Veronica J. Bray

An impact origin for Silverpit Crater, on the UK continental shelf, has been contested over the last two decades, with a lack of definitive evidence – traditionally petrographic evidence of shock metamorphism – to resolve the debate. Here we present 3D seismic, petrographic and biostratigraphic data, and numerical impact simulations to test the impact hypothesis. The seismic data provide exceptional imaging of the entire structure, confirming the presence of a central uplift, annular moat, damage zone and numerous secondary craters on the contemporaneous seabed. The distribution of normal and reverse faults in the brim, and curved radial faults around the central uplift suggest a low-angle impact from the west. The pitted, flat-topped central uplift at the top chalk horizon may indicate significant devolatilization of chalk immediately following impact. Biostratigraphic data confirm that this event occurred during the middle Eocene, between 43-46 million years ago. Petrographic analysis from the reworked ejecta sequence in the nearby 43/25-1 well reveals two grains with shock lamellae, indicating shock pressures of ~10-13 GPa, consistent with results from our numerical models. This combination of data and modelling provide compelling evidence that Silverpit Crater is an exceptionally preserved hypervelocity impact structure.

Titan’s impact craters are significantly shallower than fresh craters on Ganymede, which are often used as templates for fresh craters on Titan. The presence of a clathrate crust on Titan, which would insulate the ice shell and maintain warmer ice near the surface, may contribute to the shallowness of its craters. Research into the effects of a clathrate crust on the topographic relaxation of craters found that a 5–10 km thick clathrate crust may explain their shallowness. That previous study simulated simple, bowl-shaped craters with Ganymede-derived depths. However, if craters are initially like Ganymede’s, they will exhibit complex rather than simple morphologies. Therefore, we model the relaxation of more realistic shapes incorporating short-wavelength features (central peaks, pits, and pit-rims), floor widths, and rimwall slopes derived from Ganymede’s fresh complex craters. We simulate topographic relaxation of craters 40–100 km in diameter in ice shells with 5–20 km thick clathrate crusts. We find that most crater depths can be reproduced in 5–15 km thick clathrate crusts. Notably, simulated short-wavelength central features remain or grow during relaxation. Evidence of central features is rare, suggesting that additional processes modify Titan’s craters. To fill the remaining flexural moats, 300–1000 m of infill is required, while 200–400 m may need to erode from elevated central features to flatten the relief. We conclude that topographic relaxation may be partially responsible for the shallowness and that erosion and infill are responsible for the dearth of central features in Titan’s craters.

The icy dwarf planets of Pluto, Charon, and Ceres provide important geologic comparisons for each other as they possess similar gravities and experience impacts of similar velocities compared to larger planets and moons. Differences in their impact craters can therefore be attributed to differences in the impactor and/or target compositions. Craters on Ceres show taller rims than craters on Charon, but comparable internal crater dimensions (depth, wall slope, and peak size). It is possible that the non-ice component of the Ceres crust provides additional strength to this outer portion of the crater, maintaining less rim collapse during the modification stage of crater formation, while the warmer central crater region is controlled by the weakness of the icy component. The spreads in depth-to-diameter ratio and wall-slope values for the Pluto data set are not clearly related to preservation state, suggesting that fresh craters on Pluto possess a large variance in these crater dimensions compared to Charon. This could be due to the geologically diverse terrains and depositional histories at different latitudes and longitudes of Pluto. These provide different target properties for each impact, so that even pristine impact craters on Pluto could display a wide range in depths and wall slopes based on the different strength of the target in each area.

Spectral characterization of near-Earth asteroids (NEAs) has revealed a continuum of space-weathered states for the surfaces of S-complex NEAs, with Q-class NEAs, an S-complex subclass, most closely matching the unweathered surfaces of ordinary chondrite meteorites. Dynamical calculations of the orbital evolution of S-complex NEAs revealed that Q-class NEAs tend to have close encounters with terrestrial planets, suggesting that planetary tides may play a role in refreshing NEA surfaces. However, the exact physical mechanism(s) that drive resurfacing through tidal encounters and the encounter distance at which these mechanisms are effective have remained unclear. Through the lens of the upcoming (99942) Apophis encounter with Earth in 2029, we investigate the potential for surface mobilization through tidally driven seismic shaking over short timescales during the encounter and subsequent surface slope evolution over longer timescales driven by tumbling. We perform multiscale numerical modeling and find that the 2029 encounter will induce short-term tidally driven discrete seismic events that lead to high-frequency (>0.1 Hz) surface accelerations that reach magnitudes similar to Apophis’s gravity and that may be detectable by modern seismometers. It is still unclear if the shaking we model translates to widespread particle mobilization and/or lofting. We also find that there will be a significant change in Apophis’s tumbling spin state that could lead to longer-term surface refreshing in response to tumbling-induced surface slope changes. We propose that through these mechanisms, space-weathered S-class asteroid surfaces may become refreshed through the exposure of unweathered underlying material. These results will be tested by the future exploration of Apophis by NASA’S OSIRIS-APEX.

