Analyses of sample trenches excavated on Mars, using a theory for plowing by narrow blades, provide estimates of the angles of internal friction and the cohesions of the Martian surface materials.Angles of internal friction appear to be the same as those of many terrestrial soils because they are generally between 27° and 39°. Drift material, at the Lander 1 site, has a low angle of internal friction (near 18°). All the materials excavated have low cohesions, generally between 0.2 and 10 kPa. The occurrence of cross bedding, layers of crusts, and blocky slabs shows that these materials are heterogeneous and that they contain planes of weakness. The results reported here have significant implications for future landed missions, Martian eolian processes, and interpretation of infrared temperatures.