Table of contents

Remote sensing observations of the Moon suggest that the lunar polar regolith environment is affected by several natural processes that may cause the regolith in these regions to become more porous and fine particulate. One of these processes may be the mechanical breakdown of regolith particles through the interaction of water ice and regolith by frost wedging. We present morphological and spectral analyses of high-fidelity lunar regolith simulants LHS-1 (lunar highlands simulant-1) and LMS-1 (lunar mare simulant-1) that have been exposed to varying concentrations of water ice (1, 10, and 30 wt%) over extended periods of time (1, 3, and 6 months) to evaluate the extent at which lunar regolith may be weathered by ice-regolith interactions in the Moon's polar regions. To characterize changes in regolith particle morphology, we explored grain size and shape parameters with the CILAS ExpertShape suite and characterized the abundance and evolution of clinging fines with scanning electron microscopy and energy dispersive X-ray spectroscopy. Reflectance spectra were taken from 1.0–22.5 μm (444.4–10,000 cm⁻¹) to characterize any differences in spectral features that may occur as a result of regolith breakdown. Both the morphological and spectral investigations display trends that show simulant particle degradation as a function of composition, increasing water concentration, and freezing time. Our study demonstrates that the lunar regolith is susceptible to mechanical breakdown in the presence of water ice and that water ice is likely a contributor to the weathering environment within permanently shadowed regions on the lunar surface.

Energy balance models (EBMs), alongside radiative–convective climate models and global climate models (GCMs), are useful tools for simulating planetary climates. Historically, planetary and exoplanetary EBMs have solely been 1D latitudinally dependent models with no longitudinal dependence, until the study of Okuya et al., which focused on simulating synchronously rotating planets. Following the work of Okuya et al., I have designed the first 2D EBM (PlaHab) that can simulate N₂–CO₂–H₂O–H₂ atmospheres of both rapidly and synchronously rotating planets, including Mars, Earth, and exoplanets located within their circumstellar habitable zones. PlaHab includes physics for both water and CO₂ condensation. Regional topography can be incorporated. Here, I have specifically applied PlaHab to investigate the present Earth, early Mars, TRAPPIST-1 e, and Proxima Centauri b, representing examples of habitable (and potentially habitable) worlds in our solar system and beyond. I compare my EBM results against those of other 1D and 3D models, including those of the recent Trappist-1 Habitable Atmosphere comparison project. Overall, the EBM results are consistent with those of other 1D and 3D models, although inconsistencies among all models continue to be related to the treatment of clouds and other known differences between EBMs and GCMs, including heat transport parameterizations. Although 2D EBMs are a relatively new entry in the study of planetary/exoplanetary climates, their ease of use, speed, flexibility, wide applicability, and greater complexity (relative to 1D models) may indicate an ideal combination for the modeling of planetary and exoplanetary atmospheres alike.

Long-term magma ocean phases on rocky exoplanets orbiting closer to their star than the runaway greenhouse threshold—the inner edge of the classical habitable zone—may offer insights into the physical and chemical processes that distinguish potentially habitable worlds from others. The thermal stratification of runaway planets is expected to significantly inflate their atmospheres, potentially providing observational access to the runaway greenhouse transition in the form of a habitable zone inner edge discontinuity in radius–density space. Here, we use Bioverse, a statistical framework combining contextual information from the overall planet population with a survey simulator, to assess the ability of ground- and space-based telescopes to test this hypothesis. We find that the demographic imprint of the runaway greenhouse transition is likely detectable with high-precision transit photometry for sample sizes ≳100 planets if at least ∼10% of those orbiting closer than the habitable zone inner edge harbor runaway climates. Our survey simulations suggest that, in the near future, ESA's PLATO mission will be the most promising survey to probe the habitable zone inner edge discontinuity. We determine the survey strategies that maximize the diagnostic power of the obtained data and identify as key mission design drivers: (1) a follow-up campaign of planetary mass measurements and (2) the fraction of low-mass stars in the target sample. Observational constraints on the runaway greenhouse transition will provide crucial insights into the distribution of atmospheric volatiles among rocky exoplanets, which may help to identify the nearest potentially habitable worlds.