The Nadir Crater offshore West Africa is a recently proposed near K-Pg impact structure identified on 2D seismic. Here we present 3D seismic data that image this crater in exceptional detail, unique for any such structure, which demonstrates beyond reasonable doubt that the crater-forming mechanism was a hypervelocity impact. Seismic mapping reveals a near-circular crater rim of 9.2 km and an outer brim of ~23 km diameter defined by concentric normal faults. An extended damage zone is evident across the region, well beyond the perceived limit of subsurface deformation for impact craters, except in a ‘sheltered zone’ to the east. The paleo-seabed shows evidence for widespread liquefaction because of seismic shaking, and scars and gullies formed by tsunami wave propagation and resurge. Deformation within the ~425 m high stratigraphic uplift and annular moat allows us to reconstruct the evolution of the crater, with radial thrusts at the periphery of the uplift suggesting a low-angle impact from the east. Structural relationships are used to reconstruct the deformation processes during the crater modification stage, with the central uplift forming first, followed by centripetal flow of surrounding sediments into the evacuated crater floor in the seconds to minutes after impact.

We report the detection of 239 trans-Neptunian objects discovered through the ongoing New Horizons survey for distant minor bodies being performed with the Hyper Suprime-Cam mosaic imager on the Subaru Telescope. These objects were discovered in images acquired with either the r2 or the recently commissioned EB-gri filter using shift and stack routines. Due to the extremely high stellar density of the search region downstream of the spacecraft, new machine learning techniques had to be developed to manage the extremely high false-positive rate of bogus candidates produced from the shift and stack routines. We report discoveries as faint as r2 ∼ 26.5. We highlight an overabundance of objects found at heliocentric distances R ≳ 70 au compared to expectations from modeling of the known outer solar system. If confirmed, these objects betray the presence of a heretofore-unrecognized abundance of distant objects that can help explain a number of other observations that otherwise remain at odds with the known Kuiper Belt, including detections of serendipitous stellar occultations, and recent results from the Student Dust Counter on board the New Horizons spacecraft.

Impact crater formation is affected by the physical properties of planetary crusts, allowing comparison of crater dimensions to serve as a proxy for comparing the crustal properties of different planetary bodies. New topographic profiles of Callisto craters, derived from Galileo-based digital terrain models, are presented, and the crater dimensions recorded. These data were used to derive crater morphometry scaling trends, which were then compared to the established trends of Ganymede and the Moon. Our comparative study suggests that the upper brittle portion of Callisto’s ice crust allows for the retention of steep-sided and elevated rim scarps, while subsurface warmer ice leads to an enhanced uplift and shallowing of the crater bowl. Crater dimensions are similar between Callisto and Ganymede, suggesting that the bulk properties of their near-surface crusts are comparable. The most notable difference between craters on these two Galilean moons were the smaller central pit diameters on Callisto. This difference can be explained if the pit formation on these bodies is controlled by the presence and movement (drainage and/or volatile loss) of impact melt water: the lower impact velocity and/or lower expected crustal heat flow on Callisto will result in less impact melt generation, and thus smaller central pits.

We report the detection of 239 trans-Neptunian Objects discovered through the on-going New Horizons survey for distant minor bodies being performed with the Hyper Suprime-Cam mosaic imager on the Subaru Telescope. These objects were discovered in images acquired with either the r2 or the recently commissioned EB-gri filter using shift and stack routines. Due to the extremely high stellar density of the search region down stream of the spacecraft, new machine learning techniques had to be developed to manage the extremely high false positive rate of bogus candidates produced from the shift and stack routines. We report discoveries as faint as r2$\sim26.5$. We highlight an overabundance of objects found at heliocentric distances $R\gtrsim70$~au compared to expectations from modelling of the known outer Solar System. If confirmed, these objects betray the presence of a heretofore unrecognized abundance of distant objects that can help explain a number of other observations that otherwise remain at odds with the known Kuiper Belt, including detections of serendipitous stellar occultations, and recent results from the Student Dust Counter on-board the New Horizons spacecraft.