Jessica Claire Elizabeth Irving

Scientific exploration of the UK and Ireland's subsurface has made important contributions to scholarship and prosperity for people and the planet, including economic growth, sustainable use of natural resources, storage of greenhouse gases, and inspiring curiosity about the Earth beneath our feet. This article outlines a vision for an array of seismological instruments spanning the UK and Ireland, UKI Array, augmented by other types of geophysical sensors, to maximise the value offered by existing equipment pools. The mission is to research natural phenomena and structure in the deep and shallow Earth, to solve problems concerning hazards and resources, to connect scientists to schools and the broader public, and thus to inspire a new generation to learn about geophysics. The vision was created through a community driven process of engagement and participation. This paper describes the concept and design of the UKI-Array; a companion paper discusses related opportunities and potential applications.

The 15 January 2022 submarine volcanic eruption of Hunga Tonga‐Hunga Ha'apai released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid‐oceanic column acoustic pressure recordings from 24 freely drifting Mobile Earthquake Recorder in Marine Areas by Independent Divers sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the ocean as a 30‐min T wave with energy around 2.5–10 Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, T ‐wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray‐theoretical) Fresnel zone at a depth of 1,350 m, where the fundamental‐mode T ‐wave eigenfunction is maximal. Adjusted for geometric spreading, the cross‐correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6 dB reduction in sound pressure level per logarithm of occlusion count, explaining 88% of the T ‐wave sound pressure variance across the ocean. Source characterization requires adequate path models. Our findings describe how to correct signal amplitudes for seafloor roughness. This is important for constraining volcanic or explosive yield estimates and earthquake magnitudes, and useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.

The 15 January 2022 submarine volcanic eruption of Hunga Tonga‐Hunga Ha'apai released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid‐oceanic column acoustic pressure recordings from 24 freely drifting Mobile Earthquake Recorder in Marine Areas by Independent Divers sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the ocean as a 30‐min T wave with energy around 2.5–10 Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, T ‐wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray‐theoretical) Fresnel zone at a depth of 1,350 m, where the fundamental‐mode T ‐wave eigenfunction is maximal. Adjusted for geometric spreading, the cross‐correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6 dB reduction in sound pressure level per logarithm of occlusion count, explaining 88% of the T ‐wave sound pressure variance across the ocean. Source characterization requires adequate path models. Our findings describe how to correct signal amplitudes for seafloor roughness. This is important for constraining volcanic or explosive yield estimates and earthquake magnitudes, and useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.

SUMMARY: Seismic tomography is a principal method for studying mantle structure, but imaging of Earth’s wave speed anomalies is conditioned by seismic wave sampling. Global models use misfit criteria that may strive for balance between portions of the data set but can leave important regional domains underserved. We evaluate two full-waveform global tomography wave speed models, GLAD-M25 (Global adjoint tomography model) and SEMUCB-WM1 (whole-mantle tomography model derived from fully numerical spectral element method forward modelling), in the mantle below the Pacific Ocean. The region of the South Pacific Superswell contains multiple hotspots which may be fed by plumes anchored in the Large Low Shear-Velocity Province at the base of the mantle. The uneven distribution of seismic receivers worldwide leaves several candidate plumes beneath various hotspots poorly resolved. We assess the regional quality of GLAD-M25 relative to its global performance using a partition of the seismic waveform data used in its construction. We evaluate synthetic waveforms computed using the spectral-element method to determine how well they fit the data according to a variety of criteria measured across multiple seismic phases and frequency bands. The distributions of traveltime anomalies that remain in GLAD-M25 are wider for trans-Pacific paths than globally, suggesting comparatively insufficiently resolved seismic velocity structure in the region of interest. Hence, Pacific-centred regional inversions, based on (augmented) subsets of the global data set have the potential to enhance the resolution of velocity structure. We compare GLAD-M25 and SEMUCB-WM1 by cross-validation with a new, independent, data set. Our results reveal that short- and long-wavelength structure is captured differently by the two models. Our findings lead us to recommend focusing future model iteration on and around the Pacific Superswell and adding data that sample new corridors, especially using ocean sensors, to better constrain seismic velocity structure in this area of significant geodynamic complexity.

