**An overview of lunar and martian regolith physical properties measured
in situ**

Robert Sullivan, <rjs33@cornell.edu>, CRSR/Astronomy, Cornell
University, Ithaca NY 14853.

Characterization of physical and mechanical properties of surface
materials has been a fundamental part of in situ planetary exploration
beginning with the *Surveyor* series of lunar landers, continuing
through the *Apollo* program and extending later to Mars with the
*Viking*, *Pathfinder,* and *Mars Exploration Rover* (MER) missions. On
the Moon, disturbances in lunar soil by *Surveyor* foot pads, sampler
arms, and rocket exhaust were analyzed to measure *c*, *φ*, and other
properties of lunar soils. *Apollo* continued the investigation of
physical and mechanical properties of lunar regolith on an elaborate
scale, including the retrieval to Earth of drill core soil samples up to
3 m deep. These materials were analyzed along with photographs of boot
prints, rover tracks, and other impressions into the lunar surface. In
many instances soil mechanics analyses employed lunar soil simulants of
crushed basalt with particle-size characteristics matched to the
returned lunar soils. Regolith relative densities (\>65% below \~15 cm),
cohesions (0.1-4 kPa), friction angles (\~30-55°), and other properties
were determined, as well as the variations in these properties with
depth. Typically, lunar regolith properties change rapidly in the upper
10-15 cm, with strength increasing substantially through this depth.
Mechanical properties of martian surface materials have been estimated
by robotic spacecraft. The two *Viking* Landers utilized extendable
surface samplers to dig into martian soil, to reveal and probe surface
crusts, and to push against rocks (moving some of them). Transformation
of arm actuator currents and image measurements of soil disturbances
into standard soil mechanical parameters such as *c*, *φ*, and bulk
density required application of a plowing analogy for some activities of
the soil scoop. Trench walls, slopes of artificial piles of material,
and footpad imprints were also analyzed to determine mechanical
properties of surface materials. Drift, blocky, and crusty-to-cloddy
soil materials could be distinguished with these data. Estimated
friction angles ranged from 18-35°, cohesions from 0.7-16 kPa. Rocks
were too strong to be chipped or scratched by the surface sampler.
*Pathfinder* site soil mechanical properties were derived mainly from
analysis of *Sojourner* wheel tracks and wheel-trenching sites. Analysis
of rover suspension telemetry, frictional wheel torques indicated by
motor currents, and images of disturbed soil suggested *φ* ranges of
15-42°. High fidelity castings of wheel treads in drift material
indicated a substantial component of silt- and/or smaller-sized
particles. Currently, soil physical properties are being analyzed in a
similar fashion by the larger, more capable MER vehicles, with the
Microscopic Imagers carried by each vehicle proving to be a valuable new
tool for understanding and interpreting regolith properties. Regolith at
the MER Gusev site typically consists of a brightly dusted, thin, weak
surface crust overlying fine-grained but poorly sorted materials forming
a matrix burying larger rock clasts. Regolith at the MER Meridiani site
consists of an aeolian-derived lag of basaltic sand with coarser
hematite-enriched spherules and fragments therefrom, all overlying much
finer grained, cloddy materials whose origin is unknown. Active
mobilization of fines by wind has been observed at both MER sites.