Dr Ben Fernando

Abstract:

The return home of the OSIRIS-REx spacecraft in September 2023 marked only the fifth time that anartificial object entered the Earth’s atmosphere at interplanetary velocities. Although rare, such events serve as valuable analogs for natural meteoroid re-entries; enabling study of hypersonic dynamics, shock wavegeneration, andacoustic-to-seismic coupling. Here, wereportonthesignaturesrecordedbyadense(100mscale)11-station array located almost directly underneath the capsule’s point of peak atmospheric heat ing in northern Nevada. Seismic data are presented, which allow inferences to be made about the shape of the shock wave’s footprint on the surface, the capsule’s trajectory, and its flight parameters.

Abstract:

This exploratory study investigates whether seismic signals can be used to infer fragmentation during a fireball event. Re-entry objects, particularly sample return capsules (SRCs) such as the one from the Hayabusa2 mission, behave similarly to slow meteors during atmospheric entry and provide valuable insights into natural fireball events. In this study, we initially analyze seismic signals from the Hayabusa2 SRC re-entry, which took place on 5 December 2020, over South Australia. The SRC’s signature was captured by a dense network of seismic stations (Eakin, 2018; O’Donnell et al., 2020), offering a unique opportunity to investigate the signals’ characteristics and verify their connection to the re-entry event. The ballistic trajectory was confirmed as the source shock mechanism for this event. We isolate this signal and use it as a reference for a ballistic shock signature and compare it to three other fireball case studies, including a subor-bital re-entry and two natural meteoroids. Although factors such as local geology and atmospheric conditions were not considered in this preliminary study, our results show promise with high correlations for events with purely ballistic trajectories and lower correlations for those involving fragmentation or airbursts. This implies that seismic data may be able to disambiguate whether any particular fireball event underwent significant fragmentation or airburst, key phenomena for assessing body strengths.

Abstract:

A planet's interior is a time capsule, preserving clues to its early history. We report the discovery of kilometer-scale heterogeneities throughout Mars' mantle, detected seismically through pronounced wavefront distortion of energy arriving from deeply probing marsquakes. These heterogeneities, likely remnants of the planet's formation, imply a mantle that has undergone limited mixing driven by sluggish convection. Their size and survival constrain Mars' poorly known mantle rheology, indicating a high viscosity of 10^21.3 to 10^21.9 pascal-seconds and low temperature dependence, with an effective activation energy of 70 to 90 kilojoules per mole, suggesting a mantle deforming by dislocation creep. The limited mixing, coupled with ubiquitous, scale-invariant heterogeneities, reflects a highly disordered mantle, characteristic of the more primitive interior evolution of a single-plate planet, contrasting sharply with the tectonically active Earth.