  Celestial bodies such as the Moon and Mars are mainly covered by loose,
granular soil, a notoriously challenging terrain to traverse with (wheeled)
robotic systems. Here, we present experimental work on traversing steep,
granular slopes with the dynamically walking quadrupedal robot SpaceBok. To
adapt to the challenging environment, we developed passive-adaptive planar feet
and optimized grouser pads to reduce sinkage and increase traction on planar
and inclined granular soil. Single-foot experiments revealed that a large
surface area of 110cm2 per foot reduces sinkage to an acceptable level even on
highly collapsible soil (ES-1). Implementing several 12mm grouser blades
increases traction by 22% to 66% on granular media compared to grouser-less
designs. Together with a terrain-adapting walking controller, we validate - for
the first time - static and dynamic locomotion on Mars analog slopes of up to
25{\deg}(the maximum of the testbed). We evaluated the performance between
point- and planar feet and static and dynamic gaits regarding stability
(safety), velocity, and energy consumption. We show that dynamic gaits are
energetically more efficient than static gaits but are riskier on steep slopes.
Our tests also revealed that planar feet's energy consumption drastically
increases when the slope inclination approaches the soil's angle of internal
friction due to shearing. Point feet are less affected by slippage due to their
excessive sinkage, but in turn, are prone to instabilities and tripping. We
present and discuss safe and energy-efficient global path-planning strategies
for accessing steep topography on Mars based on our findings.
