Multispectral Pancam images of dust patterns on Opportunity's Capture and Filter magnets analyzed by a Computational Fluid Dynamics model.

The Capture and Filter magnets, located at the foot of the Pancam Mast Assembly (PMA) on the Mars Exploration Rovers, collect magnetic dust grains from the martian atmosphere. In addition to analysis by the instruments on the robotic arm, these magnets were regularly imaged by the multispectral Panoramic Camera (Pancam) on the PMA. Here we present an analysis of a multispectral set of Pancam images of Opportunity's Capture and Filter magnets from sol 37 of Opportunity's mission using a computational fluid dynamics (CFD) model. From theoretical considerations we may predict that the shape of the pattern on a magnet is determined by three quantities: These are: The local wind speed, the magnetic susceptibility of dust grains, and a characteristic response time of dust grains, determined by their size and mass density. CFD modeling of the process of dust capture on the magnets reveals in detail the influence of each of these 3 quantities on dust motion and thereby on the dust pattern on the magnet. These results are corroborated by experimental data from a wind tunnel operating under martian atmospheric conditions. Pancam data reveal color differences between the dust on the Capture and Filter magnets as well as color variations across the surface of the Capture magnet. These variations may be understood based on the CFD analysis and indicate that the airborne martian dust is not uniform but rather falls on a wide distribution, where brighter, red dust correlates with low magnetization, or small grain size, or both, whereas darker dust correlates with larger and/or more magnetic grains.