers want to design a device that produces a gas that's 99.8 percent CO2 at a rate of 2.5 liters per minute. To do that, Way says, will require quite a bit of membrane. Although the membrane is very thin - about 25 microns, one-quarter of the diametre of a strand of hair - it will probably need to be about 300 square feet in area, the size of a small room. All that will have to fit into a package of about 1 square foot.