Measuring the physical properties of geological materials is important for understanding geologic history. Yet there has never been an instrument with the purpose of measuring mechanical properties of rocks sent to another planet. The Mars Science Laboratory (MSL) rover employs the Powder Acquisition Drill System (PADS), which provides direct mechanical interaction with Martian outcrops. While the objective of the drill system is not to make scientific measurements, the drill's performance is directly influenced by the mechanical properties of the rocks it drills into. We have developed a methodology that uses the drill to indicate the uniaxial compressive strengths of rocks through comparison with performance of an identically assembled drill system in terrestrial samples of comparable sedimentary class. During this investigation, we utilize engineering data collected on Mars to calculate the percussive energy needed to maintain a prescribed rate of penetration and correlate that to rock strength. Plain Language Summary The one tool that personifies the field geologist is a rock hammer. The field geologist will use his/her hammer to expose fresh rock surfaces allowing examination of unweathered rock. He/she will also use it to determine qualitative rock strengths in the field. While Curiosity does not carry a traditional rock hammer, it does have a drill system. The ability to fail rock with a hammering mechanism and the ability to use the performance data from the drill system presented the authors with an innovative concept. The drill could be used as an instrument to indicate the strength of rocks, except that the drill's measurement is better. Where the field geologist has only his/her tactile senses, Curiosity is instrumented with sensors that measure rates of penetration, percussive energy, and weight on bit allowing a quantifiable measurement of rock strength. The article describes the methodologies that allow the drill system aboard the Mars Science Laboratory rover to also serve as a scientific instrument and reports the compressive strength of the rocks drilled on Mars using these methods. Key Points A nonstandard method was employed to provide a scientific measurement on the surface of Mars Sedimentary structure matters when determining rock strength Strengthwise, the sedimentary rocks at Gale crater are more akin to younger sedimentary rock on Earth