Particle-particle and particle-gas processes significantly impact planetary precursors such as dust aggregates and planetesimals. We investigate gas permeability (kappa) in 12 granular samples, mimicking planetesimal dust regoliths. Using parabolic flights, this study assesses how gravitational compression - and lack thereof - influences gas permeation, impacting the equilibrium state of low-gravity objects. Transitioning between micro- and hyper-gravity induces granular sedimentation dynamics, revealing collective dust-grain aerodynamics. Our experiments measure kappa across Knudsen number (Kn) ranges, reflecting transitional flow. Using mass and momentum conservation, we derive kappa and calculate pressure gradients within the granular matrix. Key findings: (i) As confinement pressure increases with gravitational load and mass flow, kappa and average pore space decrease. This implies that a planetesimal's unique dust-compaction history limits subsurface volatile outflows. (ii) The derived pressure gradient enables tensile strength determination for asteroid regolith simulants with cohesion. This offers a unique approach to studying dust-layer properties when suspended in confinement pressures comparable to the equilibrium state on planetesimals surfaces, which will be valuable for modelling their collisional evolution. (iii) We observe a dynamical flow symmetry breaking when granular material moves against the pressure gradient. This occurs even at low Reynolds numbers, suggesting that Stokes numbers for drifting dust aggregates near the Stokes-Epstein transition require a drag force modification based on permeability.

Context. The solar wind impinging on the lunar surface results in the emission of energetic neutral atoms. This particle population is one of the sources of the lunar exosphere. Aims. We present a semi-empirical model to describe the energy spectra of the neutral emitted atoms. Methods. We used data from the Advanced Small Analyzer for Neutrals (ASAN) on board the Yutu-2 rover of the Chang'E-4 mission to calculate high-resolution average energy spectra of the energetic neutral hydrogen flux from the surface. We then constructed a semi-empirical model to describe these spectra. Results. Excellent agreement between the model and the observed energetic neutral hydrogen data was achieved. The model is also suitable for describing heavier neutral species emitted from the surface. Conclusions. A semi-analytical model describing the energy spectrum of energetic neutral atoms emitted from the lunar surface has been developed and validated by data obtained from the lunar surface.

Following spacecraft encounters with comets 67P/C-G and 1P/Halley, it was surprising that O2, expected to be a very minor species in their comas, was observed to outgas at a few percent abundance during their ice sublimation phases. This challenged the direct connection suggested between comets and material in the interstellar medium (ISM), which exhibits a very low O2/H2O gas-phase abundance, leading to a number of papers suggesting novel sources for O2. Since these eccentrically orbiting comets have lost significant amounts of their evaporating surfaces over their lifetimes, the O2 observed must have been stably trapped down to significant depths in these primordial icy bodies. O2 was also seen in the coma by Rosetta, along with other volatiles, long after water ice sublimation began to subside. Here we note that the extensive observations of the icy satellites of Jupiter (Europa, Ganymede, and Callisto) exhibit radiolytic and outgassing processes that provide certain direct parallels to interpretations of recent comet observations. Given that O2 is consistently observed in the atmospheres of icy Jovian satellites, as well as stably trapped as 'bubbles' (Johnson and Jesser, 1997) in their water ice surfaces, their spectral observations can help constrain the environment in which Jupiter-family and Oort cloud comets formed given that the observed O2/H2O abundances at both types of comets and icy moons are nearly identical. Based on the approximate charged particle radiation required to produce the observed steady-state concentrations of O2, we suggest that comets likely formed in a far more energetic environment than the ISM. While grains can be irradiated for longer timescales in the neutral ISM, small grains are expected to erode before significant O2 formation and trapping occurs. Independent of celestial dynamics then, an unknown radiation source, may provide insight to the first population of oxidized water ice grains in the early solar system.