The Miniature Radio Frequency instrument (Mini-RF) on the Lunar Reconnaissance Orbiter obtained widespread synthetic aperture radar observations of the Moon in the S band (12.6 cm), including nearly complete coverage at both lunar poles. The currently archived monostatic data have spatial offsets from the lunar reference frame, making them more difficult to compare to other data sets. To address this issue, we have developed a new algorithm for spatially controlling the Mini-RF S-band monostatic data set and orthorectifying these data onto lunar topography. Additionally, as the influence of incidence angle changes on radar observations is well known, we describe an empirical approach to account for variations in observation geometry and surface topography. Individual radar swaths and mosaics produced using this method more clearly show the variability in scattering behavior due to changes in lunar regolith properties and suppress some of the behavior arising from these topographic effects alone. Once these terrain effects are taken into account, we find that areas of permanent shadow at both poles have a higher median radar reflectivity than nonpermanently shadowed regions, but the polarization behavior of shadowed versus unshadowed areas is largely similar. The higher radar reflectivity in permanent shadow is likely the result of physical or compositional differences in these unique environments, though the precise cause remains uncertain. The results here illustrate how reducing the influence of topography and geometry effects in Mini-RF radar data may enable better characterization of lunar geologic units, regolith structure, and potential areas hosting volatile deposits at the lunar poles.

With the increasing number of binary asteroid systems being discovered, ejecta studies must expand from solely investigating single-body systems to modeling more complex multiple-body systems. For example, the Double Asteroid Redirection Test provides an opportunity to study the dynamics of a debris cloud around Didymos and Dimorphos, a near-Earth binary asteroid system. Here we simulate 72 variations on the Didymos system in order to categorize types of ejecta outcomes and analyze the influence of the varying system parameters on each outcome. We have varied five parameters: the system separation, the mass ratio between the two bodies, the impact location, the target-body shape, and the target-body rotation period. The resulting provenance maps of the final ejecta distributions were blindly sorted into five categories, while the resulting cumulative distribution functions (CDFs), describing the rate at which particles hit the surface, were blindly sorted into eight categories. We count the occurrences of the parameter values in each of the categories and apply a Cramer's V statistical test to evaluate the significance of the association between each varied effect and the overall grouping of the provenance maps and CDFs. We conclude that more dominant effects, such as a small rotation period, produce notably similar ejecta distributions that result in being assigned to the same category. Less dominant effects, such as target-body location, are sorted into several categories due to the larger influence of varying dominant effects.

From heavily cratered Umbriel to extensively tectonized Miranda, Titania is an intermediary of the Uranian system: heavily cratered, yet tectonically modified. An outstanding mystery in Titania's crater population is its apparent relative lack of large (>30 km) craters. However, progress has been limited by the coverage and quality of images available. Here, we present a new map of Titania enabled by reprocessing Voyager images to reduce the effects of motion blur. Of note, we identify a network of fractures, a set of lineaments that may represent a large multi-ring impact structure, and newly identified catenae. These findings suggest Titania's crater population is missing large craters due to viscous relaxation, tectonic resurfacing, and/or planetocentric debris, and does not necessarily require cryovolcanic resurfacing. In preparation for future missions to the Uranian system, this work presents foundations for identifying imaging targets that can contribute to furthering our understanding of the history and evolution of the Uranian system in a broader context of icy satellite evolution.

Exoplanet characterization missions planned for the future will soon enable searches for life beyond our solar system. Critical to the search will be the development of life detection strategies that can search for biosignatures while maintaining observational efficiency. In this work, we adopted a newly developed biosignature decision tree strategy for remote characterization of Earthlike exoplanets. The decision tree offers a step-by-step roadmap for detecting exoplanet biosignatures and excluding false positives, based on Earth's biosphere and its evolution over time. We followed the pathways for characterizing a modern-Earth-like planet and an Archean-Earth-like planet and evaluated the observational trades associated with coronagraph bandpass combinations of designs consistent with the Habitable Worlds Observatory precursor studies. With retrieval analyses of each bandpass (or combination), we demonstrate the utility of the decision tree and evaluate the uncertainty on a suite of biosignature chemical species and habitability indicators (i.e., the gas abundances of H₂O, O₂, O₃, CH₄, and CO₂). Notably for modern Earth, less than an order of magnitude spread in the 1σ uncertainties was achieved for the abundances of H₂O and O₂, planetary surface pressure, and atmospheric temperature, with three strategically placed bandpasses (two in the visible and one in the near-infrared). For the Archean, CH₄ and H₂O were detectable in the visible with a single bandpass.

