Images from the Voyager 2 mission revealed the small Uranian satellite Miranda to be a complex, dynamic world. This is exemplified by signs of recent geological activity, including an extensive fault system and the mysterious coronae. This has led to speculation that Miranda may have been tectonically active within the geologically recent past and could have hosted a subsurface liquid water ocean at the time. In this work, we aim to constrain the thickness ranges for the ice shell and potential subsurface ocean on Miranda. Here, we present the results for our geological mapping of craters, ridges, and furrows on the surface. We also present the results for our comparison of the geographic distribution of these features to the predicted geographic distribution of maximum tidal stress based on stress models. We model eccentricity tidal stress, ice shell thickening stress, true polar wander stress, and obliquity tidal stress and compare the predicted surface stress pattern for each to what pattern can be inferred from the surface geology. Our results show that a thin crust (≤30 km) is most likely to result in sufficient stress magnitude to cause brittle failure of ice on Miranda's surface. Our results also suggest the plausible existence of a ≥100 km thick ocean on Miranda within the last 100–500 million yr. This has implications for the dynamical history of Miranda and its status as a potential ocean world.

Titan's impact craters are hundreds of meters shallower than expected, compared to similar-sized craters on Ganymede. Only 90 crater candidates have been identified, the majority of which have low certainty of an impact origin. Many processes have been suggested to shallow, modify, and remove Titan's craters, including fluvial erosion by liquid from rainfall, aeolian sand infill, and topographic relaxation induced by insulating sand infill. Here we propose an additional mechanism: topographic relaxation due to an insulating methane clathrate crustal layer in Titan's upper ice shell. We use finite element modeling to test whether a clathrate crust 5, 10, 15, or 20 km thick could warm the ice shell and relax craters to their currently observed depths or remove them completely. We model the viscoelastic evolution of crater diameters 120, 100, 85, and 40 km, with two initial depths based on depth−diameter trends of Ganymede's craters. We find that all clathrate crustal thicknesses result in rapid topographic relaxation, despite Titan's cold surface temperature. The 5 km thick clathrate crust can reproduce nearly all of the observed shallow depths, many in under 1000 yr. A 10 km thick crust can reproduce the observed depths of the larger craters over geologic timescales. If relaxation is the primary cause of the shallow craters, then the clathrate thickness is likely 5–10 km thick. Topographic relaxation alone cannot remove craters; crater rims and flexural moats remain. To completely remove craters and reproduce the observed biased crater distribution, multiple modification processes must act together.

Although the scientific principles of anthropogenic climate change are well-established, existing calculations of the warming effect of carbon dioxide rely on spectral absorption databases, which obscures the physical foundations of the climate problem. Here, we show how CO₂ radiative forcing can be expressed via a first-principles description of the molecule's key vibrational-rotational transitions. Our analysis elucidates the dependence of carbon dioxide's effectiveness as a greenhouse gas on the Fermi resonance between the symmetric stretch mode ν₁ and bending mode ν₂. It is remarkable that an apparently accidental quantum resonance in an otherwise ordinary three-atom molecule has had such a large impact on our planet's climate over geologic time, and will also help determine its future warming due to human activity. In addition to providing a simple explanation of CO₂ radiative forcing on Earth, our results may have implications for understanding radiation and climate on other planets.

Apophis's current trajectory takes it safely past our planet at a distance of several Earth radii on 2029 April 13. Here the possibility is considered that Apophis could collide with a small asteroid, like the ones that frequently and unpredictably strike Earth, and the resulting perturbation of its trajectory. The probability of an impact that could significantly displace Apophis relative to its keyholes is found to be less than one in 10⁶, requiring a Δv ≳ 0.3 mm s⁻¹, while for an impact that could significantly displace Apophis compared to its miss distance in 2029, it is less than one in 10⁹, requiring a Δv ≳ 5 cm s⁻¹. These probabilities are below the usual thresholds considered by asteroid impact warning systems. Apophis is in the daytime sky and unobservable from mid-2021 to 2027. It will be challenging to determine from single-night observations in 2027 if Apophis has moved on the target plane enough to enter a dangerous keyhole, as the deviation from the nominal ephemeris might be only a few tenths of an arcsecond. An impending Earth impact would, however, be signaled clearly in most cases by deviations of tens of arcseconds of Apophis from its nominal ephemeris in 2027. Thus, most of the impact risk could be retired by a single observation of Apophis in 2027, though a minority of cases present some ambiguity and are discussed in more detail. Charts of the on-sky position of Apophis under different scenarios are presented for quick assessment by observers.

