How is a Mars rover like a sandfish skink? Whether you’re a $2.5 billion robot carefully rolling across Martian soil or a 10-cm-long North African lizard (see figure 1) that burrows in the desert sand to evade predators, your locomotion obeys the same set of rules that govern the deformation of granular media.

For close to a decade, researchers have successfully applied an empirical scheme called resistive force theory (RFT) to describe locomotion in dry granular environments. First proposed in the 1950s to calculate speeds of sea-urchin spermatozoa swimming in seawater,1 RFT approximates locomotion in viscous fluids relatively well, but it’s far from perfect. For granular media, though, it works bafflingly well.2 

To figure out the secret behind granular RFT’s success, Ken Kamrin of MIT and his postdoc Hesam Askari (now at the University of Rochester) devised the simplest continuum-mechanics equations that could describe granular flow around...

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