Saturn's large and diffuse E ring is populated by microscopic water ice dust particles, which originate from the Enceladus plume. Cassini's Cosmic Dust Analyser sampled these ice grains, revealing three compositional particle types with different concentrations of salts and organics. Here, we present the analysis of CDA mass spectra from several orbital periods of Cassini, covering the region from interior to Enceladus' orbit to outside the orbit of Rhea, to map the distribution of the different particle types throughout the radial extent of the E ring. This will provide a better understanding of the potential impact of space weathering effects on to these particles, as the ice grains experience an increasing exposure age during their radially outward migration. In this context, we report the discovery of a new ice particle type (Type 5), which produces spectra indicative of very high salt concentrations, and which we suggest to evolve from less-salty Enceladean ice grains by space weathering. The radial compositional profile, now encompassing four particle types, reveals distinct radial variations in the E ring. At the orbital distance of Enceladus our results are in good agreement with earlier compositional analyses of E ring ice grains in the moon's vicinity. With increasing radial distance to Saturn however, our analysis suggests a growing degree of space weathering and considerable changes to the spatial distribution of the particle types. We also find that the proportion of Type 5 grains - peaking near Rhea's orbit - probably reflects particle charging processes in the E ring.

We present a combined reflectance and thermal radiance model for airless planetary bodies. The Hapke model provides the reflected component. The developed thermal model is the first to consistently use rough fractal surfaces, self-scattering, self-heating, and diskresolved bolometric albedo for entire planets. We validated the model with disk-resolved lunar measurements acquired by the Chinese weather satellite Gaofen-4 at around 3.5-4.1 mu m and measurements of the Diviner lunar radiometer at 8.25 mu m and 25-41 mu m, finding nearly exact agreement. Further, we reprocessed the thermal correction of the global lunar reflectance maps obtained by the Moon Mineralogy Mapper M3 and employed the new model to correct excess thermal radiance. The results confirm the diurnal, latitudinal, and compositional variations of lunar hydration reported in previous and recent studies with other instruments. Further, we compared the model to lunar measurements obtained by the Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) on board BepiColombo during a flyby maneuver on April 9, 2020: the measured and the modeled radiance variations across the disk match. Finally, we adapted the thermal model to Mercury for emissivity calibration of upcoming Mercury flyby measurements and in-orbit operation. Although a physical parameter must be invariant under various observation scenarios, the best lunar surface roughness fits vary between different datasets. We critically discuss possible reasons and conclude that anisotropic emissivity modeling has room for improvement and requires attention in future studies.

The study of thermal properties of frozen salt solutions representative of ice layers in Jovian moons is crucial to support the JUpiter ICy moons Explorer (JUICE) (ESA) and Europa Clipper (NASA) missions, which will be launched in the upcoming years to make detailed observations of the giant gaseous planet Jupiter and three of its largest moons (Ganymede, Europa, and Callisto), due to the scarcity of experimental measurements. Therefore, we have conducted a set of experiments to measure and study the thermal conductivity of macroscopic frozen salt solutions of particular interest in these regions, including sodium chloride (NaCl), magnesium sulphate (MgSO4), sodium sulphate (Na2SO4), and magnesium chloride (MgCl2). Measurements were performed at atmospheric pressure and temperatures from 0 to -70 degrees C in a climatic chamber. Temperature and calorimetry were measured during the course of the experiments. An interesting side effect of these measurements is that they served to spot phase changes in the frozen salt solutions, even for very low salt concentrations. A small sample of the liquid salt-water solution was set aside for the calorimetry measurements. These experiments and the measurements of thermal conductivity and calorimetry will be valuable to constrain the chemical composition, physical state, and temperature of the icy crusts of Ganymede, Europa, and Callisto.

