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Mimas' small size, lack of geologic activity, and high eccentricity suggest a frozen, inactive history. However, Cassini libration measurements are best explained by a present‐day liquid ocean under an ice shell 24–31 km thick, a configuration that tidal heating can support. These unexpected findings have motivated further study of Mimas' surface geology to constrain its interior evolution. Here, we model the formation of Mimas' large impact basin, Herschel, using iSALE‐2D. Our goal is to determine whether the current estimated ice shell thickness is sufficient to withstand the impact and produce the observed basin morphology. We find that, if Mimas had an ocean at the time of the Herschel‐forming impact, the ice shell had to have been 10s of km thicker than today. These findings are consistent with inferences from minimal tectonic activity that Mimas must be a young ocean world, if it indeed has an ocean today. Plain Language Summary: Mimas, a small moon of Saturn, is heavily cratered and lacks the typical characteristics of an ocean‐bearing moon, such as the active surface of its neighboring moon, Enceladus. However, measurements of Mimas, made by the Cassini mission, are best explained by an ocean under a relatively thick ice shell. Here, we try to understand how this ice shell and ocean may have changed with time by modeling the formation of Mimas' largest impact basin, Herschel. When such large impacts occur, the shapes and depths of the resulting basin can be affected by the presence of the ocean. We find that Herschel could not have formed in an ice shell at the present‐day thickness without obliterating the ice shell at the impact site. Rather, we conclude that, if Mimas has an ocean today, the ice shell has been thinning since the formation of Herschel, which could also explain the lack of fractures on Mimas. Mimas as an emerging ocean world places important constraints on the formation, evolution, and habitability of all of the mid‐sized moons of Saturn. Key Points: We performed numerical simulations of the formation of the Herschel impact basin to explore Mimas' evolutionThe Herschel impact basin could not have formed in an ice shell of the thickness indicated by Mimas' present‐day librationsHerschel's morphology and the lack of tectonics on Mimas are compatible with a thinning ice shell and geologically young ocean [ABSTRACT FROM AUTHOR]

Evidence from Arrokoth and comets strongly suggests a very low density for this and similar small Kuiper belt objects. Plausible compositions imply high porosities, in excess of 70%, and low compaction crush strengths. If so, impact craters on Arrokoth (especially Sky, its largest) formed largely by compaction of pore space and material displacement. This is consistent with geological evidence from New Horizons imaging. High porosity reduces cratering efficiency in the gravity regime whereas compaction moves it toward crush strength scaling and increased efficiency. Compaction also guarantees that most impactor kinetic energy is taken up as waste heat near the impact point, with momentum transferred to the rest of the body by elastic waves only. Monte Carlo simulations of Sky‐forming conditions indicate that the momentum imparted likely separated Arrokoth's two lobes, but displacement was limited by dissipation at the neck between them. Unusual strength properties are not required to preserve Arrokoth's bilobate configuration. Plain Language Summary: It has become apparent over the last few years that small asteroids and comets are very underdense compared with the materials they are made of. This means that their total porosities are likely quite high, in excess of 70%, both as tiny voids within particles (so‐called microscopic porosity) and spaces between particles (macroscopic porosity). But none are likely as porous as the distant denizens of the Kuiper belt such as Arrokoth (visited by the New Horizons spacecraft in 2019). This paper concerns impact craters on Arrokoth and similar small bodies, and the rather unusual effects expected. Imagine a fluffy (fine powder) snowball striking a much larger fluffy snowball, only that the snow is not pure ice but a mixture of porous icy, rocky, and carbon‐rich particles. Even at high velocities (>100 s of m/s) craters should mostly form by compacting pore space and pushing material away from the impact point, not the traditional blasting of ejecta back into space. Similar to crush‐up of an automobile bumper, compaction helps to protect from the potentially catastrophic effects of large impacts, such as complete disruption of the target or breakup of bilobate bodies like Arrokoth, and should be incorporated in future collisional evolution studies. Key Points: Arrokoth is likely a low density, highly porous, contact binary planetesimalImpact craters on Arrokoth, and particularly its largest, likely formed as compaction craters, with modest or little ejectaHigh porosity acted to protect Arrokoth from catastrophic disruption, ejecta recoil, and lobe dislocation and rearrangement [ABSTRACT FROM AUTHOR]

