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The Feeding Mars study was devised as a small, pilot proof of concept study to research the potential for using aquaponic effluents as an additive to regoliths which on Mars and the Moon are devoid of organic material and thus lacking microbes which assist in the delivery of water and nutrients to the plants via their roots. This research investigates aquaponics as a way to potentially produce fish and vegetal products in regoliths on Mars and the Moon as well as in extreme environments on Earth. In order to settle on Mars, settlers will have to grow their own food in systems that are self-perpetuating, with little or no inputs being brought from Earth once these systems have been established. This means that nutrients from the fish water can be used to grow plants in the hydroponic parts of the aquaponic system but also potentially in the Martian regoliths which are treated with effluents taken from aquaponic systems. Once production is established additional nutrients can be sourced from the arisings and waste, both from the fish (that are processed and eaten) and the plants, which can be used as compost to turn the regoliths into soil. In order to have fish in space, there is also the need for the systems to be self-sustaining in the production of fish feed. The key outcomes of the project were that all the species grown (potatoes, tomatoes, dwarf beans, carrots, lettuce, spring onions, chives and basil) indicate the potential to be grown in regoliths with the addition of aquaponic effluents. A significant result was that on the whole the plants that were grown with the addition of aquaponic effluents were greener than those grown in the horticultural soil, indicating that the nutrient supply was adequate. However, a key lesson learned is that germination and thus development of the plants grown in the Mars simulant and analogue was slower than those grown in the horticultural soil. Thus, developing nutrients in the soil before planting is necessary as it is with agriculture and horticulture practices on Earth, where manuring/fertilization occurs before planting. The consequence of this research, and the envisaged research to follow is not only for extra-terrestrial environments. The Earth has its own hostile environments, characterised with regoliths and other unproductive soils, and aquaponic water and aquaponic wastes can readily be envisaged as providing solutions to growing nutritious food in areas where agriculture is not currently viable. The research was undertaken in an exhibition gallery setting at the University of Greenwich in order to encourage public interest and dialogue, which it did. [ABSTRACT FROM AUTHOR]

Many upcoming lunar missions and payloads are targeting the south pole of the Moon, due to the volatiles potentially harboured in this region including ESA's PROSPECT instrument. PROSPECT is designed to sample the lunar regolith within the first meter of the surface and to analyse any volatiles found. Remote sensing methods and a range of datasets including thermal models, illumination models, LRO NAC images, LOLA DEMs and LRO NAC DEMs generated with shape-from-shading, were used to identify suitable areas for PROSPECT science within the south polar region (84–90°S). Sites identified were down selected using a science matrix and scoring sites of interest based on if and how well the point of interest met the science requirements of PROSPECT. The highest scoring sites are presented and proposed to be ideal candidate landing sites for missions targeting the lunar south polar region, especially for missions that are interested in sampling volatiles, micro cold traps and Permanently Shaded Regions (PSRs). Understanding and sampling these colder areas within the south polar region will advance the understanding of volatiles within the lunar surface and volatile transfer. • Four candidate sites that meet ESA's PROSPECT instrument scientific requirements are presented. • A candidate landing site and a reserve site are proposed in areas of geological interest in the lunar south polar region. • An assessment should be carried out to see if the proposed candidate sites meet individual mission requirements. [ABSTRACT FROM AUTHOR]

According to several orbital reflectance spectroscopy studies, numerous widely scattered lunar surface locales feature remarkably high (>98%) abundance of plagioclase. These "pure" anorthosite claims are based on an absorption band at ∼1.25 μm, associated with minor Fe2+ within plagioclase. But utilization of the 1.25 μm band as a direct gauge of plagioclase abundance requires an underlying assumption that plagioclase FeO is uniform across all measured materials. Available data for FeO in lunar plagioclase are in many cases suspect because electron-probe FeO measurements may be inflated by secondary fluorescence from nearby mafic phases. We studied plagioclase FeO in a set of 13 anorthositic lunar rocks, taking care to avoid proximity to mafic phases; or in cases where complete avoidance was not possible without sacrificing representativeness (i.e., fine-grained impact melt rocks with zoned silicates), correcting close-to-mafic analyses for secondary fluorescence (Sugawara, T. [2001], Japanese Mag. Mineral. Petrol. Sci. 30, 159–163). Results for rock-average plagioclase FeO range from 0.030 wt% in plutonic troctolitic anorthosite 76335, to 0.23 wt% in impact-melt rock 14310. In general, as shown by exsolution of mafic silicates from plagioclase within several samples, final plagioclase FeO in lunar plutonic anorthosites is determined by reequilibration during slow postigneous cooling and/or metamorphism. In contrast, four fast-cooled anorthositic impact melt rocks have consistently higher plagioclase FeO, averaging 1.9 times higher in comparison to average plutonic anorthositic rock. Thus, impact melted lunar anorthosite will have far more prominent 1.25 μm absorption than plutonic anorthosite of the same plagioclase abundance. In view of the inconstancy of plagioclase FeO, orbital spectral reflectance using the 1.25 μm absorption band, or the ratio between that band and one or more mafic silicate bands, cannot be precise enough to justify claims of ability to resolve "purest" (98 vol% +) anorthosite from compositions that are high-plagioclase but not that extreme. • Measuring abundance of lunar plagioclase using its 1.25 μm iron band is problematic. • Plagioclase Fe content in lunar highland rocks is highly diverse. • New electron probe data show plutonic plagioclase Fe ranging from 0.030 to 0.137 wt%. • Impact melted (fast-cooled) highland rocks have even higher plagioclase Fe. • Depth of the plagioclase 1.25 μm band is a function of both abundance and Fe content. [ABSTRACT FROM AUTHOR]

The passive retroreflector arrays placed on the moon by Apollo 11, 14 and 15 astronauts continue to produce valuable Earth–Moon range measurements that enable high-precision tests of gravitational physics, as well as studies of geo- and selenophysics. The optical throughput of these retroreflectors has declined since their deployment, with an additional signal loss at full moon when the reflectors experience direct solar illumination. We show that the loss in return rate can be attributed to the accumulation of a thin layer of lunar dust on the surfaces of the corner cube retroreflectors. First, a careful analysis of the optical link budget for the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) experiment reveals that the lunar return rate is 15–20 times smaller than predicted, a deficit that can be explained by a reflector dust covering fraction of ∼ 50 %. Second, range measurements taken during a lunar eclipse indicate that the solar illumination of the retroreflectors degrades their throughput by an additional factor of ∼ 15. Finally, a numerical simulation of heat transfer in dust-coated reflectors is able to model the resulting thermal lensing effect, in which thermal gradients in the retroreflectors degrade their far-field diffraction pattern. A comparison of this simulation to eclipse measurements finds a dust coverage fraction of ∼ 50 %. Taken together, the link analysis, eclipse observations and thermal modeling support the claim that the retroreflectors are obscured by lunar dust, with both link budget and simulation independently finding the dust fraction to be ∼ 50%. • APOLLO regularly observes a decreased return rate during full moon. • This improves by a factor of 15 during a lunar eclipse, confirmed by observations. • APOLLO's link budget shows the observed rate is 15–20 times smaller than predicted. • We find that a dust obscuration factor of 50% best explains the observed deficit. [ABSTRACT FROM AUTHOR]

Our understanding of the lunar farside from orbital remote sensing is limited by the lack of ground-truth constraints from returned samples. China's Chang'e-6 (CE-6) mission plans to land at the southern rim of the Apollo basin in the South Pole-Aitken basin on the lunar farside and return ∼2 kg samples. In this study, we conduct analyses on the local geological context of the CE-6 candidate landing regions, with a focus on the non-mare ejecta. The CE-6 candidate landing region has been divided into three subunits. According to our analyses, the northeastern subunit is adjacent to the lowest elevation (and one of the highest excavation depth) of this region where the materials from the deepest crust may exist. The northwestern subunit is affected by the magnesian ejecta from young crater. Returning these non-mare ejecta will provide key insights into the magnesian lithologies that occurred in the lunar farside, whose petrogenesis is long debated. As for the southern light plains unit, a topographic positive feature (unofficially termed as Mons White) located at the west of the light plains is identified in this work. Three scenarios were proposed for its origin: (1) A volcanic complex built on special magmatism in the SPA basin; (2) Remnants of the central uplift of an ancient impact basin; (3) Target surface folded and uplifted by the Apollo basin impact event. Gradual lithological variations in the Mons White are discovered. The White crater has likely ejected materials with these gradual lithologies into the CE-6 candidate landing regions. The Orientale basin ejecta also exists on the light plains. These non-mare materials, if could be sampled by the CE-6 mission, will significantly help refine the lunar chronostratigraphic system from the pre-Nectarian/Aitkenian to the late Imbrian. Thus, we suggest that landing on the southern light plains will achieve the maximum scientific outcomes for the CE-6 mission, from a perspective of sample diversity. • We conduct analyses on the local geological context of the CE-6 candidate landing regions. • The northeastern subregion is adjacent to the Chawla crater where deep-seated materials may exist. • The northwestern subunit is affected by the magnesian ejecta from young crater. • A topographic positive feature located at the west of the candidate landing region is identified. • Three scenarios were proposed for the origin of the topographic positive feature. [ABSTRACT FROM AUTHOR]

