The Sands of Mars

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.

Photo credit: Stein Sture An experimental Elastic Loop Mobility System that might work on worlds with dusty soil like Mars and the Moon.

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.