Impact ejecta and crater formation on asteroid surfaces

Abstract

Asteroids in the Solar System are numerous and have varied composition. Analysis of impact crater sizes and morphologies on asteroids can provide a direct diagnosis of the surface material properties and near-surface structures. This thesis describes numerical simulations of impacts into low-gravity asteroid surfaces using the iSALE shock physics code to inform this diagnosis. Asteroids may pose a future catastrophic threat to Earth and to avoid it, the incoming asteroid can be deflected by a spacecraft impact. However, the efficiency of the deflection is determined by target properties. This work considered different target scenarios to determine the sensitivity of crater morphology, ejecta mass-velocity distribution and momentum transferred, to asteroid surface properties and shallow structures. For homogeneous targets, the surface cohesion, initial porosity, and internal friction were found to greatly influence ejecta mass/velocity distributions and the amount an asteroid can be deflected. In a two-layer target scenario, the presence of a less porous, stronger lower layer can cause both amplification and reduction of ejected mass and momentum relative to the homogeneous case. Impacts into targets with decreasing porosity with depth only produced an enhancement in the ejected momentum for sharp exponential decreases in porosity. Using reasonable estimates for the material properties of the Double Asteroid Redirection Test (DART) asteroid target, the simulations show that the ejecta produced from the impact can enhance the deflection 2 to 4 times. Simulations of impacts into possible target structures on Psyche show large diversity in possible crater morphologies that the ‘Psyche’ mission could encounter. If Psyche’s interior is homogeneous, then the mission will find simple bowl-shaped craters, with a depth-diameter ratio diagnostic of rock or iron. If Psyche has a layered structure, the spacecraft could find craters with more complex morphologies, e.g. concentric or flat-floored craters. Based on 3-4 proposed large craters on Psyche’s surface, model size-frequency distributions suggest that Psyche could be at least 3 billion years old if rocky and more than 4 billion years old if metal-rich.Open Acces