Groundwater exploration in Basement Complex zones is somewhat difficult due to the erratic nature of groundwater occurrence in these zones. This has led to high failure rates in wells constructed in these settings. This problem has, however, been further complicated by the high cost of geophysical surveys to locate suitable drilling points. Therefore, there is the need to explore cheaper and more systematic techniques for optimal results. This study focused on the delineation of groundwater potential zones (GPZs) in Miri and environs, northeastern Nigeria. Remote sensing and geographic information system (GIS) methods were employed for the identification of structural and geomorphological controls of groundwater. Five thematic maps comprising lithology, lineament density, drainage density, slope, and elevation were generated while thirty-three vertical electric soundings (VESs) were employed for characterization of the saprolitic units in the study area. Data evaluation entailed integration of thematic maps using overlay analysis to generate a groundwater potential map of the study area and estimation of geoelectric parameters to verify the resulting GIS-based map. The geospatial studies revealed three GPZ which include good zones (3%), moderate zones (65%), and poor zones (32%). The VES revealed 3–4 geoelectric layers, with dominantly clayey sand (thickness of 1–65 m and resistivity of 5.4–396 ohm- m) aquifer units of moderate potential. The validation of GPZ map with the VES data showed that the low potential zones are attributable to poorly developed regoliths in the northern parts of the study area. The study showed that groundwater potential in the Miri area is medium to low and the clayey sand saprolitic units and saprocks constitute the aquifer units. Groundwater recharge in the area is influenced by geomorphological features. [ABSTRACT FROM AUTHOR]

We present a newly developed method which combines the nonlinear spectral mixing model of Shkuratov et al. (1999) with a machine learning algorithm to map the lunar regolith composition using spectral data. The new method performs orders of magnitude faster than the traditionally used numerical optimization approaches, allowing the mapping of regolith properties (including mineralogical composition, average grain size and optical maturity) over large areas of the lunar surface. A new set of basic mineral spectra of the lunar soil for using with spectral mixing models is proposed. Used together with the nonlinear mixing model (Shkuratov et al., 1999), the set is able to describes Chandrayaan-1 M3 instrument spectra collected from test areas which includes the Shapley crater with its surroundings containing mare and highland terrains well. The new set includes a virtual "gray component" with a "flat" (constant) spectrum, accounting for the factors that change general surface albedo, such as spectrally neutral components (e.g., agglutinate glasses), errors in the photometric reduction, uncertainties in estimations of lunar regolith porosity q and the mean grain size S of the basic minerals. The proposed new method takes into account the influence of space weathering and nonlinear correlation between the compositional and spectral parameters of the lunar soils delivering values for the optical properties and mineralogical abundance determination of the lunar regolith which are compatible with the results found from lunar samples measurements in the laboratory. The proposed approach can be used for analyzing spectral observations not only of the lunar surface but also for other surfaces with are covered by regolith. • A new method of mineralogy mapping combining the Shkuratov spectral mixing model with machine learning is proposed. • The method is computationally efficient and removes ambiguities in the spectral unmixing results. • The method allows a mapping of the mineralogical composition, grain size and maturity over large lunar surface areas. • The method was tested on a lunar area near the crater Shapley using the Chandrayaan-1 M3 hyperspectral imager data. • The method provides regolith parameters values which are in agreement with the values found through lunar sample analysis. [ABSTRACT FROM AUTHOR]

Shear strength and cohesion of granular materials are important geotechnical properties that play a crucial role in the stability and behavior of lunar and Martian regolith, as well as their terrestrial analog materials. To characterize and predict shear strength and cohesion for future space missions, it is also important to understand the effects of particle size distribution and density on these fundamental geotechnical properties. Generalized equations have been established using empirical data from direct shear measurements of lunar and Martian regolith simulants to quantify the effects of particle size distribution and density on cohesion and shear strength. Preliminary results are also presented highlighting the effects of atmospheric absorbed water on shear strength and cohesion when conducting experiments in atmospheric conditions on Earth. The results of this study show that cohesion increases exponentially with bulk density, while the exponential growth constant is also dependent on particle size distribution. The presence of absorbed atmospheric water can also produce non-monotonic effects on the shear strength of regolith, and it's influence on shear strength varies based on sample density for Earth-based experiments and applications. Together, these results highlight the importance of particle size distribution, bulk density, and atmospheric water content on the shear strength and cohesion of lunar and Martian simulants. This study also establishes generalized equations for predictive, testing, and modeling efforts for future space missions. • Cohesion of lunar and Martian regolith simulants varies exponentially with density. • Exponential growth of cohesion with density depends on particle size distribution. • Absorbed atmospheric water has a non-monotonic increase on shear strength. • Particle shapes and sizes of regolith simulants closely match lunar samples. [ABSTRACT FROM AUTHOR]

Due to the lack of in-situ geotechnical data from lunar Permanently Shadowed Regions (PSRs), it is important that a versatile icy regolith simulant be used in terrestrial development of rovers, excavators, and water extractors intended for operating in lunar PSRs. Fine tuning of existing icy regolith simulant properties is not possible; for a given water percentage they either exhibit strength similar to high strength concrete or to sand, but nothing in between. In this work we present a novel method for creating icy lunar regolith simulant, called Pressure Sintered icy lunar regolith Simulant. PSS is notable for being created from solid phase water, and for being tailorable to a wide range of mechanical properties through the sintering of ice and regolith grains, induced by applied uniaxial pressure. Samples were produced at 0%, 2%, 5%, and 10% ice content as measured by weight, and were pressed at four different pressure levels. Penetration resistance was measured for each of these samples, and it was observed that a continuous distribution of penetration resistance levels could be achieved by varying the applied pressure and ice content. Significant relaxation of samples during the pressing process was also observed. The production method for PSS is included, and is followed by the penetration resistance and density results along with some qualitative observations. Finally, we make recommendations for the use of PSS in terrestrial testing activities. • The mechanical properties of Pressure Sintered icy lunar regolith simulant can be tuned without changing ice content. • Icy regolith simulant mechanical properties are primarily driven by the interaction of ice and regolith grains, rather than the interaction of regolith grains with each other. • Bulk density and penetration resistance cannot predict water content in Pressure Sintered icy lunar regolith Simulant. [ABSTRACT FROM AUTHOR]

Active jets of water vapour and ice grains discovered by the Cassini spacecraft at the South Pole of Enceladus result in the deposit of freshly erupted materials on the icy surface of the moon, potentially coming directly from its subsurface water ocean. As suggested by sporadic detection of plume activity, similar processes may be at work on Europa as well. Such fresh ice deposit areas are primary targets for future landing and sampling missions for the search of potential bio-signatures. Determining the mechanical properties of such fresh icy powder-like materials is essential to identify safe landing sites and appropriate sampling techniques. Ice surface energy or cohesion is a key parameter driving the porosity of the material, its strength, stickiness and flowability, which determine the stability of fresh surface deposits. In this work we performed an experimental study on laboratory analogues of fresh icy regolith, consisting in investigating the flowability of ice powders through a rotating drum device at several temperatures. Our experimental results show that the flowability of fresh icy deposits significantly decreases with increasing temperatures over the range of 90 to 150 K. Using a simple scaling analysis, we derive the surface energy of ice grains from the observed angle of the flowing surface inside the drum and we show that the surface energy increase by a factor of 5 to 10 between 90 K to 150 K, a behaviour not observed in any other powder material. This result indicates that the flowability of fresh icy deposits at the surface of Enceladus and Europa should significantly change depending on the local surface temperature, which can vary spatially and temporally on the same range of temperature explored here. Due to lower gravity, ice powder will behave as a more cohesive powder on Enceladus than on Europa. Higher cohesion on Enceladus may result in the formation of a very loose regolith with high porosity, making stable landing more challenging. On Europa, the reduction of ice cohesiveness with decreasing temperature should help the flow of freshly eruptive granular ice materials, potentially explaining numerous flow features observed throughout the surface. • Ice powders are characterised by a custom rotating drum at very low temperatures. • Ice powder cohesion increases with the temperature over the range of 90 to 150 K. • A mechanical model allows to estimate the grains surface energy from flow behaviour. • Icy moons' regolith could see drastic cohesion change with diurnal cycles/location. [ABSTRACT FROM AUTHOR]