Understanding the interior structures of icy moons will be crucial in untangling narratives of formation and evolution, both within our solar system and beyond it. Seismology is a proven and unparalleled methodology for investigating the deep interiors of planetary bodies but has never been deployed on an icy moon. To improve future mission design, we conduct seismic simulations for Saturn's icy moon Enceladus which account for the unique seismic responses of icy ocean worlds. We discover that even with high surface temperatures at the south pole and 3D ice thickness models, seismic amplitudes are two orders of magnitude higher than the self‐noise of mission‐candidate instrumentation. We compare the effects of a 2D and 3D ice shell to determine the detail of seismic inversion for ice shell properties and how this varies with source and receiver location. We also compare the travel time differences caused by ice shell variation with potential effects from the uncertain core structure and discover that these two sources of travel time perturbation have similar magnitudes but could be distinguished through careful inversion strategy. We investigate varied source types to represent focal mechanisms likely to be present at the south pole of Enceladus. We finally make recommendations supporting landing sites between 20 and 30 from the south pole that should enable observation of a wide range of seismic phases, including core‐transmitted phases that could constrain core velocities.

Encouraging diversity in planetary science requires making a particular effort to bring a broader range of people onto the mission teams that are the backbone of the field. Observer programmes, which offer early-career researchers the chance to embed within a mission team during a science meeting, are one way of doing this. Here we present a quantitative analysis of the effectiveness of two observer programmes: InSightSeers and DART Boarders, linked respectively to the InSight and the Double Asteroid Redirection Test (DART) missions, using a mixture of one-group pre-test/post-test and one-group post-test only evaluation methods, with a total of 56 participants. We find substantial educational value added to participants from both programmes, with particular strengths being the effectiveness of these programmes at providing an introduction to mission teams and international collaborations. This work demonstrates that mission observer programmes can be an effective way of exposing early-career researchers to planetary science missions.

Understanding the interior structures of icy moons will be crucial in untangling narratives of formation and evolution, both within our solar system and beyond it. Seismology is a proven and unparalleled methodology for investigating the deep interiors of planetary bodies but has never been deployed on an icy moon. To improve future mission design, we conduct seismic simulations for Saturn’s icy moon Enceladus which account for the unique seismic responses of icy ocean worlds. We discover that even with high surface temperatures at the south pole and 3D ice thickness models, seismic amplitudes are two orders of magnitude higher than the self-noise of mission-candidate instrumentation. We compare the effects of a 2D and 3D ice shell to determine the detail of seismic inversion for ice shell properties and how this varies with source and receiver location. We also compare the travel time differences caused by ice shell variation with potential effects from the uncertain core structure and discover that these two sources of travel time perturbation have similar magnitudes but could be distinguished through careful inversion strategy. We investigate varied source types to represent focal mechanisms likely to be present at the south pole of Enceladus. We finally make recommendations supporting landing sites between 20 - 30° from the south pole that should enable observation of a wide range of seismic phases, including core-transmitted phases that could constrain core velocities.

Recent seismological studies challenge the traditional view that the interface between the core and mantle is a straightforward discontinuity. As seismology is pushed to its observational limits, a complex - potentially compositionally layered - region between the core and mantle is emerging.

