Introduction: The Rock Abrasion Tool (RAT) serves as the sample preparation device on the Mars Exploration Rover (MER) science payload. As an end-effector on the rover’s robotic arm the RAT [1] grinds a circular spot on the order of millimeters deep into a rock face to remove surface fines and weathered layers, preparing the rock for imaging and spectral observations. As of 5 January 2005, 12 grinding operations have been performed at Gusev Crater by Spirit and 22 at Meridiani Planum by Opportunity. Basic Abrasion Properties: Once the robotic arm places and loads the RAT onto a rock surface, the RAT affects high rotational speed face grinding. A grinding bit with two diamond grain embedded phenolic resin pads rotates near 3000 RPM and feeds slowly in a circular path across the rock, creating the larger abrasion area. The 45.5 mm outer diameter of the abrasion is a fixed aspect of the mechanism. The overall depth of penetration varies with operation due to rock properties and available rover energy, typically no greater than 10 millimeters. The depth of penetration also varies across the abrasion area due to the rock’s original surface geometry and the RAT’s approach angle. Abrasion areas can be less than fully circular and can include irregular unabraded areas internally. Often an overall depth is reported with a grind operation, describing the distance from the highest point on the original rock face within the circular grind area to the floor of the abrasion.