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission to carbonaceous asteroid (101955) Bennu performed detailed mapping with a suite of instruments to characterize the composition and geology of its surface. Here we use data from the OSIRIS-REx Thermal Emission Spectrometer (OTES) instrument to investigate the relationship of OTES-derived spectral indices to other derived data products from OTES, the OSIRIS-REx Camera Suite (OCAMS), and the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) at global and local scales. We quantitatively confirm that high values of the OTES silicate stretching slope (from ∼10 to ∼12 μm) in midday spectra that are indicative of thin and/or patchy dust cover are strongly associated with low thermal inertia (high porosity), low albedo boulders on Bennu. These high porosity boulders have brecciated textures with embedded clasts that likely originated on Bennu's parent body or during its disruption. The high porosity of these boulders is a key factor in the local production of the dust or its entrapment, as some large, brecciated boulders with a lower porosity have little evidence of dust. A second OTES spectral parameter, the silicate bending band depth near 22.7 μm applied to early evening spectra, is not correlated to thermal inertia, but is weakly to strongly correlated to albedo, OVIRS-derived 1.05 μm and 2.74 μm band depths, OVIRS-derived hydrogen abundance, and modeled nanophase troilite abundance. In several regions on Bennu there is a strong spatial relationship between these parameters, whereby areas with shallower silicate bending bands also have shallower 1.05 μm and 2.74 μm bands and lower albedo with higher nanophase troilite abundances. These correlations, combined with analysis of the silicate bending band in laboratory experiments of space weathered and mildly heated carbonaceous chondrites, suggests that decreased silicate bending band depths signify decomposition of phyllosilicates, likely Fe-bearing, due to space weathering or mild heating (<600 °C) via solar radiation during Bennu's time in near-Earth space. There is a strong association of larger silicate bending band depths in areas dominated by small rocks and unresolved material and in areas with small (≤ 25 m) craters identified as the spectrally reddest on Bennu, suggesting that this material has been more recently exposed due to impact and/or mass wasting processes. The shallowest silicate bending depths are associated with larger rocks and boulders that appear to have the longest surface exposure history, although there is band depth variation among them suggesting either initial composition variation that resulted in different responses to space weathering or heating, or varied exposure history of individual boulders themselves. We predict that any grains returned from Bennu with a significant surface exposure history will be characterized by shallower 22.7 μm, 1.05 μm and 2.7 μm band depths and increased sulfide (troilite) abundance, as well as textural and chemical evidence for phyllosilicate dehydration. • Spectral evidence suggests phyllosilicate decomposition and dehydration has occurred on Bennu. • This phyllosilicate decomposition appears to be related to the surface exposure age of the material. • Phyllosilicate decomposition on Bennu is most likely caused by space weathering or mild solar radiative heating (<600 °C). [ABSTRACT FROM AUTHOR]

In 2018, the Japanese spacecraft Hayabusa2, arrived at the small asteroid Ryugu. The surface of this C-type asteroid is covered with numerous boulders whose size and shape distributions are investigated in this study. Using a few hundred Optical Navigation Camera (ONC) images with a pixel scale of approximately 0.65 m, we focus on boulders greater than 5 m in diameter. Smaller boulders are also considered using five arbitrarily chosen ONC close-up images with pixel scales ranging from 0.7 to 6 cm. Across the entire surface area (~2.7 km2) of Ryugu, nearly 4400 boulders larger than 5 m were identified. Boulders appear to be uniformly distributed across the entire surface, with some slight differences in latitude and longitude. At ~50 km−2, the number density of boulders larger than 20 m is twice as large as on asteroid Itokawa (or Bennu). The apparent shapes of Ryugu's boulders resemble laboratory impact fragments, with larger boulders being more elongated. The ratio of the total volume of boulders larger than 5 m to the total excavated volume of craters larger than 20 m on Ryugu can be estimated to be ~94%, which is comparatively high. These observations strongly support the hypothesis that most boulders found on Ryugu resulted from the catastrophic disruption of Ryugu's larger parent body, as described in previous papers (Watanabe et al., 2019; Sugita et al., 2019). The cumulative size distribution of boulders larger than 5 m has a power-index of −2.65 ± 0.05, which is comparatively shallow compared with other asteroids visited by spacecraft. For boulders smaller than 4 m, the power-index is even shallower and ranges from −1.65 ± 0.05 to −2.01 ± 0.06. This particularly shallow power-index implies that some boulders are buried in Ryugu's regolith. Based on our observations, we suggest that boulders near the equator might have been buried by the migration of finer material and, as a result, the number density of boulders larger than 5 m in the equatorial region is lower than at higher latitudes. • The cumulative boulder size distribution on the entire surface of Ryugu has a power-index of −2.65 ± 0.05. • The apparent shapes of Ryugu's boulders resemble laboratory impact fragments. • Most boulders are likely to be produced by the disruption of the Ryugu's larger parent body. [ABSTRACT FROM AUTHOR]

Regolith on Mars exchanges water with the atmosphere on a diurnal basis and this process causes significant variation in the abundance of water vapor at the surface. While previous studies of regolith-atmosphere exchange focus on the abundance, recent in-situ experiments and remote sensing observations measure the isotopic composition of the atmospheric water. We are therefore motivated to investigate isotopic water exchange between the atmosphere and the regolith and determine its effect on the deuterium to hydrogen ratio (D/H) of the atmosphere. We model transport of water in the regolith and regolith-atmosphere exchange by solving a transport equation including regolith adsorption, condensation, and diffusion. The model calculates equilibrium fractionation between HDO and H 2 O in each of these processes. The fractionation in adsorption is caused by the difference in the latent heat of adsorption, and that of condensation is caused by the difference in the vapor pressure. Together with a simple, bulk-aerodynamic boundary layer model, we simulate the diurnal variation of the D/H near the planetary surface. We find that the D/H can vary by 300–1400‰ diurnally in the equatorial and mid-latitude locations, and the magnitude is greater at a colder location or season. The variability is mainly driven by adsorption and desorption of regolith particles, and its diurnal trend features a drop in the early morning, a rise to the peak value during the daytime, and a second drop in the late afternoon and evening, tracing the water vapor flow into and out from the regolith. The predicted D/H variation can be tested with in-situ measurements. As such, our calculations suggest stable isotope analysis to be a powerful tool in pinpointing regolith-atmosphere exchange of water on Mars. • We predict diurnal variation of the D/H at the surface of Mars of ∼1000‰. • The variation is mainly driven by adsorption and desorption of regolith particles. • Stable isotopes can be used to pinpoint regolith-atmosphere exchange on Mars. [ABSTRACT FROM AUTHOR]

Abstract We present a new method for the determination of the surface properties of airless bodies from measurements of the emitted infrared flux. Our approach uses machine learning techniques to train, validate, and test a neural network representation of the thermophysical behavior of the atmosphereless body given shape model, illumination and observational geometry of the remote sensors. The networks are trained on a dataset of thermal simulations of the emitted infrared flux for different values of surface rock abundance, roughness, and values of the thermal inertia of the regolith and of the rock components. These surrogate models are then employed to retrieve the surface thermal properties by Markov Chain Monte Carlo Bayesian inversion of observed infrared fluxes. We apply the method to the inversion of simulated infrared fluxes of asteroid (101195) Bennu – according to a geometry of observations similar to those planned for NASA's OSIRIS-REx mission – and infrared observations of asteroid (25143) Itokawa. In both cases, the surface properties of the asteroid – such as surface roughness, thermal inertia of the regolith and rock component, and relative rock abundance – are retrieved; the contribution from the regolith and rock components are well separated. For the case of Itokawa, we retrieve a rock abundance of about 85% for pebbles larger than the diurnal skin depth, which is about 2 cm. The thermal inertia of the rock is found to be lower than the expected value for LL chondrites, indicating that the rocks on Itokawa could be fractured. The average thermal inertia of the surface is around 750 J s−1/2 K−1 m−2 and the measurement of thermal inertia of the regolith corresponds to an average regolith particle diameter of about 10 mm, consistently with in situ measurements as well as results from previous studies. Highlights • Thermophysical simulations can be streamlined in neural-network response functions. • Neural nets can quickly predict infrared fluxes from surface properties of asteroids. • Bayesian inference using neural nets allow retrieving rock abundance of the surface. • Inversion of fluxes of (25143) Itokawa gives results consistent with previous works. [ABSTRACT FROM AUTHOR]

Abstract Understanding the heat transfer mechanism within dust aggregates is of great importance for many subjects in planetary science. We calculated the coordination number and the thermal conductivity through the solid network of compressed dust aggregates. We found a simple relationship between the coordination number and the filling factor and revealed that the thermal conductivity through the solid network of aggregates is represented by a power-law function of the filling factor and the coordination number. Highlights • The coordination number Z is Z = 2 + 9.38 φ 1.62, where φ is the filling factor. • The solid thermal conductivity is given by a power-law function of φ and Z (φ). • The radiative thermal conductivity is inversely proportional to the opacity. • Our novel formula well explains the experimental data of the thermal conductivity. [ABSTRACT FROM AUTHOR]

