The strength of ice-saturated extraterrestrial rock analogs

In situ resource utilization (ISRU) activities have been identified by the National Research Council as one of the most important scientific endeavors of the coming decade. The failure of the MUPUS-PEN experiment on Rosetta's Philae lander to penetrate the surface of comet 67P demands knowledge of the mechanical properties of extraterrestrial materials in such environments. To such an end we conducted laboratory strength measurements at cryogenic conditions of two terrestrial rocks, Bishop tuff and Indiana limestone as potential cometary and asteroid analogs, respectively. We measured failure strength of dried and fully saturated samples under constant displacement rate compressional loading at temperatures of 77 K-295 K and (for most) confining pressure of 5 MPa. The strength of saturated samples increased dramatically with decreasing temperature below the ice point: saturated limestone from 30 MPa at 295 K to >200 MPa at 150 K and below, and saturated tuff from 26 MPa at 240 K to 160 MPa at 150 K. Some ductility or distributed fracturing was evident in both saturated rock types above 150 K. The saturated tuff was stronger than pure ice at all conditions but its transition from brittle to ductile deformation with temperature closely paralleled that of ice. The results of this study will be useful to future sample retrieval missions or ISRU maneuvers. The fully saturated state of our samples and use of Bishop tuff as an unconsolidated cometary analog likely means our results are upper estimates for substrates we may encounter in our solar system. Pending corroboration and refinement of these preliminary measurements, the large increase in compressive strength we observed in the saturated materials at cryogenic temperatures suggests that future missions must prepare technology with the energetic and mechanical capability to penetrate very hard substrates.