It has been predicted that high equatorial temperatures on Mercury could promote thermal annealing by Ostwald ripening, where nanophase metal particles (a product of space weathering) coalesce and grow into larger, microphase particles, resulting in lower albedo. Here, we test this prediction by studying the correlation between albedo and temperature in 1° spatial bins using newly recalibrated 1064 nm reflectance data acquired by the Mercury Laser Altimeter (MLA), low incidence angle data from the Mercury Dual Imaging System (MDIS), and newly modeled maximum surface temperatures (MSTs). Accounting for local geology and latitude, we compare the reflectance values of surfaces with MSTs >675 K (where Ostwald ripening is predicted to be most effective) to surfaces with MSTs <473 K (where ripening is predicted not to be effective). Smooth plain surfaces >675 K are 10% and 12% darker than surfaces <473 K in MLA and MDIS data, respectively, and nonsmooth plain surfaces >675 K are 8% and 7% darker than surfaces <473 K. However, open questions remain regarding the causation of this darkening; statistical tests cannot distinguish whether the reflectance differences are systematic or the result of compositional variations that happen to correlate with MST. Along Mercury's thermal longitudes, we find that reflectance is typically lower along hot poles than along the 90°E cold pole in the low-to-midlatitudes, especially in the smooth plains, consistent with previous work identifying a decrease in optical maturity along the 90°E cold pole. Longitudinal reflectance variations correlate with temperature variations, rather than variations in micrometeoroid or solar wind fluxes.

The amount of vapor in the impact-generated protolunar disk carries implications for the dynamics, devolatilization, and moderately volatile element isotope fractionation during lunar formation. The equation of state (EoS) used in simulations of the giant impact is required to calculate the vapor mass fraction (VMF) of the modeled protolunar disk. Recently, a new version of M-ANEOS (Stewart M-ANEOS) was released with an improved treatment of heat capacity and expanded experimental Hugoniot. Here, we compare this new M-ANEOS version with a previous version (N-SPH M-ANEOS) and assess the resulting differences in smoothed particle hydrodynamics (SPH) simulations. We find that Stewart M-ANEOS results in cooler disks with smaller values of VMF and in differences in disk mass that are dependent on the initial impact angle. We also assess the implications of the minimum "cutoff" density (ρ_c), similar to a maximum smoothing length, that is set as a fast-computing alternative to an iteratively calculated smoothing length. We find that the low particle resolution of the disk typically results in >40% of disk particles falling to ρ_c, influencing the dynamical evolution and VMF of the disk. Our results show that the choice of EoS, ρ_c, and particle resolution can cause the VMF and disk mass to vary by tens of percent. Moreover, small values of ρ_c produce disks that are prone to numerical instability and artificial shocks. We recommend that future giant impact SPH studies review smoothing methods and ensure the thermodynamic stability of the disk over simulated time.

New high-precision disk-integrated measurements of the polarization of Io and Ganymede in the UBVRI bands are presented. The observations were obtained using polarimeters mounted on the Crimean Astrophysical Observatory and the Peak Terskol Observatory in 2019–2023. For Io, the negative polarization branch (NPB) reaches a minimum of P_min ≈ −0.25 ± 0.02% in the V band at a phase angle of α_min = 21 ± 05. The inversion angle is α_inv = 26° ± 6° in the V and R bands. The NPB for Ganymede is an asymmetric curve, with P_min = −0.34 ± 0.01% at α_min = 052 ± 006 and α_inv = 85 ± 02 in the V band. Although Io and Europa have similar geometric albedos (0.63 and 0.67, respectively), their NPB shapes differ. The NPB of Ganymede (albedo of 0.43) is morphologically similar to that of Europa, although it is described by different parameter values (P_min, α_min, and α_inv). This discrepancy is likely due to the compositions of their surfaces: Europa's with H₂O ice, Io's with sulfuric/silicate composition, and Ganymede's with H₂O ice and silicates. Numerical computations using the radiative transfer coherent backscattering method demonstrated a match to the polarimetric observations and to the geometric albedos for Ganymede with the single-scattering albedo ≈ 0.943 and mean free path length kl = 2πl/λ_eff ≈ 150, where λ_eff is the wavelength. For Io's regolith, the single-scattering albedo was found to be ≈ 0.979 and kl ≈ 40.