Abstract Space weathering is an important process on airless bodies, and it must be considered when interpreting data from planetary missions. Previous work has shown that solar energetic particles may cause dielectric breakdown in regolith within permanently shadowed regions near the poles of the Moon. Here, we predict that dielectric breakdown weathering could have melted and/or vaporized 2 − 9 % of gardened (i.e., thoroughly mixed) regolith at the equator and 5 − 11 % near the poles. If so, then ∼ 3 − 10 % of all gardened regolith on the Moon may have experienced dielectric breakdown, and this process must be considered when analyzing remote sensing data or soil samples returned by the Luna and Apollo missions. [ABSTRACT FROM AUTHOR]

We use observations of Bennu's impact craters, and measurements of particle sizes seen in images from the OSIRIS-REx (the Origins, Spectral Interpretation, Resource Identification Regolith Explorer sample-return mission) sampling event, to conclude there is a ∼ global sub-surface layer of finer-grained material on Bennu. The layer is evident in particle measurements from areas inside and around craters, roughness assessments of crater interiors vs. surrounding terrain, crater colors, crater depth-to-diameter (d/D) ratios, and the global crater size-frequency distribution. All data sets show a transition in behavior at crater diameters between 20 m and 40 m, with the most abrupt changes at ∼25 m diameter. Based on the depth of ∼25 m diameter craters, we infer that the layer is 1–4 m thick. The reddest craters are the youngest craters, and there is a trend to older ages as redness decreases. The decrease in redness is characterized by an initial, rapid fade, followed by a slower fade. The reduction in expected crater abundance with diameters ∼40–80 m, when extrapolating from larger sizes, is consistent with impact armoring caused by deeper, large sub-surface boulders depressing crater diameter. The existence of the layer appears to be a natural byproduct of impacts across a wide energy range and can be described as a "punctuated equilibrium": small, frequent impacts contribute to the local production of finer-grained particles and locally mix smaller particles into the subsurface; larger, less-frequent large impacts eject the fine-grained material, exposing interior boulders, and resetting the finer-grained particle population in that area. Other processes, e.g. thermal fracturing, could also contribute to the production of fine-grained material on rapid timescales. Extrapolating to the broader asteroid population and considering the production and preservation of small particles is related to target strength, surface acceleration, and impact speeds, we expect to find such layers on other asteroids, and that the layer thickness and depth will vary with composition, size, and orbital parameters. • Impact crater features on asteroid (101955) Bennu indicate subsurface layering. • Particles visible during the sampling event at Bennu support this conclusion. • A near-surface layer consists of finer-grained particles, and is 1–4 m thick. • We predict a layer of finer-grained particles should exist on other asteroids. • Layering has implications for the evolution of asteroid surfaces and interiors. [ABSTRACT FROM AUTHOR]

The topographic slopes of the Moon host different types of mass wasting features, in which boulder falls are abundant globally. Boulder falls are characterized by trails that are produced by rolling, bouncing and sliding of boulders on the lunar surface, in response to ground shaking originating from shallow moonquakes, impact events and other surface processes. Our previous work showed evidence for moonquake-triggered boulder falls at hundreds of sites on the interior of Orientale multi-ring basin. In this study, we document new evidence for ejecta boulder falls around 135 fresh simple impact craters with diameters ranging from 0.1 to 2.5 km in the same basin. Our detailed characterization of ejecta boulder falls around eight selected impact craters reveal their modes of occurrences, morphological properties, formation ages and clues to their impact origins. The ejecta boulder falls have the characteristic boulder trails arranged radially around the host impact craters, shorter trail lengths, narrow trail widths and shallow trail depths, when compared to the moonquake-triggered boulder falls in the same basin. Some impact craters exhibit radial asymmetry of boulder trails around these craters because of oblique impacts, in which longer trails are abundant in down-range directions, while fewer and shorter trails characterize up-range directions. In some cases, local slopes also contribute to increased trail lengths and deviation from radial distribution of trails. Formation ages of a few host impact craters, determined based on conventional crater counting method, are in the range of 0.94–6.4 Ma, and the age of ejecta boulder falls is consistent with the host impact crater ejecta. These characteristics suggest that the trails were produced by the ejecta boulders, during the ejecta blanket formation stage of the host impact craters. The relationship between the host impact crater diameter and the average size of large ejecta boulders present in their ejecta blankets suggests that the host impact craters are primary impact craters. Hence, the initial ejecta boulder fall velocities are expected to be in the ejection velocity range of ∼5–100 m/s. During ballistic sedimentation, these ejecta boulders erode the local target materials including regolith and mix them along their trails. It is also observed that ejecta boulder falls occur around small fresh simple impact craters (<2–3 km) at the Schrödinger and Lorentz basins. Around large size impact craters and basins, ejecta boulder falls produced large-size radial grooves, lineaments and sculptures on the Moon. We conclude that the characteristics of ejecta boulder falls and moonquake boulder falls are distinctly different from each other, reflecting their distinct formation conditions, but these form contemporaneously on the Moon. • Hundreds of small fresh impact craters developed trail forming ejecta boulder falls at the Orientale, Schrödinger and Lorentz basins • The ejecta spall boulders produced trails radially around fresh lunar impact craters during the ejecta emplacement • Oblique impacts produced radial asymmetry in the ejecta boulder trail distribution and variation of trail lengths with respect to the impact trajectory • The ejecta boulder trails are characterized by shorter lengths, narrow widths, and shallow trail depths than the moonquake triggered boulder trails • The ejecta boulder falls are produced by ballistic sedimentation processes during ejecta emplacement of host impact craters [ABSTRACT FROM AUTHOR]

The Surface Electrical Properties (SEP) experiment deployed at the Apollo 17 landing site in the Taurus-Littrow valley was a continuous-wave, radiofrequency (1–32 MHz) interferometer designed to probe the subsurface from depths of meters to kilometers. Signals were transmitted by orthogonal electric dipoles laid out near the lunar module and three components of the magnetic field were measured on the lunar rover at useful distances up to 1.6 km. The gross range decay of the signals is well fit by a simple equation combining spherical spreading, first-order interference, and attenuation. The derived loss tangents ∼0.01 are consistent with laboratory-measured radiofrequency absorption in Apollo 17 basalts. Thus the attenuation in the SEP data can be explained by absorption with negligible small-scale scattering; the latter can be quantified as a dearth of 10-m scale lateral heterogeneity and a mean-free path of kilometers. Numerical modeling was used to compare predicted vs. observed waveforms and to invert for vertically varying dielectric structure. The dielectric constant is readily converted to density and porosity for lunar rocks. The preferred model shows a sharp decrease in porosity in the top 20 − 30 m, with little change below 300 m. This likely tracks the transition from fully gardened regolith, to impact-fractured rock, to largely intact 3.7-Ga basalt. The bottom of the basalt was not detected by the SEP to a depth of ∼2 km or more, which is consistent with reanalysis of Apollo 17 seismic and gravity data. [ABSTRACT FROM AUTHOR]

Impact craters across Mars, that are otherwise similar, commonly exhibit vastly different thermal inertia values on their rims. We focused on the Tisia Valles region because the terrain is mostly flat, and craters of various ages are present within the same target geologic unit. We present an investigation of two hypotheses for why these thermal inertia variations exist. The first hypothesis is that crater rim thermal inertia is affected by crater degradation state, and the second hypothesis is that thermal inertia is affected by the amount of regolith mantling the rims, regardless of degradation state. To investigate the first hypothesis, we used a series of multilevel regression analyses to test for correlations between crater rim thermal inertia and depth-diameter ratio, rim irregularity, and radii variation, which are indicators of crater degradation state. To investigate the second hypothesis, we tested for a correlation between thermal inertia and mantled crater rim percentage, which is indicative of the amount of regolith atop the rims. Our results did not support the first hypothesis but did support the second hypothesis. Therefore, we concluded that regolith rim mantling affects crater rim thermal inertia variations in the Tisia Valles region. Crater degradation may not have an effect on rim thermal inertia because regolith is transported down the crater rim slopes faster than local regolith is produced from erosion in this region. An implication of this work is that the spatial extent of regolith-rich regions near Tisia Valles can be mapped using the spatial extent of proximal craters with low thermal inertia rims. Additionally, the timing of mantling events can be quantified by comparing counts of craters with high thermal inertia rims to those of low thermal inertia rims within a region. [ABSTRACT FROM AUTHOR]

