Extracting scientific results from robotic arm support operations: A technique for estimating the density and composition of rocks on Mars

Background: Robotic arms on landed spacecraft are typically designed to either (a) retrieve surface samples for analyses by the main spacecraft or (b), bring a mobile instrument package into contact with surface components. Yet the engineering data returned by a robotic arm while conducting these science support operations can themselves be used to investigate the physical properties of surface materials. The Viking Lander 2 (VL2) displaced several nearby rocks with its sampling arm to obtain regolith samples from the protected environment beneath them. Method: The masses of displaced rocks are estimated using measurements of the pushing force exerted by the sampling arm by assuming a basic block-sliding model. Rocks densities are estimated by dividing the mass estimates by rock volumes determined from stereo pairs of images. Conclusion: Although the precision of the VL2 sampling arm motor current records is insufficient to derive unambiguous results, the bulk densities of pushed rocks appear to be low (≤2.6 g/cm), consistent with rocks that are vesicular throughout their volumes (i.e., vesicular impact or volcanic glasses). If the bulk density and vesicularity are known with high enough precision, the method demonstrated here can roughly constrain rock composition. Although the Viking measurements are ultimately inconclusive, the robotic sampling arm on the Mars Phoenix lander could execute a similar pushing strategy on nearby rocks to obtain more accurate density and compositional estimates.