Space Forum / Space Flight / August 2003

http://science.nasa.gov/headlines/y2003/20aug_supercriticalco2.htm

Harvesting Mars
NASA Science News
August 20, 2003

A NASA-supported scientist is learning how to use carbon dioxide--the main
gas in Mars' atmosphere--to harvest rocket fuel and water from the red
planet.

August 20, 2003: 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.

"We might have to do what explorers have done for ages: live off the land,"
says chemical engineer Ken Debelak of Vanderbilt University.

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 edible. 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,
Debelak believes it can be achieved. He's working on a NASA project to make
it happen. The key, he says, lies in the Martian atmosphere.

It's a meager 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, says Debelak, can be used to harvest almost everything else.

Inside martian rocks and soil lies a bounty of useful elements: magnesium
and hydrogen for rocket fuel, oxygen to breath, 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 solvent," says
Debelak.

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 supercritical 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 turbine blades.

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,
Debelak has found. "That's an experiment that we're quite excited about at
the moment," he says. Magnesium, which is likely to be found in martian
soil, ignites easily and can be used to fuel rockets. In fact, says Debelak,
one Mars exploration scenario called for a lander to be made of
magnesium--"the legs and so on." When the astronauts were ready to go home,
"you could chop it up, pack it into a rocket engine, and then add some other
oxidizer to fire it off." Using CO2 as a 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," marvels
Debelak. "You can actually use it to form water."

Pulling water from rocks will probably have the biggest payoff, at least in
the short term, says Debelak. 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.

A supercritical fluid has some advantages over other solvents: Its
solubility changes dramatically when you alter the temperature or the
pressure. You can control it, so that sometimes it's a solvent for a
particular substance, and sometimes it's not. That makes it easy to recover
the material that has been dissolved. Let's say you have caffeine dissolved
in supercritical carbon dioxide. To recover the caffeine (caffeine recovered
from coffee beans is often put in soft drinks), you just lower the pressure
of the CO2 and the caffeine drops out.

Currently, Debelak is trying to pin down the way a variety of substances
behave in supercritical CO2. He's looking at which minerals are easily
soluble and which are not. And if they're not, he's trying to determine how
their solubility can be improved. Adding other substances to the CO2
sometimes helps, he says.

Debelak's work could be useful on Earth, too. Carbon dioxide is often
spotlighted because of its damaging role in global warming. But as a
solvent, it's benign. Many solvents common in industry are toxic. They cause
cancer, and if they get into the water system, they stay for a long time. So
there's interest, says Debelak, in learning how to use CO2 as a 'green'
alternative.

Carbon dioxide plays widely different roles on Earth and on Mars. "That's
what's intriguing," points out Debelak. "Mars is a totally foreign
environment to us. The rules are different."

"So that's what we're doing--trying to figure out the rules," he says. "And
then we can figure out how to play the game ... on both planets."