In this work we update the regolith mixing model presented by Gault et al. (1974) , including new input values and reworking key parameters. Much as Gault et al. did, we present a way to calculate the rate at which lunar regolith is overturned at depth. The model describes a mixing front that proceeds downward from the surface following a power-law function of time. Our most important update is the inclusion of secondary impacts. Our calculations show that secondaries are necessary to produce the reworking rate inferred from the depth distribution of surface-correlated material in Apollo cores (Fruchter et al., 1977; Morris, 1978; Blanford, 1980), from the rate at which splotches rework the top 3 cm of regolith (Speyerer et al., 2016), and from the rate at which Diviner cold spots (Bandfield et al., 2014) and crater rays (Pieters et al., 1985; Hawke et al., 2004; Werner and Medvedev, 2010) are reworked into background regolith. Overturn calculations that only consider the impact of primaries fail to describe observed reworking rates at all depths and timescales. We conclude that secondary impacts dominate mixing in the top meter of lunar regolith. [ABSTRACT FROM AUTHOR]

Highlights • Solar energetic particles may cause dielectric breakdown near the poles of Phobos. • We predict breakdown has affected 5–10% of impact gardened regolith near the poles. • Dielectric breakdown may help explain why Phobos appears so space weathered. Abstract The origins of Phobos and Deimos are uncertain; both are so space weathered that their surface compositions are difficult to determine using spectral reflectance measurements. We show how the winter temperatures and associated conditions in the polar regions of Phobos could make the regolith susceptible to space weathering from dielectric breakdown caused by solar energetic particles (SEPs). During SEP events, charged particles accumulate throughout the top ∼1 mm of the regolith, which has low conductivity, and create subsurface electric fields that act to dissipate any net buildup of charge. The faster the net charge accumulates, the larger the electric field needed to dissipate it. If the magnitude of the subsurface electric field exceeds ∼ 10 6 V m−1, then dielectric breakdown is likely. This process rapidly dissipates the buildup of charge by vaporizing electrically conducting channels through the regolith. Dielectric breakdown is expected to be more prevalent in colder regions, where the electrical conductivity of the regolith is lower and the dissipation of charge is consequently slower. If the regolith on Phobos is made of silicates, or possibly phyllosilicates, we predict that dielectric breakdown weathering has melted or vaporized 5–10% of the impact gardened regolith in the polar regions, although this percentage depends on how long the regolith has been exposed to SEPs. This, in addition to the long exposure time of the regolith to other forms of space weathering, may help explain why both Phobos and Deimos are highly space weathered compared to other airless bodies in the Solar System, such as Earth's Moon. [ABSTRACT FROM AUTHOR]

Many air-less planetary bodies, including the Moon, asteroids, and comets, are covered by regolith. The thermal conductivity of the regolith is an essential parameter controlling the surface temperature variation. A thermal conductivity model applicable to natural soils as well as planetary surface regolith is required to analyze infrared remote sensing data. In this study, we investigated the temperature and compressional stress dependence of the thermal conductivity of the lunar regolith simulant JSC-1A, and the temperature dependence of sieved JSC-1A samples under vacuum conditions. We confirmed that a series of the experimental data for JSC-1A are fitted well by our analytical model of the thermal conductivity (Sakatani et al., 2017). Comparison with the calibration data of the sieved samples with those for original JSC-1A indicates that the thermal conductivity of natural samples with a wide grain size distribution can be modeled as mono-sized grains with a volumetric median size. The calibrated model can be used to estimate the volumetric median grain size from infrared remote sensing data. Our experiments and the calibrated model indicates that uncompressed JSC-1A has similar thermal conductivity to lunar top-surface materials, but the lunar subsurface thermal conductivity cannot be explained only by the effects of the density and self-weighted compressional stress. We infer that the nature of the lunar subsurface regolith grains is much different from JSC-1A and lunar top-surface regolith, and/or the lunar subsurface regolith is over-consolidated and the compressional stress higher than the hydrostatic pressure is stored in the lunar regolith layer. [ABSTRACT FROM AUTHOR]

The variations in near-surface properties and regolith structure of asteroids are currently not well constrained by remote sensing techniques. Radar is a useful tool for such determinations of Near-Earth Asteroids (NEAs) as the power of the reflected signal from the surface is dependent on the bulk density, ρ bd , and dielectric permittivity. In this study, high precision complex permittivity measurements of powdered aluminum oxide and dunite samples are used to characterize the change in the real part of the permittivity with the bulk density of the sample. In this work, we use silica aerogel for the first time to increase the void space in the samples (and decrease the bulk density) without significantly altering the electrical properties. We fit various mixing equations to the experimental results. The Looyenga–Landau--Lifshitz mixing formula has the best fit and the Lichtenecker mixing formula, which is typically used to approximate planetary regolith, does not model the results well. We find that the Looyenga--Landau–Lifshitz formula adequately matches Lunar regolith permittivity measurements, and we incorporate it into an existing model for obtaining asteroid regolith bulk density from radar returns which is then used to estimate the bulk density in the near surface of NEA’s (101955) Bennu and (25143) Itokawa. Constraints on the material properties appropriate for either asteroid give average estimates of ρ b d = 1.27 ± 0.33 g/cm 3 for Bennu and ρ b d = 1.68 ± 0.53 g/cm 3 for Itokawa. We conclude that our data suggest that the Looyenga-Landau-Lifshitz mixing model, in tandem with an appropriate radar scattering model, is the best method for estimating bulk densities of regoliths from radar observations of airless bodies. [ABSTRACT FROM AUTHOR]

Carbonaceous chondrites (CCs) are important materials for understanding the early evolution of the solar system and delivery of volatiles and organic material to the early Earth. Presumed CC-like asteroids are also the targets of two current sample return missions: OSIRIS-REx to asteroid Bennu and Hayabusa-2 to asteroid Ryugu, and the Dawn orbital mission at asteroid Ceres. To improve our ability to identify and characterize CM2 CC-type parent bodies, we have examined how factors such as particle size, particle packing, and viewing geometry affect reflectance spectra of the Murchison CM2 CC. The derived relationships have implications for disc-resolved examinations of dark asteroids and sampleability. It has been found that reflectance spectra of slabs are more blue-sloped (reflectance decreasing toward longer wavelengths as measured by the 1.8/0.6 µm reflectance ratio), and generally darker, than powdered sample spectra. Decreasing the maximum grain size of a powdered sample results in progressively brighter and more red-sloped spectra. Decreasing the average grain size of a powdered sample results in a decrease in diagnostic absorption band depths, and redder and brighter spectra. Decreasing porosity of powders and variations in surface texture result in spectral changes that may be different as a function of viewing geometry. Increasing thickness of loose dust on a denser powdered substrate leads to a decrease in absorption band depths. Changes in viewing geometry lead to different changes in spectral metrics depending on whether the spectra are acquired in backscatter or forward-scatter geometries. In backscattered geometry, increasing phase angle leads to an initial increase and then decrease in spectral slope, and a general decrease in visible region reflectance and absorption band depths, and frequent decreases in absorption band minima positions. In forward scattering geometry, increasing phase angle leads to small non-systematic changes in spectral slope, and general decreases in visible region reflectance, and absorption band depths. The highest albedos and larger band depths are generally seen in the lowest phase angle backscattering geometry spectra. The reddest spectra are generally seen in the lowest phase angle backscatter geometry spectra. For the same phase angle, spectra acquired in forward scatter geometry are generally redder and darker and have shallower absorption bands than those acquired in backscatter geometry. Overall, backscatter geometry-acquired spectra are flatter, brighter, and have deeper 0.7 µm region absorption band depths than forward scatter geometry-acquired spectra. It was also found that the 0.7, 0.9, and 1.1 µm absorption bands in Murchison spectra, which are attributable to various Fe electronic processes, are ubiquitous and can be used to recognize CM2 chondrites regardless of the physical properties of the meteorite and viewing geometry. [ABSTRACT FROM AUTHOR]

In 2010, fine regolith particles on asteroid Itokawa were recovered by the Hayabusa mission. The three-dimensional microstructure of 48 Itokawa particles smaller than 120 µm was examined in previous studies. The shape distribution of Itokawa particles is distributed around the mean values of the axial ratio 2:√2:1, which is similar to laboratory impact fragments larger than several mm created in catastrophic disruptions. Thus, the Itokawa particles are considered to be impact fragments on the asteroid's surface. However, there have never been any laboratory impact experiments investigating the shapes of fine fragments smaller than 120 µm, and little is known about the relation between the shapes of fine fragments and the petrographic textures within those fragments. In this study, in order to investigate the relation between the petrographic textures and the shapes of fine fragments by impacts, the shapes of 2163 fine fragments smaller than 120 µm are examined by synchrotron radiation-based microtomography at SPring-8. Most samples are fine fragments from basalt targets, obtained in previous laboratory impact experiments by Michikami et al. (2016). Moreover, two impacts into L5 chondrite targets were carried out and the shapes of their fine fragments are examined for comparison. The results show that the shape distributions of fine fragments in basalt targets are similar regardless of impact energy per target mass (in contract to the shape distribution of relatively large fragments, which are affected by impact energy), and are similar to those in L5 chondrite targets and Itokawa regolith particles. The physical process producing these fine fragments would be due to multiple rarefaction waves in the target. Besides, the petrographic textures do not significantly affect the shapes of fine fragments in our experiments. On the other hand, according to Molaro et al. (2015), the shapes of the fragments produced by thermal fatigue by the day-night temperature cycles on the asteroid surface are influenced by the petrographic textures. Therefore, we conclude that the Itokawa particles are not the products of thermal fatigue but impact fragments on the asteroid surface. [ABSTRACT FROM AUTHOR]