Jupiter's Great Red Spot (GRS) is known to exhibit oscillations in its westward drift with a 90-day period. The GRS was observed with the Hubble Space Telescope on eight dates over a single oscillation cycle in 2023 December to 2024 March to search for correlations in its physical characteristics over that time. Measured longitudinal positions are consistent with a 90-day oscillation in drift, but no corresponding oscillation is found in latitude. We find that the GRS size and shape also oscillate with a 90-day period, having a larger width and aspect ratio when it is at its slowest absolute drift (minimum date-to-date longitude change). The GRS's UV and methane gas absorption-band brightness variations over this cycle were small, but the core exhibited a small increase in UV brightness in phase with the width oscillation; it is brightest when the GRS is largest. The high-velocity red collar also exhibited color changes, but out of phase with the other oscillations. Maximum interior velocities over the cycle were about 20 m s⁻¹ larger than minimum velocities, slightly larger than the mean uncertainty of 13 m s⁻¹, but velocity variability did not follow a simple sinusoidal pattern as did other parameters such as longitude width or drift. Relative vorticity values were compared with aspect ratios and show that the GRS does not currently follow the Kida relation.

The number of planetary satellites around solid objects in the inner solar system is small either because they are difficult or unlikely to form or because they do not survive for astronomical timescales. Here we conduct a pilot study on the possibility of satellite capture from the process of collision-less binary exchange and show that massive satellites in the range 0.01–0.1 M_⊕ can be captured by Earth-sized terrestrial planets in a way already demonstrated for larger planets in the solar system and possibly beyond. In this process, one of the binary objects is ejected, leaving the other object as a satellite in orbit around the planet. We specifically consider satellite capture by an "Earth" in an assortment of hypothetical encounters with large terrestrial binaries at 1 au around the Sun. In addition, we examine the tidal evolution of captured objects and show that orbit circularization and long-term stability are possible for cases resembling the Earth–Moon system.

Two fundamental questions face lunar scientists: (1) What is the absolute age of each lunar impact basin and thus the early impact flux curve? (2) To what degree did basin impact melt seas undergo differentiation? We compiled a 1:200,000-scale geological map of the lunar Orientale basin, focusing on identifying the most widespread and accessible occurrences of impact melt deposits from the basin-forming impact to help guide sample-return missions to Orientale and especially to other undated lunar basins using the identification and interpretation strategies for Orientale. We assess the size of craters excavating through basalt cap rock that may have exhumed datable basin impact melt, and we assess the possibility of impact melt sampling and melt differentiation for the large complex crater Maunder. We also provide guidance for distinguishing impact melt produced by larger complex craters from excavated basin melt and determining whether such craters may have also sampled through the entire melt deposit. Our analysis finds six such sites that are predicted to have the same age—that of the Orientale-forming event—and provides guidance for assessing possible melt differentiation. Future missions could collect samples from these sites for in situ age dating and petrologic assessment and/or for return to Earth and subsequent age dating and analysis. By sampling and dating impact melt of known provenance from the Moon's dozens of large basins, future work can anchor the chronostratigraphy of the Moon's formative years. Such information could be scaled to infer Earth's large impactor flux around the time of life's first emergence.