Thermal inertia estimates are available for a limited number of a few hundred objects, and the results are practically solely based on thermophysical modeling (TPM). We present a novel thermal inertia estimation method, the Asteroid Thermal Inertia Analyzer (ASTERIA). The core of the ASTERIA model is the Monte Carlo approach, based on the Yarkovsky drift detection. We validate our model on asteroid Bennu plus 10 well-characterized near-Earth asteroids (NEAs) for which a good estimation of the thermal inertia from TPM exists. The tests show that ASTERIA provides reliable results consistent with the literature values. The new method is independent of TPM, allowing an independent verification of the results. As the Yarkovsky effect is more pronounced in small asteroids, the noteworthy advantage of ASTERIA compared to TPM is the ability to work with smaller asteroids, for which TPM typically lacks input data. We used ASTERIA to estimate the thermal inertia of 38 NEAs, with 31 of them being sub-kilometer-sized asteroids. Twenty-nine objects in our sample are characterized as potentially hazardous asteroids. On the limitation side, ASTERIA is somewhat less accurate than TPM. The applicability of our model is limited to NEAs, as the Yarkovsky effect is yet to be detected in main-belt asteroids. However, we can expect a significant increase in high-quality measurements of the input parameters relevant to ASTERIA with upcoming surveys. This will surely increase the reliability of the results generated by ASTERIA and widen the model's applicability.

Using images from Cassini, we analyzed the north–south albedo asymmetry that has been observed in the atmosphere of Saturn's moon, Titan. Suitable images from the Cassini Imaging Science Subsystem taken at 889 nm spanned from 2004 to 2017—around half of a Titan year—and revealed seasonal changes in the characteristics and orientation of the north–south asymmetry boundary. Such circumglobal features provide insight into the dynamics and circulation of the atmosphere more broadly. The albedo asymmetry has been observed to reverse for part of the Titan year, inverting the brighter and darker hemispheres; we also observed this inversion, along with the formation of additional banding briefly during the transition (around 2014–2016). A tilt in the rotation axis of Titan's atmosphere with respect to the solid body rotation has previously been noted. Using robust edge-detection techniques, we likewise identified a tilt offset of a few degrees in the albedo transition boundaries. The azimuth of this tilt axis remained roughly fixed in inertial space, with some smaller possible seasonal fluctuations around the fixed direction noted.

Observational data suggest that the ice shell on Enceladus is thicker at the equator than at the pole, indicating an equator-to-pole ice flow. If the ice shell is in an equilibrium state, the mass transport of the ice flow must be balanced by the freezing and melting of the ice shell, which in turn is modulated by the ocean heat transport. Here we use a numerical ocean model to study the ice–ocean interaction and ocean circulation on Enceladus with different salinities. We find that salinity fundamentally determines the ocean stratification. A stratified layer forms in the low-salinity ocean, affecting the ocean circulation and heat transport. However, in the absence of tidal heating in the ice shell, the ocean heat transport is found to always be toward lower latitudes, resulting in freezing at the poles, which cannot maintain the ice shell geometry against the equator-to-pole ice flow. The simulation results suggest that either the ice shell on Enceladus is not in an equilibrium state or tidal dissipation in the ice shell is important in maintaining the ice shell geometry. The simulations also suggest that a positive feedback between cross-equatorial ocean heat transport and ice melting results in spontaneous symmetry breaking between the two hemispheres. This feedback may play a role in the observed interhemispheric asymmetry in the ice shell.

