Exploring iron asteroid magnetospheres

The NASA Psyche mission will be the first to explore a metallic planetary body: the iron-rich asteroid (16) Psyche. A major objective of the mission is to test the hypothesis that Psyche formed when a differentiated body with a metallic core that was stripped of its mantle by impacts. Evidence for such an origin would be identification of remanent magnetization on the asteroid, which would indicate that it once generated a core dynamo magnetic field. If such magnetization is detected, Psyche would constitute the smallest planetary body confirmed to have an intrinsic magnetic field, enabling study of past dynamo generation in the small-body limit and giving rise to possibly a new type of present-day solar wind interaction with a planetary body. Here, we study the range of possible plasma dynamics that can exist around (16) Psyche using three-dimensional hybrid simulations. We show that for the strongest conceivable remanent magnetization consistent with known iron meteorites, the present body could form an obstacle to the solar wind and possess a magnetosphere. For weaker magnetizations, the magnetic obstacle is small and becomes comparable to the scale of individual ion motions. If remanent magnetization is absent, the metallic body may form an induced magnetosphere. Measurements of the induced signal could be used to constrain the metal content and interior structure of the asteroid. We analyze the possible plasma structures around such bodies, from shocks, magnetopause current layers, particle trapping, and waves. We present a framework for detecting and reconstructing the internal magnetic field from orbital measurements and test it for different simulated magnetospheres to estimate the limits and sensitivity of these methods. We also show that even the largest conceivable asteroid magnetospheres would not stably trap energetic particles to form hazardous radiation belts. We compare our results to observations of other small magnetized airless bodies including Mercury and Ganymede.