The role played by transient impact-induced endogenous brines in the formation of geomorphic features has been proposed on airless worlds such as Europa, Vesta, and Ceres, as well as on worlds with thin atmospheres such as Mars. After liquefaction, the hypothesized brines flow in a debris-flow-like process, incising curvilinear gullies and constructing lobate deposits within newly formed craters. Here we investigate what parameters (if any) could enable liquid to be transiently present for a sufficient time (∼tens of minutes) under postimpact transient atmospheric pressures (10⁻⁴–10⁻⁵ torr) to form curvilinear gullies and lobate deposits, as have been seen on Vesta and more tentatively Ceres. We report that water likely vacuum-freezes too quickly, while NaCl brine enables flow longevity. We also find that frozen lid formation facilitates longer liquid lifetimes, as with lava in terrestrial lava tubes and model predictions for cryovolcanic flows on Europa. This work provides additional contributions to the growing body of literature that investigates the role of transient brines in sculpting the surfaces of airless worlds.

Body tides reveal information about planetary interiors and affect their evolution. Most models to compute body tides rely on the assumption of a spherically symmetric interior. However, several processes can lead to lateral variations of interior properties. We present a new spectral method to compute the tidal response of laterally heterogeneous bodies. Compared to previous spectral methods, our approach is not limited to small-amplitude lateral variations; compared to finite element codes, this approach is more computationally efficient. While the tidal response of a spherically symmetric body has the same wavelength as the tidal force; lateral heterogeneities produce an additional tidal response with a spectra that depends on the spatial pattern of such variations. For Mercury, the Moon, and Io, the amplitude of this signal is as high as 1%–10% of the main tidal response for long-wavelength shear modulus variations higher than ∼10% of the mean shear modulus. For Europa, Ganymede, and Enceladus, shell-thickness variations of 50% of the mean shell thickness can cause an additional signal of ∼1% and ∼10% for the Jovian moons and Encelaudus, respectively. Future missions, such as BepiColombo and JUICE, might measure these signals. Lateral variations of viscosity affect the distribution of tidal heating. This can drive the thermal evolution of tidally active bodies and affect the distribution of active regions.

Asteroid impacts potentially represent a substantial threat to humanity, but one that we can plan for and mitigate. To design an effective asteroid mitigation mission, however, it is important to have as detailed knowledge of the asteroid threat as possible. Our understanding of a newly discovered object will generally derive from our understanding of the near-Earth object population, and in cases where there is no time for a reconnaissance mission prior to deflection or disruption, we may need to lean heavily on any existing data of similar objects. The Tour of Asteroids for Characterization Observations (TACO) mission concept would fill key gaps in the characterization knowledge needed to plan an effective response to an asteroid threat. A tour targeting potentially hazardous asteroids and focused on reconnaissance objectives specifically relevant for planetary defense would also test instruments and technologies (e.g., autonomous navigation, high-rate gimbals) ahead of when they are actually required in response to a threat. Testing these capabilities is identified as a need in the National Near-Earth Object Preparedness Strategy and Action Plan. The TACO tour concept is specifically designed to measure the most important asteroid properties for planetary defense, including mass, size/shape, surface and near-surface structure, presence of satellites, and composition. These measurements can be obtained using a nominal payload, including a narrow-angle camera, a thermal infrared imager, and deployed test masses for gravity science.

Comparing how an asteroid appears in space to its ablation behavior during atmospheric passage and finally to the properties of associated meteorites represents the ultimate probe of small near-Earth objects. We present observations from the Lowell Discovery Telescope and multiple meteor camera networks of 2022 WJ1, an Earth impactor that was disrupted over the North American Great Lakes on 2022 November 19. As far as we are aware, this is only the second time an Earth impactor has been specifically observed in multiple passbands prior to impact to characterize its composition. The orbits derived from telescopic observations submitted to the Minor Planet Center and ground-based meteor cameras result in impact trajectories that agree to within 40 m, but no meteorites have been found as of yet. The telescopic observations suggest a silicate-rich surface and thus a moderate-to-high albedo, which results in an estimated size for the object of just D = 40−60 cm. Modeling the fragmentation of 2022 WJ1 during its fireball phase also suggests an approximate 0.5 m original size for the object as well as an ordinary chondrite-like strength. These two lines of evidence both support that 2022 WJ1 was likely an S-type chondritic object and the smallest asteroid compositionally characterized in space. We discuss how best to combine telescopic and meteor camera data sets, how well these techniques agree, and what can be learned from studies of ultrasmall asteroids.