The shapes of asteroid phase curves are influenced by the physical properties of asteroid surfaces. The variation of an asteroid's brightness as a function of the solar phase angle can tell us about surface properties such as grain size distribution, roughness, porosity, and composition. Phase curves are traditionally derived from photometric observations at visible wavelengths, but phase curves using infrared data can also provide useful information about an asteroid surface. Using photometric observations centered near ∼3.4 μm from the W1 band of the Near-Earth Object Wide-field Infrared Survey Explorer mission, we construct thermally and rotationally corrected infrared phase curves for a sample of main-belt asteroids, which includes asteroids observed by the AKARI satellite, as well as subsets of the Themis and Flora dynamical families. We calculate the linear slope of the phase curves as a measure of their shape and compare W1 phase slopes to band depths of absorption features associated with hydrated materials, spectral slopes, visible albedos, W1 albedos, and diameters. We observe a steepening of the W1 phase slope of C-type asteroids with increasing 2.7 μm band depth but little correlation between the phase slope and 3 μm band depth or 3 μm spectral slope. The C-types in our sample exhibit steeper average W1 phase slopes than M- or S-types, similar to visible-light phase slopes. We also observe steeper W1 phase slopes for smaller-diameter objects within the Themis family and explore comparisons to Jupiter-family comets in phase slope versus albedo space.

Titan's ice shell floats on top of a global ocean, as revealed by the large tidal Love number k₂ = 0.616 ± 0.067 registered by Cassini. The Cassini observation exceeds the predicted k₂ by one order of magnitude in the absence of an ocean, and is 3σ away from the predicted k₂ if the ocean is pure water resting on top of a rigid ocean floor. Previous studies demonstrate that an ocean heavily enriched in salts (salinity S ≳ 200 g kg⁻¹) can explain the 3σ signal in k₂. Here we revisit previous interpretations of Titan's large k₂ using simple physical arguments and propose a new interpretation based on the dynamic tidal response of a stably stratified ocean in resonance with eccentricity tides raised by Saturn. Our models include inertial effects from a full consideration of the Coriolis force and the radial stratification of the ocean, typically neglected or approximated elsewhere. The stratification of the ocean emerges from a salinity profile where the salt concentration linearly increases with depth. We find multiple salinity profiles that lead to the k₂ required by Cassini. In contrast with previous interpretations that neglect stratification, resonant stratification reduces the bulk salinity required by observations by an order of magnitude, reaching a salinity for Titan's ocean that is compatible with that of Earth's oceans and close to Enceladus' plumes. Consequently, no special process is required to enrich Titan's ocean to a high salinity as previously suggested.

We apply basic principles of magma ascent from deep source regions and its eruption into a low-gravity vacuum environment to develop a theoretical treatment of the fluid dynamics and thermodynamics of mare basalt lava flow emplacement and evolution on the Moon. The vacuum conditions influenced the release of volatiles in magma passing through lava fountains, thus controlling the syn- and post-emplacement vesicularity of the resulting deposits. To explain observed lengths and volumes of Mare Imbrium–type flows, high (10⁶–10⁵ m³ s⁻¹) initial magma eruption rates were needed. Combined with low lunar magma viscosity, these caused flows to be initially turbulent. Resulting high radiative heat loss and consequent high crystallization rates caused rapid non-Newtonian rheological evolution and suppression of turbulence at tens of kilometers from vents. Slower cooling rates in the subsequent laminar parts of flows imply distinctive crystal growth rate histories. In a four-phase sequence, (i) initial transient dike-tip gas release followed by (ii) Hawaiian fire fountain activity with efficient volatile loss (iii) transitioned to (iv) Strombolian explosions in a lava lake. Late-stage lava now able to retain volatiles intruded and inflated existing flow deposits after flow front advance ceased. Volatiles forced out of solution by second boiling as lava cooled caused additional inflation. Low gravity and lack of atmospheric pressure commonly produced very vesicular lava. Escape of such lava through cracks in flow crusts is a possible source of ring-moat dome structures; collapse of such lava may explain irregular mare patches.

