The Mechanical Strength of Sedimentary Rocks at Meridiani Planum, Mars

The physical properties of rocks such as their strength, hardness, and density can help inform our understanding of the formation and modification history of rock units. For sedimentary rocks, their strength is inherently linked to factors such as porosity and degree of induration, which are in turn controlled by factors such as burial depth and water-rock interaction. On Earth, rock strength is typically assessed as unconfined compressive strength (UCS) and can be measured directly or inferred via indirect strength tests (e.g., the Schmidt hammer test). On Mars, instruments such as the Rock Abrasion Tool (RAT) onboard the Opportunity rover can be used in a similar manner to indirectly infer UCS. Specific grind energy (SGE) is a ratio of energy consumed by the RAT normalized to the volume of material removed and has been shown to correlate with UCS. Here, we use engineering data returned by the Opportunity rover to evaluate the strength of sedimentary rocks abraded by the RAT at Meridiani Planum, Mars. Results exhibit a range of rock strengths from 1.4–79.9 MPa, but most rock targets appear to be very weak (less than 25 MPa), being similar in strength to highly weathered or weakly cemented sedimentary rock. These results suggest limited burial depths, little compaction, and a low degree of induration by weak sulfate material in contrast to well-lithified sedimentary rocks on Earth, which have UCS values on the order of 50–100 MPa. Similar strength values are seen elsewhere on Mars, such as Peace class materials at Gusev crater as well as the Stimson fFormation at Gale crater. In general, these results help us better understand the emplacement of the bedrock units at Meridiani Planum by elucidating the extent of burial and water-rock interactions when considering geochemical and mineralogical observations.