Scientists are learning how to use carbon dioxide—the main gas in Mars’ atmosphere—to harvest rocket fuel and water from the red planet.
When astronauts first go to Mars, it’ll be difficult for them to bring everything they need to survive. Even the first tentative explorations could last as long as two years—but spaceships can only carry a limited amount.
Explorers on Earth could usually count on finding what they needed. The animals might be strange, but they’d be there, and they’d be edible①. Mars is barren. But the challenge is the same.
Astronauts will want to pull what they need from the planet itself. And although that goal seems improbable, it is believed that it can be achieved. The key lies in the Martian atmosphere.
It’s a meager② atmosphere, compared to Earth’s, and it’s about 95 percent carbon dioxide (CO2). But that turns out to be an advantage. The carbon dioxide can be used to harvest almost everything else.
Inside Martian rocks and soil lies a bounty③ of useful elements: magnesium and hydrogen for rocket fuel, oxygen to breathe, water to drink. What’s needed is a solvent to get them out, and that’s where the carbon dioxide comes in handy.
When CO2 is compressed to a pressure of 73 atm and heated to 31.1 degrees Celsius, it becomes a supercritical fluid—and a marvelous solvent④.
A supercritical fluid is a high-pressure, high-temperature state of matter perhaps best described as a liquid—like gas. Almost anything can become supercritical. Water, for instance, becomes a supercritical⑤ fluid in the high pressures and temperatures of steam turbines. Ordinary water is a good solvent. Supercritical water is a great solvent—maybe even a little too good. It dissolves the tips of the turbine⑥ blades⑦.
Supercritical carbon dioxide behaves much the same. CO2 molecules flow into solid matter, surrounding atoms, pulling them apart and away.
On Earth, supercritical CO2 is not used much to dissolve things because there are less expensive, more effective solvents close at hand. It is, however, used to remove the caffeine from coffee beans, and sometimes to dry—clean clothes. On Mars, Debelak believes, supercritical CO2 will play a much more important role.
For example: Magnesium can be dissolved quite easily by supercritical CO2. Magnesium, which is likely to be found in Martian soil, ignites easily and can be used to fuel rockets. In fact, one Mars exploration scenario called for a lander to be made of magnesium—when the astronauts were ready to go home, he could chop it up, pack it into a rocket engine, and then add some other oxidizer to fire it off. Using CO2 as solvent, magnesium could instead be harvested directly from Mars.
Supercritical CO2 might also be used to generate water. Certain Martian rocks (like some of Earth’s rocks) contain hydrogen. When these rocks are submerged in supercritical carbon dioxide, a chemical reaction takes place. The CO2’s carbon becomes “fixed” in the rock, leaving the oxygen free to find another partner: hydrogen. The process kicks out water, and you can actually use it to form water.
Pulling water from rocks will probably have the biggest payoff, at least in the short term. In addition to drinking, you can split water into hydrogen for fuel, and oxygen for breathing—or as an oxidizer for some sort of engine. Eventually, colonists could set up plants that use CO2 from the Martian atmosphere to process hundreds of kilograms of raw material a day.