The NASA Double Asteroid Redirection Test spacecraft successfully impacted the Didymos–Dimorphos binary asteroid system on 2022 September 26 UTC. We provide an update to its preimpact mutual orbit and estimate the postimpact physical and orbital parameters, derived using ground-based photometric observations taken from 2022 July to 2023 February. We found that the total change of the orbital period was −33.240 ± 0.072 minutes (all uncertainties are 3σ). We obtained the eccentricity of the postimpact orbit to be 0.028 ± 0.016 and the apsidal precession rate was 7.3 ± 2.0 degrees day⁻¹ from the impact to 2022 December 2. The data taken later in 2022 December to 2023 February suggest that the eccentricity dropped close to zero or the orbit became chaotic approximately 70 days after the impact. Most of the period change took place immediately after the impact, but in the few weeks following the impact it was followed by an additional change of $-{27}_{-58}^{+19}$ s or −19 ± 18 s (the two values depend on the approach we used to describe the evolution of the orbital period after the impact—an exponentially decreasing angular acceleration or the assumption of a constant orbital period, which changed abruptly some time after the impact, respectively). We estimate the preimpact Dimorphos–Didymos size ratio was 0.223 ± 0.012 and the postimpact is 0.202 ± 0.018, which indicate a marginally significant reduction of Dimorphos' volume by (9 ± 9)% as the result of the impact.

We have monitored the Didymos–Dimorphos binary system in imaging polarimetric mode before and after the impact from the Double Asteroid Redirection Test mission. A previous spectropolarimetric study showed that the impact caused a dramatic drop in polarization. Our longer-term monitoring shows that the polarization of the post-impact system remains lower than the pre-impact system even months after the impact, suggesting that some fresh ejecta material remains in the system at the time of our observations, either in orbit or settled on the surface. The slope of the post-impact polarimetric curve is shallower than that of the pre-impact system, implying an increase in albedo of the system. This suggests that the ejected material is composed of smaller and possibly brighter particles than those present on the pre-impact surface of the asteroid. Our polarimetric maps show that the dust cloud ejected immediately after the impact polarizes light in a spatially uniform manner (and at a lower level than pre-impact). Later maps exhibit a gradient in polarization between the photocentre (which probes the asteroid surface) and the surrounding cloud and tail. The polarization occasionally shows some small-scale variations, the source of which is not yet clear. The polarimetric phase curve of Didymos–Dimorphos resembles that of the S-type asteroid class.

The quest to find extraterrestrial life is a critical scientific endeavor with civilization-level implications. Icy moons in our solar system are promising targets for exploration because their liquid oceans make them potential habitats for microscopic life. However, the lack of a precise definition of life poses a fundamental challenge to formulating detection strategies. To increase the chances of unambiguous detection, a suite of complementary instruments must sample multiple independent biosignatures (e.g., composition, motility/behavior, and visible structure). Such an instrument suite could generate 10,000× more raw data than is possible to transmit from distant ocean worlds like Enceladus or Europa. To address this bandwidth limitation, Onboard Science Instrument Autonomy (OSIA) is an emerging discipline of flight systems capable of evaluating, summarizing, and prioritizing observational instrument data to maximize science return. We describe two OSIA implementations developed as part of the Ocean World Life Surveyor (OWLS) prototype instrument suite at the Jet Propulsion Laboratory. The first identifies life-like motion in digital holographic microscopy videos, and the second identifies cellular structure and composition via innate and dye-induced fluorescence. Flight-like requirements and computational constraints were used to lower barriers to infusion, similar to those available on the Mars helicopter, "Ingenuity." We evaluated the OSIA's performance using simulated and laboratory data and conducted a live field test at the hypersaline Mono Lake planetary analog site. Our study demonstrates the potential of OSIA for enabling biosignature detection and provides insights and lessons learned for future mission concepts aimed at exploring the outer solar system.

Ultraviolet Imaging Spectrograph (UVIS) observations show the Enceladus auroral footprint on Saturn on 2017 September 14, near the end of the Cassini mission. A series of Saturn north polar auroral images were obtained by slowly slewing the Cassini spacecraft at right angles to the UVIS long slit. The images were limb-fit to improve the spacecraft geometry. Enhanced extreme-ultraviolet 88–118 nm channel emissions due to electron impact on atomic and molecular hydrogen were seen in the expected location for the Enceladus auroral footprint on five successive images spanning almost 4 hr. Enhanced emissions were also seen in simultaneously obtained far-ultraviolet 111–165 nm images in at least two of these images, with the spectral signature expected for auroral emissions. While most Cassini UVIS auroral images do not show the Enceladus auroral footprint, these 2017 images support the earlier detection of an Enceladus-linked spot on Saturn in 2008 Cassini UVIS data.