This work revisits the hypothesis of the so-called ‘lunar terminal cataclysm’ suggested by Tera et al. (1973, 1974) as a strong peak in the meteorite bombardment of the Moon around 3.9 Ga ago. According to the hypothesis, most of the impact craters observed on the lunar highlands formed during this short time period and thus formed the majority of the lunar highland impact breccias and melts. The hypothesis arose from the observation that the ages of highland samples from all the lunar missions are mostly grouped around 3.9–4.0 Ga. Since those missions, however, radiometric dating techniques have progressed and many samples, both old and new, have been re-analyzed. Nevertheless, the debate over whether there was a terminal cataclysm persists. To progress in this problem we summarized results of 269 K–Ar datings (mostly made using the 40 Ar– 39 Ar technique) of highland rocks represented by the Apollo 14, 15, 16, 17 and Luna 20 samples and 94 datings of clasts of the highland rocks from 23 lunar meteorites representing 21 localities on the lunar surface, and considered them jointly with the results of our modelling of the cumulative effect of the impact gardening process on the presence of impact melt of different ages at the near-surface of the Moon. The considered results of K–Ar dating of the Apollo-Luna samples of lunar highland rocks confirmed a presence of strong peak centered at 3.87 Ga. But since the time when the hypothesis of terminal cataclysm was suggested, it has become clear that this peak could be a result of sampling bias: it is the only prominent feature at the sites with an apparent domination of Imbrium basin ejecta (Apollo 14 and 15) and the age pattern is more complicated for the sites influenced not only by Imbrium ejecta but also that of other basins (Nectaris at the Apollo 16 site and Serenitatis at the Apollo 17 site). Our modelling shows that the cataclysm, if it occurred, should produce a strong peak in the measured age values but we see in the considered histograms and relative probability plots not only the 3.87 Ga peak (due to Imbrium basin), but also several secondary peaks caused by the formation of other basins distributed between 3.87 and 4.25 Ga. The lunar terminal cataclysm hypothesis is in disagreement with the distribution of K–Ar ages for the highland rocks of the lunar meteorites. The population of lunar meteorites representing localities randomly distributed over the lunar surface, and thus free from the mentioned sampling bias, shows no ∼3.9 Ga peak as it should, if the cataclysm did occur. We conclude that the statistics of sample ages contradict the terminal cataclysm scenario in the bombardment of the Moon. We also see evidence for the formation of several impact basins between 3.87 and 4.25 Ga which is likewise incompatible with the hypothesis of a short interval cataclysm. There remain other basins, including the largest South Pole – Aitken, the ages of which should be determined in future studies to further clarify the impact history. Sample-return missions targeted to date several key basins need to be planned, and the continued study of lunar meteorites may also bring new details to the general view of the impact bombardment of the Moon. [ABSTRACT FROM AUTHOR]

The Lunar Regolith Penetrating Radar (LRPR) onboard the Chang'E 5 lander is the first antenna-array radar deployed for the exploration of an extraterrestrial object. It is able to measure the subsurface structure of the moon at a resolution of ∼ 10 cm. This work presents a method for calibrating the variable cable delay of the LRPR and an imaging method based on synthesis imaging from Radio Astronomy. The cable delay calibration method effectively calibrates the cable delay shift of the LRPR with a nanosecond-level precision. To validate the accuracy of the cable delay method, we use the data obtained from both the ground experiment and the moon-exploration mission of the LRPR. The imaging method is used to visualize the subsurface structure of the Chang'E-5 (CE-5) landing site. Moreover, Finite-Difference Time-Domain (FDTD) simulation is carried out to help interpret the imaging results. The radar images derived from the LRPR observations show consistency with each other and match the bit force record during the drilling process. The results suggest that the rocks interacted with the CE-5 drill feature a size of 15 to 20 cm. • We have developed a cable calibration method based on the LRPR designation. • We have proposed an imaging method to visualize the LRPR observations. • The performance of the LRPR and the imaging method is tested with FDTD simulation. • The buried rocks which interacted with the CE-5 drill feature a size of 15 to 20 cm. [ABSTRACT FROM AUTHOR]

Water is critical in the future of space exploration and development of planetary bodies. Benchtop nuclear magnetic resonance (NMR) is a geophysical measurement technique with the potential to be used for identification and characterisation of water resources on planetary bodies, such as the Moon and Mars. In this work, we explore the potential of NMR for space exploration by conducting measurements on Lunar and Martian regolith simulants using two main NMR pulse sequences. The first sequence is a decay due to internal fields (DDIF) pulse sequence which probes the pore size of the porous structure created by the regolith simulants. We then use a simple pore-to-particle size model to estimate the particle size distribution of the simulants and validate this against laser particle size analysis (LPSA) data. The DDIF type sequence can also be used to resolve the material surface relaxivity: which is useful in understanding the concentration of paramagnetic species as well as for correlating the length scales of various water volumes. The second pulse sequence utilised is a multi-echo sequence used to quantify fluid volumes as well as total moisture content within the porous media. The fluid volumes observed include adsorbed or hydrated water bound to clay minerals, as well as interparticle water between the regolith simulant grains. The NMR measured moisture content showed reasonably good agreement to corresponding gravimetric measurements used for validation. Finally, we discuss the implications of the current measurements and provide suggested focal points for potential future developments of NMR systems for space exploration. [Display omitted] • Characterisation of water distribution within regolith simulants. • Estimated particle size distributions of regolith simulants. • Identifying clay-bound and interparticle water volumes. • Reasonable agreement of NMR measurements to gravimetric water content. [ABSTRACT FROM AUTHOR]

We assessed the morphologies of more than 930 simple impact craters (diameters 40 m–10 km) on the Moon using digital terrain models (DTMs) of a variety of terrains in order to characterize the variability of fresh crater morphology as a function of crater diameter. From Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) DTMs, we determined depth-to-diameter (d/D) ratios for an extremely fresh set of these craters with diameters less than 400 m and found that their d/D ratios range from 0.11 to 0.17. Using both NAC and Kaguya Terrain Camera DTMs, we also determined the d/D ratios for the set of fresh simple craters larger than 400 m in diameter. The d/D ratios of these larger craters are typically near 0.21, as expected of gravity-dominated crater excavation. Fresh craters less than ∼400 m in diameter, on the other hand, exhibit significantly lower d/D ratios. Various possible factors affect the morphologies and relative depths (d/D ratios) of small strength-dominated craters, including impactor and target properties (e.g., effective strength, strength contrasts, porosity, pre-existing weaknesses), impact angle and velocity, and degradation state. While impact conditions resulting from secondary impacts can also affect crater morphologies, we found that d/D ratio alone was not a unique discriminator of small secondary craters. To investigate the relative influences of degradation and target properties on the d/D ratios of small strength-dominated craters, we examined a subset of fresh craters located on the geologically young rim deposits of Tycho crater. These craters are deeper and steeper than other craters of similar diameter and degradation state, consistent with their relative freshness and formation in the relatively coherent, melt-rich deposits in this region. The d/D ratios of globally distributed small craters of similar degradation state and size range, on the other hand, are relatively shallow with lower average wall slopes, consistent with crater excavation in a weak or poorly cohesive layer. The widespread predominance of these small, shallow craters is consistent with the pervasive, poorly cohesive upper regolith. [ABSTRACT FROM AUTHOR]