Knowledge of the shock remanent magnetization (SRM) property is crucial to interpret the spatial changes in magnetic anomalies observed over the impact crater. This study reports the spatial distributions of SRM intensity and stability of single‐domain titanomagnetite‐bearing basalt based on the SRM acquisition experiments using a two‐stage light gas gun with Al projectiles, remanence measurements for divided subsamples, and impact simulations. The SRM properties systematically change with increasing pressure, and three distinctive aspects are recognized at different pressure ranges: (1) constant intensity below 0.1 GPa, (2) linear trend as intensity is proportional to pressure up to 1.1 GPa, and (3) constant intensity and increasing stability above 1.9 GPa. The SRM intensity and stability distributions suggest that the crustal rocks containing the single‐domain titanomagnetite originally had an SRM intensity distribution according to the distance from the impact point, which changed depending on the remanence stability after the impact. Plain Language Summary: Shock remanent magnetization (SRM) is acquired as a result of the shock wave propagation in a magnetic field. Knowledge of a three‐dimensional distribution of the SRM intensity and stability is crucial for interpreting the spatial change in magnetic anomalies observed over the impact crater and reconstructing the paleo‐planetary field based on the anomaly data. Here, we report the SRM intensity and stability structures of fine‐grained single‐domain titanomagnetite‐bearing basalt based on the newly designed study protocol such as the SRM acquisition experiments, remanence measurements for divided subsamples, and impact simulations. The SRM properties systematically change with increasing pressure, and three distinctive aspects are recognized at different pressure ranges. The systematic changes in the SRM intensity and stability suggest that the crustal rocks containing the single‐domain titanomagnetite originally had an SRM intensity structure at the time of impact, and this structure changed depending on the remanence stability after the impact. Key Points: The shock remanent magnetization (SRM) acquisition experiments and impact simulations were conducted to reveal the SRM intensity and stability distributionsThe SRM properties systematically change with pressure, and three distinctive aspects are recognized at different pressure rangesThe SRM intensity varied with the distance from the impact point, which changed depending on the remanence stability after the impact [ABSTRACT FROM AUTHOR]

Recent analysis of New Horizons data revealed an ∼1,000‐km‐diameter region of large‐scale lineations antipodal to Sputnik Planitia, the 1,200 × 2,000 km elliptical impact basin on Pluto's anti‐Charon hemisphere. At the available resolution, these lineations are similar to antipodal terrains associated with large impact basins elsewhere in the Solar System. Here, we simulate the Sputnik Planitia‐forming impact and track stress waves through a Pluto‐like target body to the antipode. We find that the Sputnik Planitia‐forming impact is capable of producing significant antipodal terrains and the observed lineations may be extensional graben. The extent and mode of deformation at the antipode, however, is sensitive to ocean thickness and core composition. Simulations that best reproduce the observed terrains imply that Pluto may have hosted a >150‐km‐thick ocean and a hydrated core at the time of impact. Plain Language Summary: Pluto's interior structure remains largely a mystery, including the composition of the core and the existence and extent of a subsurface ocean. Though seismological study is not yet possible on Pluto, the reaction of a planet's interior structure to stress waves produced by large impacts can be used to indirectly probe the planet's interior—seismology by impact. We use impact modeling to simulate the formation of Sputnik Planitia, Pluto's massive impact basin, to determine how the dwarf planet's interior controls transmission of stress waves to the opposite hemisphere. Our results suggest that a large subsurface ocean beneath the ice shell, as well as a core that has experienced substantial alteration from interactions with an ocean above, are critical to match the size of the lineated terrain observed antipodal to Sputnik Planitia. Key Points: We model the formation of Pluto's Sputnik Planitia basin from an impact origin to assess the dwarf planet's interior structureOur simulations illustrate the Sputnik Planitia‐forming impact is capable of producing significant antipodal terrains on PlutoThe extent and mode of deformation at the antipode is most consistent with a thick (>150‐km) ocean and a hydrated core [ABSTRACT FROM AUTHOR]

We examined the spectral properties of a selection of Titan's impact craters that represent a range of degradation states. The most degraded craters have rims and ejecta blankets with spectral characteristics that suggest that they are more enriched in water ice than the rims and ejecta blankets of the freshest craters on Titan. The progression is consistent with the chemical weathering of Titan's surface. We propose an evolutionary sequence such that Titan's craters expose an intimate mixture of water ice and organic materials, and chemical weathering by methane rainfall removes the soluble organic materials, leaving the insoluble organics and water ice behind. These observations support the idea that fluvial processes are active in Titan's equatorial regions.