The Double Asteroid Redirection Test (DART) mission impacted Dimorphos, the moonlet of the binary asteroid 65803 Didymos, on 2022 September 26 and successfully tested a kinetic impactor as an asteroid deflection technique. The success of the deflection was partly due to the momentum of the excavated ejecta material, which provided an extra push to change Dimorphos's orbital period. Preimpact images provided constraints on the surface but not the subsurface morphology of Dimorphos. DART observations indicated that Dimorphos contained a boulder-strewn surface, with an impact site located between a cluster of large surface boulders. In order to better understand the momentum enhancement factor (β) resulting from the impact, we performed impact simulations into two types of targets: idealized homogeneous targets with a single boulder of varying size and buried depth at the impact site and an assembly of boulders at the impact site with subsurface layers. We investigated the relative effects of surface morphology to subsurface morphology to put constraints on the modeling phase space for DART following impact. We found that surface features created a 30%–96% armoring effect on β, with large surface boulders measuring on the order of the spacecraft bus creating the largest effect. Subsurface effects were more subtle (3%–23%) and resulted in an antiarmoring effect on β, even when layers/boulders were close to the surface. We also compared our 2D axisymmetric models to a 3D rectilinear model to understand the effects of grid geometry and dimension on deflection efficiency computational results.

The lunar south pole regions are subjected to global stresses that result in contractional deformation and associated seismicity. This deformation is mainly expressed by lobate thrust fault scarps; examples are globally distributed, including polar regions. One small cluster of lobate scarps falls within the de Gerlache Rim 2 Artemis III candidate landing region. The formation of the largest de Gerlache scarp, less than 60 km from the pole, may have been the source of one of the strongest shallow moonquakes recorded by the Apollo Passive Seismic Network. The scarp is within a probabilistic space of relocated epicenters for this event determined in a previous study. Modeling suggests that a shallow moonquake with an M_w of ∼5.3 may have formed the lobate thrust fault scarp. We modeled the peak ground acceleration generated by such an event and found that strong to moderate ground shaking is predicted at a distance from the source of at least ∼40 km, while moderate to light shaking may extend beyond ∼50 km. Models of the slope stability in the south polar region predict that most of the steep slopes in Shackleton crater are susceptible to regolith landslides. Light seismic shaking may be all that is necessary to trigger regolith landslides, particularly if the regolith has low cohesion (on the order of ∼0.1 kPa). The potential of strong seismic events from active thrust faults should be considered when preparing and locating permanent outposts and pose a possible hazard to future robotic and human exploration of the south polar region.

Over 4 billion years ago, Earth is thought to have been a hazy world akin to Saturn's moon Titan. The organic hazes in the atmosphere at this time could have contained a vast inventory of life's building blocks and thus may have seeded warm little ponds for life. In this work, we produce organic hazes in the lab in atmospheres with high (5%) and low (0.5%) CH₄ abundances and analyze the solid particles for nucleobases, amino acids, and a few other organics using GC/MS/MS to obtain their concentrations. We also analyze heated (200°C) samples from the high methane organic haze experiment to simulate these particles sitting on an uninhabitable surface. Finally, we use our experimental results and estimates of atmospheric haze production as inputs for a comprehensive numerical pond model to calculate the concentrations of nucleobases from organic hazes in these environments. We find that organic hazes typically provide up to 0.2–6.5 μM concentrations of nucleobases to warm little ponds for potentially habitable Hadean conditions. However, without seepage, uracil and thymine can reach ∼100 μM concentrations, which is the present lower experimental limit to react these species to form nucleotides. Heating samples leads to partial or complete decay of biomolecules, suggesting that biomolecule stockpiling on the hot surface is unlikely. The ideal conditions for the delivery of life's building blocks from organic hazes would be when the Hadean atmosphere is rich in methane, but not so rich as to create an uninhabitable surface.