The heat flux incident upon the surface of an airless planetary body is dominated by solar radiation during the day, and by thermal emission from topography at night. Motivated by the close relationship between this heat flux, the surface temperatures, and the stability of volatiles, we consider the effect of the slope distribution on the temperature distribution and hence prevalence of cold-traps, where volatiles may accumulate over geologic time. We develop a thermophysical model accounting for insolation, reflected and emitted radiation, and subsurface conduction, and use it to examine several idealized representations of rough topography. We show how subsurface conduction alters the temperature distribution of bowl-shaped craters compared to predictions given by past analytic models. We model the dependence of cold-traps on crater geometry and quantify the effect that while deeper depressions cast more persistent shadows, they are often too warm to trap water ice due to the smaller sky fraction and increased reflected and reemitted radiation from the walls. In order to calculate the temperature distribution outside craters, we consider rough random surfaces with a Gaussian slope distribution. Using their derived temperatures and additional volatile stability models, we estimate the potential area fraction of stable water ice on Earth’s Moon. For example, surfaces with slope RMS ∼15° (corresponding to length-scales ∼10 m on the lunar surface) located near the poles are found to have a ∼10% exposed cold-trap area fraction. In the subsurface, the diffusion barrier created by the overlaying regolith increases this area fraction to ∼40%. Additionally, some buried water ice is shown to remain stable even beneath temporarily illuminated slopes, making it more readily accessible to future lunar excavation missions. Finally, due to the exponential dependence of stability of ice on temperature, we are able to constrain the maximum thickness of the unstable layer to a few decimeters. [ABSTRACT FROM AUTHOR]

The SEIS (Seismic Experiment for Interior Structure) instrument onboard the InSight mission will be the first seismometer directly deployed on the surface of Mars. From studies on the Earth and the Moon, it is well known that site amplification in low-velocity sediments on top of more competent rocks has a strong influence on seismic signals, but can also be used to constrain the subsurface structure. Here we simulate ambient vibration wavefields in a model of the shallow sub-surface at the InSight landing site in Elysium Planitia and demonstrate how the high-frequency Rayleigh wave ellipticity can be extracted from these data and inverted for shallow structure. We find that, depending on model parameters, higher mode ellipticity information can be extracted from single-station data, which significantly reduces uncertainties in inversion. Though the data are most sensitive to properties of the upper-most layer and show a strong trade-off between layer depth and velocity, it is possible to estimate the velocity and thickness of the sub-regolith layer by using reasonable constraints on regolith properties. Model parameters are best constrained if either higher mode data can be used or additional constraints on regolith properties from seismic analysis of the hammer strokes of InSight's heat flow probe HP are available. In addition, the Rayleigh wave ellipticity can distinguish between models with a constant regolith velocity and models with a velocity increase in the regolith, information which is difficult to obtain otherwise. [ABSTRACT FROM AUTHOR]

This work investigates the macroscopic thermomechanical behavior of lunar boulders by modeling their response to diurnal thermal forcing. Our results reveal a bimodal, spatiotemporally-complex stress response. During sunrise, stresses occur in the boulders’ interiors that are associated with large-scale temperature gradients developed due to overnight cooling. During sunset, stresses occur at the boulders’ exteriors due to the cooling and contraction of the surface. Both kinds of stresses are on the order of 10 MPa in 1 m boulders and decrease for smaller diameters, suggesting that larger boulders break down more quickly. Boulders ≤ 30 cm exhibit a weak response to thermal forcing, suggesting a threshold below which crack propagation may not occur. Boulders of any size buried by regolith are shielded from thermal breakdown. As boulders increase in size (>1 m), stresses increase to several 10 s of MPa as the behavior of their surfaces approaches that of an infinite halfspace. As the thermal wave loses contact with the boulder interior, stresses become limited to the near-surface. This suggests that the survival time of a boulder is not only controlled by the amplitude of induced stress, but also by its diameter as compared to the diurnal skin depth. While stresses on the order of 10 MPa are enough to drive crack propagation in terrestrial environments, crack propagation rates in vacuum are not well constrained. We explore the relationship between boulder size, stress, and the direction of crack propagation, and discuss the implications for the relative breakdown rates and estimated lifetimes of boulders on airless body surfaces. [ABSTRACT FROM AUTHOR]

Crater count equilibrium occurs when new craters form at the same rate that old craters are erased, such that the total number of observable impacts remains constant. Despite substantial efforts to understand this process, there remain many unsolved problems. Here, we propose an analytical model that describes how a heavily cratered surface reaches a state of crater count equilibrium. The proposed model formulates three physical processes contributing to crater count equilibrium: cookie-cutting (simple, geometric overlap), ejecta-blanketing, and sandblasting (diffusive erosion). These three processes are modeled using a degradation parameter that describes the efficiency for a new crater to erase old craters. The flexibility of our newly developed model allows us to represent the processes that underlie crater count equilibrium problems. The results show that when the slope of the production function is steeper than that of the equilibrium state, the power law of the equilibrium slope is independent of that of the production function slope. We apply our model to the cratering conditions in the Sinus Medii region and at the Apollo 15 landing site on the Moon and demonstrate that a consistent degradation parameterization can successfully be determined based on the empirical results of these regions. Further developments of this model will enable us to better understand the surface evolution of airless bodies due to impact bombardment. [ABSTRACT FROM AUTHOR]

Remote observations of the surfaces of airless planetary objects are fundamental to inferring the physical structure and compositional makeup of the surface material. A number of forward models have been developed to reproduce the photometric behavior of these surfaces, based on specific, assumed structural properties such as macroscopic roughness and associated shadowing. Most work of this type is applied to geometric albedos, which are affected by complicated effects near zero phase angle that represent only a tiny fraction of the net energy reflected by the object. Other applications include parameter fits to resolved portions of some planetary surface as viewed over a range of geometries. The spherical albedo of the entire object (when it can be determined) captures the net energy balance of the particle more robustly than the geometric albedo. In most treatments involving spherical albedos, spherical albedos and particle phase functions are often treated as if they are independent, neglecting the effects of roughness. In this paper we take a different approach. We note that whatever function captures the phase angle dependence of the brightness of a realistic rough, shadowed, flat surface element relative to that of a smooth granular surface of the same material, it is manifested directly in both the integral phase function and the spherical albedo of the object. We suggest that, where broad phase angle coverage is possible, spherical albedos may be easily corrected for the effects of shadowing using observed (or assumed) phase functions, and then modeled more robustly using smooth-surface regolith radiative transfer models without further imposed (forward-modeled) shadowing corrections. Our approach attributes observed “powerlaw” phase functions of various slope (and “linear” ranges of magnitude- vs. -phase angle) to shadowing, as have others, and goes in to suggest that regolith-model-based inferences of composition based on shadow-uncorrected spherical albedos overestimate the amount of absorbing material contained in the regolith. [ABSTRACT FROM AUTHOR]

We present new results of polarimetric observations of 15 main belt asteroids of different composition. By merging new and published data we determined polarimetric parameters characterizing individual asteroids and mean values of the same parameters characterizing different taxonomic classes. The majority of asteroids show polarimetric phase curves close to the average curve of the corresponding class. We show that using polarimetric data it is possible to refine asteroid taxonomy and derive a polarimetric classification for 283 main belt asteroids. Polarimetric observations of asteroid (21) Lutetia are found to exhibit possible variations of the position angle of the polarization plane over the surface. [ABSTRACT FROM AUTHOR]

The second channel (CH2) of the Lunar Penetrating Radar (LPR) carried on the Chang'e-3 (CE-3) Yutu Rover was used to determine the thickness and structure of the lunar regolith. Accurately revealing the true structure beneath the surface requires knowledge of the dielectric permittivity of the regolith, which allows one to properly apply migration to the radar image. In contrast to simple assumptions in previous studies, this paper takes account of heterogeneity of the regolith and derives regolith's permittivity distribution laterally and vertically by a method widely used in data processing of terrestrial Ground Penetrating Radar (GPR). We find that regolith permittivity at the landing site increases with depth more quickly than previously recognized. At a depth of ∼2.5–3 m, the dielectric constant reaches the value of solid basalt. The radar image was migrated on the basis of the permittivity profile. We do not find any continuous distinct layers or an apparent regolith/rock interface in the migrated radargram, which implies that this area is covered by relatively young, poorly layered deposits. [ABSTRACT FROM AUTHOR]

We present a method of conversion of the lunar neutron counting rate measured by the Lunar Reconnaissance Orbiter (LRO) Lunar Exploration Neutron Detector (LEND) instrument collimated neutron detectors, to water equivalent hydrogen (WEH) in the top ∼1 m layer of lunar regolith. Polar maps of the Moon's inferred hydrogen abundance are presented and discussed. [ABSTRACT FROM AUTHOR]

