Role of Magnetic Forces On Regolith Behavior on Metallic Asteroids with a Residual Magnetic Field

Abstract

We have visited and observed various traditional (stony) asteroids to study the origin and evolution of terrestrial planets.Recently, the OSIRIS-REx spacecraft returned samples from the asteroid Bennu. These missions and studies have shown that stony asteroids are rubble-pile structures held together by self-gravity and cohesion. This cohesive force among regolith grains influences the geological and morphological features on asteroids' surfaces. Magnetic fields have also been detected on asteroids such as Braille and Gaspra. While these detections were indirect, the Psyche mission is the first to carry a magnetometer onboard. The spacecraft will detect the potential magnetic field on the metallic asteroid Psyche. It is one of the most unique objects in the solar system, as its main constituents are Iron and Nickel. The leading theory is that Psyche is a protoplanet - the building block of planets. Researchers argue that a magnetic field would be locked into the asteroid during the cooling period post-formation. Studying such M-type asteroids provides insight into planetary formation. The presence of a magnetic field on Psyche would have significant implications for the behavior of the surface regolith. When placed in a uniform magnetic field, paramagnetic particles experience a mutual force. Therefore, the regolith on metallic asteroids will experience an additional magnetic cohesion. Magnetic forces could influence mass-wasting processes, such as avalanching, which shape the asteroid's surface. Understanding these processes is crucial for interpreting the geological features observed by the Psyche mission and for planning future missions to metallic asteroids, including potential sample return and mining activities. In this dissertation, we investigate the role of magnetic forces in regolith behavior on metallic asteroids with residual magnetic fields. First, we develop an empirical model for the magnetic force between paramagnetic particles in a uniform magnetic field. We compared various models for the magnetic force existing in the literature. The dipole-dipole model is the most commonly used model for magnetic force, but it is only accurate for particles that are far apart. We find that the dipole-dipole model significantly underestimates the magnetic force when particles are in close proximity, which is the case for regolith grains on asteroid surfaces. We implemented a Spherical Harmonics Approximation (SHA) model for magnetic force, which provides the exact solution for the two-particle case. Using the SHA model, we generated data for the magnetic force between two particles for a range of particle sizes and magnetic field strengths. We then developed an empirical model of magnetic force using a polynomial fit to the SHA data. This empirical model provides a more accurate representation of magnetic forces for computations such as the magnetic bond number, enabling comparisons of forces between regolith grains on small bodies. Second, we implemented a magnetic force model in LIGGGHTS, an open-source Discrete Element Method (DEM) software, to simulate regolith behavior on metallic asteroids. The model uses the Mutual Dipole Model (MDM) for far-field interactions, and closer-range interactions are calculated using the Spherical Harmonics Approximation (SHA). We also developed a characteristic magnetic time step for DEM simulations to ensure numerical stability while maintaining accuracy. This inclusion model implementation was validated for two-, three-, and eight-particle chains, as per data available in the literature. We then performed avalanching simulations with steel balls to validate the model against experimental data for the angle of repose with varying magnetic field strengths. All three flow regimes (granular, correlated, and plastic) were observed in the simulations, and the angle of repose and surface roughness trends were consistent with experimental data. These validations confirmed that our implementation of magnetic forces in DEM simulations is accurate and results in the first experimental validation of magnetic forces in granular simulations. Finally, we performed avalanching simulations of regolith on 16-Psyche, varying the magnetic field strength and the composition of the regolith grains. We find that magnetic forces can significantly influence the angle of repose and the final settled configuration of the regolith after an avalanche. At higher magnetic field strengths, the regolith becomes plastic, potentially leading to steeper slopes and more stable regolith on Psyche's surface. The simulations showed several interesting characteristics of the flow, including unstable, steep cliffs, regolith grains forming chains, and the formation of hanging clusters of regolith. These results suggest that magnetic forces could play a significant role in shaping the surface features of metallic asteroids, and these findings will be crucial for interpreting the data from the Psyche mission and for planning future missions to metallic asteroids.