Dimorphos was the target of the Double Asteroid Redirection Test (DART) mission. This paper summarizes the properties of an updated shape model of Dimorphos, describes the differences between the updated shape model and an earlier version published by Daly, Ernst, Barnouin et al. (doi:10.1038/s41586-023-05810-5), summarizes the data products associated with this model, and explains where the products can be accessed. The updated shape model benefited from improved methods of incorporating limb information, which will accelerate future shape modeling efforts for other objects with limited imaging data. The updated shape model is similar to the earlier model but slightly smaller (−2.8% change in volume) than the previous Dimorphos global shape model, and the updated shape is slightly more elongated. The additional analysis reported here supports an oblate preimpact shape for Dimorphos. This result indicates that the postimpact elongation of Dimorphos derived from ground-based observations is evidence for a large crater or global reshaping of the asteroid due to the DART impact. The updated global shape model of Dimorphos, as well as the earlier version, will be available in the Planetary Data System Small Bodies Node and through the public Small Body Mapping Tool.

Jupiter-family comets (JFCs) exhibit a wide range of activity levels and mass loss over their orbits. We analyzed high-cadence observations of 42 active JFCs with the wide-field Asteroid Terrestrial-impact Last Alert System (ATLAS) in 2020–2021. We measured the dust production rates of the JFCs using the Afρ parameter and its variation as a function of heliocentric distance. There is a tendency for our JFC sample to exhibit a maximum Afρ after perihelion, with 254P/McNaught and P/2020 WJ5 (Lemmon) having their maximum Afρ over a year after perihelion. On average, the rate of change of activity postperihelion was shallower than preperihelion. We also estimated the maximum mass-loss rate for 17 of the JFCs in our sample, finding 4P/Faye to be the most active. We present a subset of comets whose measured Afρ have been interpolated and extrapolated to a common distance of 2 au preperihelion and postperihelion. From these measurements we found no correlation of intrinsic activity with current perihelion distance. For three of the JFCs in our sample, 6P/d'Arrest, 156P/Russell–LINEAR, and 254P/McNaught, there was no visible coma but a constant absolute magnitude, which we attribute to a probable detection of the nucleus. We derived upper limits for the nuclear radii of ≤2.1 ± 0.3 km, ≤2.0 ± 0.2 km, and ≤4.0 ± 0.8 km, respectively. Finally, we found that 4P/Faye, 108P/Ciffreo, 132P/Helin–Roman–Alu 2, 141P/Machholz 2, and 398P/Boattini experienced outbursts between 2020 and 2022.

The depth-to-diameter (d/D) ratios of small lunar craters (D < 400 m) can be used to determine important properties of the upper regolith, specifically material strength or thickness. The d/D is also an important component of topographic diffusion models that describe how different erosive processes influence and change the topography of a surface over time, and these models have been applied to estimate surface ages. These models must make assumptions regarding rates of erosion and the initial d/D of a crater. Previous works investigating d/D of small craters, which use various methodologies to calculate depth, typically assume that a fresh appearing crater is a young crater. Work presented here provides d/D measurements of known—rather than assumed—young, meter-scale craters to provide better constraints on small crater depths and to help further our understanding of lunar surface ages and upper regolith properties. Given the interest in impact crater modification at small, human scales on the Moon and the wide range of assumptions built into topographic diffusion models and their predictions, understanding whether the results for initial d/D from past work hold up under different analyses is critical. We observed no distinct trends in d/D for small, young craters and report a wide range of d/D from 0.08 to 0.215, in contrast with past work that derived different averages based on crater size. The variation in d/D may correspond to heterogeneous regolith properties or be a result of a data source artifact.

We analyze the polarization observations of the Didymos–Dimorphos system before and after the impact by the NASA Double Asteroid Redirection Test spacecraft on Dimorphos. We fit empirical polarization phase curve models and statistically confirm the discovery by Gray et al. about the degree of linear polarization of the system decreasing on the impact and remaining altered for at least 30 days post-impact. With numerical simulations of particles in the geometric optics domain, we estimate the dominant size of the particles either in the regolith of Didymos and Dimorphos or in the impact-driven ejecta cloud to be several hundred micrometers. The observed change between the pre-impact and post-impact systems indicates either a decrease in average particle size of some tens of micrometers or a decreased level of space weathering.