Some groups of carbonaceous chondrites are dominated by phyllosilicates, specifically the serpentine group, which are characterized as alternating layers of silicates, metal cations, and hydroxyl (OH). Both antigorite, an Mg-rich member, and cronstedtite, an Fe-rich member, can be found in hydrated carbonaceous chondrites. Cronstedtite is found in substantial amounts in CM chondrites. The hydroxyl can make up to a quarter of the molecular mass, and part of it can turn into molecular water through thermal-induced crystal structure breakup. Understanding how the water is released through thermal effects is important for describing the history of certain near-Earth asteroids, as well as for ascertaining the viability of in situ resource utilization (ISRU) for a given object. Here we investigate the temperature of dehydration of the main serpentine constituents of carbonaceous chondrites—antigorite, lizardite, and cronstedtite—under both atmospheric and vacuum conditions. Our results show that the mass loss for cronstedtite starts at about 150 °C lower temperature than for the Mg serpentines. Additionally, vacuum does not affect significantly the onset of the mass loss, but it affects its dynamics. Finally, our observations suggest that cronstedtite loses additional oxygen molecules besides the ones contained within the hydroxyl. These results provide an important new perspective on the orbital history of certain asteroids and the required proximity to the Sun in order for them to lose water. Additionally, it puts the CM-type asteroids on the center stage for ISRU.

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.

We generalize the crater equilibrium concept, a terminal state on a cratered surface where the balance of crater production and erasure apparently limits the crater population from further growth. Assuming the crater production consists of a single power law, our model identifies four classes of crater equilibrium. The first class is the most common state, where the power-law slope for the equilibrium size–frequency distribution is independent of the crater production slope power. The second class arises when there is efficient degradation of larger craters by smaller crater production, which results in dependence of the crater equilibrium slope power on the crater production slope power. The third class is another common state when a shallow production function causes a crater equilibrium state with a similarly shallow slope. This class results from the enhanced degradation of smaller craters by larger crater production. The fourth class is a combination of the second and third classes. We further compare the concept of geometric saturation, which has been widely used to quantify the level of crater equilibrium, and that of cookie-cutter saturation. We present a crucial update to the cookie-cutter saturation concept that brings models closer to the reality of crater accumulation over a range of sizes than the geometric saturation concept. Our model offers simpler analytical formulae for cookie-cutter saturation and proposes this concept as a more meaningful reference to argue the crater equilibrium level. Our work and earlier studies confirm the consistency of the crater equilibrium concepts, enabling deeper interpretations of crater equilibrium.

Two fundamental questions face lunar scientists: (1) What is the absolute age of each lunar impact basin and thus the early impact flux curve? (2) To what degree did basin impact melt seas undergo differentiation? We compiled a 1:200,000-scale geological map of the lunar Orientale basin, focusing on identifying the most widespread and accessible occurrences of impact melt deposits from the basin-forming impact to help guide sample-return missions to Orientale and especially to other undated lunar basins using the identification and interpretation strategies for Orientale. We assess the size of craters excavating through basalt cap rock that may have exhumed datable basin impact melt, and we assess the possibility of impact melt sampling and melt differentiation for the large complex crater Maunder. We also provide guidance for distinguishing impact melt produced by larger complex craters from excavated basin melt and determining whether such craters may have also sampled through the entire melt deposit. Our analysis finds six such sites that are predicted to have the same age—that of the Orientale-forming event—and provides guidance for assessing possible melt differentiation. Future missions could collect samples from these sites for in situ age dating and petrologic assessment and/or for return to Earth and subsequent age dating and analysis. By sampling and dating impact melt of known provenance from the Moon's dozens of large basins, future work can anchor the chronostratigraphy of the Moon's formative years. Such information could be scaled to infer Earth's large impactor flux around the time of life's first emergence.