Using the near-infrared spectral reflectance data of the Chandrayaan-1 Moon Mineralogy Mapper (M-3) instrument, we report an unusually bright structure of 30 x 60 km(2) on the lunar equatorial farside near crater Dufay. At this location, the 3-mu m absorption band feature, which is commonly ascribed to hydroxyl (OH) and /or water (H2O), at local midday is significantly (similar to 30%) stronger than on the surrounding surface and, surprisingly, stronger than in the illuminated polar highlands. We did not find a similar area of excessively strong 3-mu m absorption anywhere else on the Moon. A possible explanation for this structure is the recent infall of meteoritic or cometary material of high OH /H2O content forming a thin layer detectable by its pronounced 3-mu m band, where a small amount of the OH /H2O is adsorbed by the surface material into binding states of relatively high activation energy. Detailed analysis of this structure with next-generation spacecraft instrumentation will provide further insight into the processes that lead to the accumulation of OH /H2O in the lunar regolith surface.

Lunar OH/H2O has been confirmed and mapped by analyzing the 3 mu m absorption band in spectra acquired by the Moon Mineralogy Mapper (M-3) instrument. Space weathering leads to accumulation of submicroscopic iron particles in the uppermost layer of the regolith which gradually changes the spectral signature of airless planetary bodies and thus may affect the detection of lunar OH/H2O. The contribution of this paper is twofold. (1) Our new technique combines Hapke reflectance modeling and ab initio Mie scattering calculations to model the scattering behavior of submicroscopic iron which governs the optical effects due to space weathering. (2) Thermally corrected M-3 spectra of mature and immature sample points in mare and highland regions are used to assess the performance of the simulation framework and are analyzed to understand maturity-related changes of the OH/H2O band depth. We find that the simulation method can convincingly reproduce the spectral changes of maturing lunar soil. It becomes clear that there is only a minor effect on the 3 mu m absorption feature. This finding makes the analysis of the lunar OH/H2O mapping largely invariant with respect to space weathering. In general, the absorption features around 1 and 2 mu m are more strongly obstructed than the feature around 3 mu m. Further, we discuss agglutination as the main cause for slight deviations found around the 2 mu m band and layering/clustering as a likely reason to explain predicted iron particle sizes that are larger than observed.

While the Earth and Moon are generally similar in composition, a notable difference between the two is the apparent depletion in moderately volatile elements in lunar samples. This is often attributed to the formation process of the Moon, and it demonstrates the importance of these elements as evolutionary tracers. Here we show that paleo space weather may have driven the loss of a significant portion of moderate volatiles, such as sodium and potassium, from the surface of the Moon. The remaining sodium and potassium in the regolith is dependent on the primordial rotation state of the Sun. Notably, given the joint constraints shown in the observed degree of depletion of sodium and potassium in lunar samples and the evolution of activity of solar analogs over time, the Sun is highly likely to have been a slow rotator. Because the young Sun's activity was important in affecting the evolution of planetary surfaces, atmospheres, and habitability in the early Solar System, this is an important constraint on the solar activity environment at that time. Finally, as solar activity was strongest in the first billion years of the Solar System, when the Moon was most heavily bombarded by impactors, evolution of the Sun's activity may also be recorded in lunar crust and would be an important well-preserved and relatively accessible record of past Solar System processes.

Molecular dynamics simulations are used to analyse the effects after 20 MeV sulfur ion impact into an ice mixture consisting of water, carbon dioxide, ammonia, and methanol. By using a so-called REAX, i.e., reactive, potential, the chemical processes occurring after the impact can be studied. Such impacts may occur in Jupiter's magnetosphere, where energetic S ions originate from Io's surface and irradiate ice surfaces of Jupiter's moons, of comets or ice dust particles entering the magnetosphere. By segmenting the ion trajectory to smaller pieces that fit into our simulation box, we can follow the ion from its impact point at the surface down to the depth where it is stopped. Electronic stopping is modelled by a thermal track model; it is necessary to use a sufficiently small track radius R in order to be able to include the hot-chemistry reactions occurring in the track volume. We find that the number of dissociations and ensuing reactions scales approximately linearly with the deposited energy density. In consequence, the total number of molecules produced is approximately proportional to the impact energy. In addition, the most complex molecules are formed at the highest energy densities. Smaller molecules such as formaldehyde and hydrogen peroxide, in contrast, are produced all along the ion track.