The Miniature Radio Frequency (Mini-RF) instrument aboard NASA's Lunar Reconnaissance Orbiter (LRO) is a hybrid dual-polarized synthetic aperture radar (SAR) that operated in concert with the Arecibo Observatory to collect bistatic radar data of the lunar nearside from 2012 to 2015. The purpose of this bistatic campaign was to characterize the radar scattering properties of the surface and near-surface, as a function of bistatic angle, for a variety of lunar terrains and search for a coherent backscatter opposition effect indicative of the presence of water ice. A variety of lunar terrain types were sampled over a range of incidence and bistatic angles; including mare, highland, pyroclastic, crater ejecta, and crater floor materials. Responses consistent with an opposition effect were observed for the ejecta of several Copernican-aged craters and the floor of the south-polar crater Cabeus. The responses of ejecta material varied by crater in a manner that suggests a relationship with crater age. The response for Cabeus was observed within the portion of its floor that is not in permanent shadow. The character of the response differs from that of crater ejecta and appears unique with respect to all other lunar terrains observed. Analysis of data for this region suggests that the unique nature of the response may indicate the presence of near-surface deposits of water ice. [ABSTRACT FROM AUTHOR]

Currently, few thermal infrared measurements exist of fine particulate (<63 µm) analogue samples (e.g. minerals, mineral mixtures, rocks, meteorites, and lunar soils) measured under simulated lunar conditions. Such measurements are fundamental for interpreting thermal infrared (TIR) observations by the Diviner Lunar Radiometer Experiment (Diviner) onboard NASA's Lunar Reconnaissance Orbiter as well as future TIR observations of the Moon and other airless bodies. In this work, we present thermal infrared emissivity measurements of a suite of well-characterized Apollo lunar soils and a fine particulate (<25 µm) San Carlos olivine sample as we systematically vary parameters that control the near-surface environment in our vacuum chamber (atmospheric pressure, incident solar-like radiation, and sample cup temperature). The atmospheric pressure is varied between ambient (1000 mbar) and vacuum (<10 −3 mbar) pressures, the incident solar-like radiation is varied between 52 and 146 mW/cm 2 , and the sample cup temperature is varied between 325 and 405 K. Spectral changes are characterized as each parameter is varied, which highlight the sensitivity of thermal infrared emissivity spectra to the atmospheric pressure and the incident solar-like radiation. Finally spectral measurements of Apollo 15 and 16 bulk lunar soils are compared with Diviner thermal infrared observations of the Apollo 15 and 16 sampling sites. This comparison allows us to constrain the temperature and pressure conditions that best simulate the near-surface environment of the Moon for future laboratory measurements and to better interpret lunar surface compositions as observed by Diviner. [ABSTRACT FROM AUTHOR]

Large solar energetic particle events may cause dielectric breakdown in the upper 1 mm of regolith in permanently shadowed regions (PSRs). We estimate how the resulting breakdown weathering compares to meteoroid impact weathering. Although the SEP event rates measured by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) are too low for breakdown to have significantly affected the regolith over the duration of the LRO mission, regolith gardened by meteoroid impacts has been exposed to SEPs for ∼10 6 yr. Therefore, we estimate that breakdown weathering’s production rate of vapor and melt in the coldest PSRs is up to 1.8 − 3.5 × 10 − 7 kg m − 2 yr − 1 , which is comparable to that produced by meteoroid impacts. Thus, in PSRs, up to 10–25% of the regolith may have been melted or vaporized by dielectric breakdown. Breakdown weathering could also be consistent with observations of the increased porosity (“fairy castles”) of PSR regolith. We also show that it is conceivable that breakdown-weathered material is present in Apollo soil samples. Consequently, breakdown weathering could be an important process within PSRs, and it warrants further investigation. [ABSTRACT FROM AUTHOR]

The Powdered Sample Dosing and Distribution System (PSDDS) of the ExoMars rover will be required to handle and contain samples of Mars regolith for long periods of time. Cementation of the regolith, caused by water and salts in the soil, results in clumpy material and a duricrust layer forming on the surface. It is therefore possible that material residing in the sampling system may cement, and could potentially hinder its operation. There has yet to be an investigation into the formation of duricrusts under simulated Martian conditions, or how this may affect the performance of sample handling mechanisms. Therefore experiments have been performed to create a duricrust and to explore the cementation of Mars analogues, before performing a series of tests on a qualification model of the PSDDS under simulated Martian conditions. It was possible to create a consolidated crust of cemented material several millimetres deep, with the material below remaining powder-like. It was seen that due to the very low permeability of the Montmorillonite component material, diffusion of water through the material was quickly blocked, resulting in a sample with an inhomogeneous water content. Additionally, samples with a water mass content of 10% or higher would cement into a single solid piece. Finally, tests with the PSDDS revealed that samples with a water mass content of just 5% created small clumps with significant internal cohesion, blocking the sample funnels and preventing transportation of the material. These experiments have highlighted that the cementation of regolith in Martian conditions must be taken into consideration in the design of sample handling instruments. [ABSTRACT FROM AUTHOR]

On Mars, in the present and past, highly concentrated liquid brine is suggested to have formed repeatedly through ice melting and/or deliquescence. Such repeated seepages of brine could generate characteristic geomorphic features on Mars; however, the dynamics of repeated brine flows have been little investigated. Here we report results of laboratory experiments that investigate the flow behavior of brine (MgCl 2 solution) upon repeated cycles of seepage and drying in glass beads on slopes. We compare the flow behavior with those of ultrapure water and ethylene glycol. We found that at a low flow rate, comparable to ice melting on Mars, both ultrapure water and ethylene glycol infiltrate radially into glass beads as infiltration flows. In contrast, even at a low flow rate, surface flows appear due to repeated seepages of MgCl 2 solution. The surface flows move downward on slopes, forming elongated flow features. This happens because a decrease in the porosity and permeability of the glass beads caused by precipitated salts prevents infiltration in subsequent brine seepages. We suggest that precipitated salts have a role in the formation of transient surface flows on Mars, even at low seepage rates. • We experimentally investigated the roles of precipitated salts in brine seepage. • Precipitated salts in soils prevent infiltration of brine in subsequent seepages. • This leads to saturation of brine in soils and generates surface flow. • Surface flow at low fluid flow rates can generate flow features on slopes on Mars. [ABSTRACT FROM AUTHOR]

This paper considers a small region in Chryse Planitia where several subparallel chains of cones are observed. Despite many papers on small Martian cones, the mechanism of their formation is not unequivocally explained, nor the reason for their arrangement in subparallel chains. In this paper, we test a few hypotheses stating that most of the cones under consideration are the result of outgassing Martian regolith when atmospheric pressure has become too low for liquid water to be stable. This mechanism explains the existence of a series of cones outside of the main volcanic zones in the vast lowlands of Mars. Magma heating might be another factor, but our calculations show that its role was often rather limited. Some of the cones may be rootless cones. Subparallel chains of cones might be formed above several types of linear subparallel structures: e.g., outcrops of aquifers, linear outcrops, fissures resulting during landslides, linear heat sources, etc. We also found that thermal advection (i.e. gas or liquid flow) could transport heat from deep aquifers to shallow aquifers. It could enabled rapid loss of volatiles. Our research on the cycloidal structure indicates that it is an outcrop of a layer that probably extends under several hills. Moreover, our preliminary research points that similar processes may have taken place in other regions of Chryse Planitia. This conclusion is based on the observation of similar structures, e.g., cones, chains of cones, arcs of outcrops, etc. in the region. • Some cones are the result of degassing due to the drop in atmospheric pressure. • Full degassing required additional heat. It can have been supplied from deep aquifers. • Some subparallel chains of cones may have formed along the sediment outcrops. • We identified a cycloidal line that can be an outcrop of the sediment layer. [ABSTRACT FROM AUTHOR]

Water ice in permanently shadowed regions on the Moon is exposed to galactic cosmic rays (GCRs) and solar energetic particles (SEPs). Because this radiation alters the chemistry of the ice, constraining the total radiation dose is important for understanding both the origin and evolution of the ice. The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) onboard the Lunar Reconnaissance Orbiter (LRO) has measured the energetic charged particle dose rate for more than a solar cycle, providing the longest continuous dataset of radiation in the lunar environment. CRaTER's unique design enables us to measure the dose rates behind three amounts of mass shielding and thus constrain the GCR and SEP dose rates as a function of depth in the regolith. In a further improvement on prior studies, we combine these dose rates with a model for how impact gardening affects the exposure time of the regolith. We can thus calculate the total dose received by water ice in gardened regolith and find that impact-gardened ice has received a dose of ∼ 0. 1 – 1 eV molecule−1 over the past 1 Gyr. This dose is one to two orders of magnitude lower than the doses calculated in studies that do not incorporate the effects of gardening. Relatively undisturbed ice may have received a higher dose, but no more than ∼ 10 eV molecule−1 in the top centimeter. This result provides a valuable constraint for researchers studying radiation processing of lunar water ice. • Water ice on the Moon is exposed to energetic charged particle radiation. • We use LRO/CRaTER to measure the dose rate over a full solar cycle. • We model how impact gardening affects the exposure time of the regolith. • After ∼ 1 Gyr, impact-gardened water ice has received a dose of ∼ 0.1–1 eV/molecule. [ABSTRACT FROM AUTHOR]