Engineers at the University of Wisconsin-Madison uncovered a critical flaw in how lunar and Martian rovers are tested on Earth. Simulations revealed that test results have been misleading for decades because researchers only adjusted rover weight to simulate low gravity—but ignored how Earth’s gravity affects the terrain itself. Using a powerful simulation tool called Chrono, the team showed that sandy surfaces behave very differently on the Moon, where they’re fluffier and less supportive.

Through simulation, Dan Negrut, a professor of mechanical engineering at UW-Madison, and his collaborators determined that Earth’s gravity pulls down on sand much more strongly than the gravity on Mars or the moon does. On Earth, sand is more rigid and supportive – reducing the likelihood it will shift under a vehicle’s wheels. But the moon’s surface is “fluffier” and therefore shifts more easily – meaning rovers have less traction, which can hinder their mobility.