Wheel Abrasion Experiment on Pathfinder.
See pages 6,8, and 9 here:
Pathfinder's Finds.
http://pr.caltech.edu/periodicals/EandS/articles/Pathfinder%20Feature.pdf
The experiment was meant to determine soil material properties by how they effected the coatings on the Sojourner wheels. But it was also indirectly gave evidence of the hardness of the white material underlying the Pathfinder site.
Wheel Abrasion Experiment (WAE)
http://powerweb.grc.nasa.gov/pathfinder/wae.html
Bob Clark