John Gregory Ruck, Cristina G Wilson, Thomas Shipley, Daniel Koditschek, Feifei Qian, Douglas J Jerolmack 

Moving down a hillslope from ridge to valley, soil develops and becomes increasingly weathered. Downslope variation in clay content, organic matter, and porosity should produce concomitant changes in soil strength that influence slope stability and erosion. This has yet to be demonstrated, however, because in-situ measurements of soil rheology are challenging and rare. Here we employ a robotic leg as a mechanically sensitive and time-efficient penetrometer to map soil strength along a canonical temperate hillslope profile. We observe a systematic downslope weakening, and increasing heterogeneity, of soil strength associated with a transition from sand-rich ridge materials to cohesive valley bottom soil aggregates. Weathering-induced changes in soil composition lead to physically distinct mechanical behaviors in cohesive soils that depart from the behavior observed for sand. We also demonstrate the promise that legged robots may use their limbs to sense and improve mobility in complex environments, with implications for planetary exploration.