Echoing in your ears are the final words from mission control: "Your mission, should you care to accept it, is to return to Earth--if possible using fuel and oxygen you mine from the sands of Mars. Good luck!"

It sounds simple enough, mining raw materials from a rocky, sandy planet. We do it here on Earth, why not on Mars, too? But it's not as simple as it sounds. Nothing about granular physics ever is.

Granular physics is the science of grains, everything from kernels of corn to grains of sand to grounds of coffee. These are common everyday substances, but they can be vexingly difficult to predict. One moment they behave like solids, the next like liquids. Consider a dump truck full of gravel.

When the truck begins to tilt, the gravel remains in a solid pile, until at a certain angle it suddenly becomes a thundering river of rock.

Understanding granular physics is essential for designing industrial machinery to handle vast quantities of small solids - like fine Martian sand.

The problem is, even here on Earth "industrial plants don't work very well because we don't understand equations for granular materials as well as we understand the equations for liquids and gases," says James T. Jenkins, professor of theoretical and applied mechanics at Cornell University in Ithaca, N.Y.

"That's why coal-fired power plants operate at low efficiencies and have higher failure rates compared to liquid-fuel or gas-fired power plants."

So "do we understand granular processing well enough to do it on Mars?" he asks.

Let's start with excavation: "If you dig a trench on Mars, how steep can the sides be and remain stable without caving in?" wonders Stein Sture, professor of civil, environmental, and architectural engineering and associate dean at the University of Colorado in Boulder. There's no definite answer, not yet. The layering of dusty soils and rock on Mars isn't well enough known.

Some information about the mechanical composition of the top meter or so of Martian soils could be gained by ground-penetrating radar or other sounding devices, Sture points out, but much deeper and you "probably need to take core samples."

NASA's Phoenix Mars lander (landing 2008) will be able to dig trenches about a half-meter deep; the 2009 Mars Science Laboratory will be able to cut out rock cores. Both missions will provide valuable new data.

To go even deeper, Sture (in connection with the University of Colorado's Center for Space Construction) is developing innovative diggers whose business ends vibrate into soils. Agitation helps break cohesive bonds holding compacted soils together and can also help mitigate the risk of soils collapsing. Machines like these might one day go to Mars, too.

Another problem is "hoppers" - the funnels miners use to guide sand and gravel onto conveyor belts for processing. Knowledge of Martian soils would be vital in designing the most efficient and maintenance-free hoppers.

"We don't understand why hoppers jam," Jenkins says. Jams are so frequent, in fact, that "on Earth, every hopper has a hammer close by." Banging on the hopper frees the jam. On Mars, where there would be only a few people around to tend equipment, you'd want hoppers to work better than that. Jenkins and colleagues are researching why granular flows jam.

And then there's transportation: The Mars rovers Spirit and Opportunity have had little trouble driving miles around their landing sites since 2004. But these rovers are only about the size of an average office desk and only about as massive as an adult.

They're go-carts compared to the massive vehicles possibly needed for transporting tons of Martian sand and rock. Bigger vehicles are going to have a tougher time getting around.

Sture explains: As early as the 1960s when scientists were first studying possible solar-powered rovers for negotiating loose sands on the Moon and other planets, they calculated "that the maximum viable continuous pressure for rolling contact pressure over Martian soils is only 0.2 pounds per square inch (psi)," especially when traveling up or down slopes.

This low figure has been confirmed by the behavior of Spirit and Opportunity.

A rolling contact pressure of only 0.2 psi "means that a vehicle has to be light-weight or has to have a way of effectively distributing the load to many wheels or tracks. Reducing contact pressure is crucial so the wheels don't dig into soft soil or break through duricrusts [thin sheets of cemented soils, like the thin crust on windblown snow on Earth] and get stuck."

That requirement implies that a vehicle for moving heavier loads - people, habitats, equipment -might be "a huge Fellini-type thing with wheels 4 to 6 meters (12 to 18 feet) in diameter," says Sture, referring to the famous Italian director of surreal films.

Or it might have enormous open-mesh metal treads like a cross between highway-construction backhoes on Earth and the lunar rover used during the Apollo program on the Moon. Thus, tracked or belted vehicles seem promising for carrying large payloads.

A final challenge facing granular physicists is to figure out how to keep equipment operating through Mars' seasonal dust storms.

Martian storms whip fine dust through the air at velocities of 50 m/s (100+ mph), scouring every exposed surface, sifting into every crevice, burying exposed structures both natural and manmade, and reducing visibility to meters or less.

Jenkins and other investigators are studying the physics of aeolian [wind] transporting of sand and dust on Earth, both to understand the formation and moving of dunes on Mars, and also to ascertain what sites for eventual habitats might be best protected from prevailing winds (for example, in the lee of large rocks).

Returning to Jenkins's big question, "do we understand granular processing well enough to do it on Mars?" The unsettling answer is: we don't yet know.

Working with imperfect knowledge is okay on Earth because, usually, no one suffers much from that ignorance. But on Mars, ignorance could mean reduced efficiency or worse preventing the astronauts from mining enough oxygen and hydrogen to breathe or use for fuel to return to Earth.

Granular physicists analyzing data from the Mars rovers, building new digging machines, tinkering with equations, are doing their level best to find the answers. It's all part of NASA's strategy to learn how to get to Mars ... and back again.

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