Introduction Enceladus' interior structure comprises an icy outer shell, a global water ocean, and a rocky core, with the ice shell thinning significantly at the south pole, and to a lesser extent at the north pole, and with its greatest thickness around the equator. However, there is a large amount of debate over the thicknesses, compositions, and densities of each of these layers. Central to this uncertainty is an imbalance in its thermodynamic energy budget; the energy required to maintain the global ocean and south polar vents is significantly more than that generated by tidal heating assuming a consolidated core, and heating from radiogenic elements is expected to be limited. A porous, hydrothermally altered, and mechanically weaker core has been proposed to increase tidal heating in the core and help sustain the global ocean. This theory is supported by the chemical composition of ejecta from the south polar vents, which indicates water-rock interactions. Seismic research on Enceladus will benefit from the satellite's innate seismic sources, with high levels of eruptive and seismic activity at the south pole, as well as creaking along existing faults in the ice shell and potential for waves in the subsurface ocean, all generating distinctive seismic signals. The recent increase in Earth-based cryoseismology research can provide useful analogues to assist in characterising these sources and predicting wave propagation through the ice. Enceladus' small size and anticipated axisymmetry both help to minimise the computational resources necessary to produce detailed seismic wavefield simulations. Its 252km radius leads to short propagation distances that should enable even low-amplitude signals to be detected anywhere on the moon, and produces low interior pressures that help reduce the problem of extrapolation of material properties commonly associated with Earth-based seismology. Models Initially, an array of existing structural models were collected and quantified in order to draw comparisons between these results. These models, developed by numerous different authors, take various approaches including isostasy, libration, thermodynamics, and compositional constraints, and produce a wide range of results that demonstrate the inability of existing data to constrain a single coherent interior model. Three of these models were then recreated for analysis using the TauP toolkit software [1]: that of Čadek et al. [2], model 5 of Vance et al. [3], and model A1 of Neumann and Kruse [4], which are compared in Figure 1. Seismic velocities estimated by Stähler et al. [5] were adopted for these models, using the higher, consolidated core velocity for the entire Čadek core, and the maximum ice thickness was assumed for the 1D Čadek model. Ray theoretical results Travel time predictions are shown in Figure 2. In general, the Vance model, with its thicker ice shell and hydrated upper core layer, enables a larger number of phases to travel further around the moon. Results for the Neumann model are fairly similar, though they show slightly less expansive wavefields. Meanwhile, the Čadek model's thinner ice shell terminates most phases by 90 degrees and reduces the difference in travel time between direct phases, such as the direct shear waves (S), and reflected phases, such as shear waves reflected once from the ice-ocean interface (ScS). If detectable, PKJKP shear core phases, which appear at large distances in all three models, are likely to be useful in distinguishing between attenuation structures in the core to make inferences regarding its porosity. In addition, the thinner porous upper core layer in the Neumann model produces fewer core-underside reflected (PKIIKP) arrivals at any given distance than the thicker layer in the Vance model which, in contrast, produces an array of PKIIKP waves, some of which do not enter the deeper core and are instead reflected within the upper core. The boundary between the hydrated upper core and the consolidated deeper core has been specified to be sharp; however, a more gradational interface may suppress the number of these PKIIKP phases, and could be a useful diagnostic tool. Ray theoretical travel time predictions alone cannot provide insight into how detectable, and therefore useful, each phase may be. In addition, the 1D models necessarily omit known complexities such as variations in ice shell thickness with latitude, as well as smaller length-scale heterogeneities suggested by surface features. However, this early stage of modelling has already demonstrated the effects that different hypothesised seismic structures may have on a seismic survey of Enceladus, and these discrepancies are likely to become more reliably diagnostic with a higher-dimension model. Full wavefield simulations Enceladus' main structural heterogeneities appear to vary primarily with latitude, for example in the thinning of the ice shell at the poles. In addition, the anticipated primary sources of seismic activity, in the form of fault reactivation and plume emissions, are located at the south pole. Beyond using ray theory, this research also employs the axisymmetric full waveform simulation code AxiSEM [6] to assess the seismic wavefields generated by quakes at the south pole of Enceladus. Initial investigations assess the detectability of different phases in the 1D models described above. Input parameters to AxiSEM can be modified to investigate the effect of variables such as attenuation properties and degree of core porosity or serpentinisation. The introduction of attenuation structures based on terrestrial measurements and expectations will improve the prediction of geographical variation in the detectability of key phases. Different types of source mechanisms can be explored in AxiSEM to describe the sources present on Enceladus. Comparison of the 2.5D wavefields generated by AxiSEM with the 1D TauP models will help to evaluate the impact of...