Geology, the interdisciplinary study of the Earth (and other celestial bodies), their compositions, and how they have changed throughout time, is deeply connected to the study of physics. This talk will begin by discussing the practical applications of various principles that we learn in undergraduate physics, which have helped explain the structure and composition of Earth and the other terrestrial planets in our solar system. I will then focus on the planet Mars and the instrumentation that has been key in developing our understanding of this planet. Special attention will be paid towards the Alpha Particle X-ray Spectrometer (APXS) and its contribution to Martian exploration. Three generations of APXS instruments have been sent to Mars and the most recent APXS (on the Mars Science Laboratory Rover, Curiosity) was developed here at the University of Guelph with operations and research ongoing. I will finish my talk by discussing how we determine the bulk chemistry of Martian samples analyzed by the APXS and the issues we face in doing x-ray spectroscopy on another planet.