Regolith Materials Research Articles

Enceladus' extreme heat flux as revealed by its relaxed craters

Enceladus' cratered terrains contain large numbers of unusually shallow craters consistent with deformation by viscous relaxation of water ice under conditions of elevated heat flow. Here we use high‐resolution topography to measure the relaxation fraction of craters on Enceladus far from the active South Pole. We find that many craters are shallower than expected, with craters as small as 2 km in diameter having relaxation fractions in excess of 90%. These measurements are compared with numerical simulations of crater relaxation to constrain the minimum heat flux required to reproduce these observations. We find that Enceladus' nominal cold surface temperature (70 K) and low surface gravity strongly inhibit viscous relaxation. Under such conditions less than 3% relaxation occurs over 2 Ga even for relatively large craters (diameter 24 km) and high, constant heat fluxes (150 mW m−2). Greater viscous relaxation occurs if the effective temperature at the top of the lithosphere is greater than the surface temperature due to insulating regolith and/or plume material. Even for an effective temperature of 120 K, however, heat fluxes in excess of 150 mW m−2are required to produce the degree of relaxation observed. Simulations of viscous relaxation of Enceladus' largest craters suggest that relaxation is best explained by a relatively short‐lived period of intense heating that decayed quickly. We show that infilling of craters by plume material cannot explain the extremely shallow craters at equatorial and higher northern latitudes. Thus, like Enceladus' tectonic terrains, the cratered regions of Enceladus have experienced periods of extreme heat flux.

Experimental Determination of Hydrodynamic Properties of Weathered Granite

Weathering processes transform hard fresh rock into regolith materials, composed of weathered bedrock and soils. The thickness of the weathered bedrock layer varies greatly, attaining several meters under certain climatic and lithologic conditions. The hydrology of catchments on bedrock is strongly controlled by the hydrologic functioning of these weathered bedrock layers, and the understanding and modeling of this functioning requires a knowledge of their hydraulic properties, such as their water retention curves. There are few references available on the water retention curve of weathered bedrock. To fill this gap, we measured the water retention characteristic of highly weathered Plomelin leucogranodiorite in replicate core samples. The study of weathered granite samples showed that their mean water retention curves differed significantly from those estimated for soil samples of the same grain size distribution using a pedotransfer function and for other granites with the same weathering index. Even if one can generate hypotheses to explain the observed differences, the three approaches we used to compare our results cannot replace the experimental approach to generate the water retention curve of a material such as weathered granite. In addition to enriching the body of work on water retention curves of weathered bedrocks, the results of this work suggest that there is a need for developing a database of retention properties of weathered bedrocks in parallel with the development of a model based on factors more appropriate than the weathering index.

Optimizing statistical classification accuracy of satellite remotely sensed imagery for supporting fast flood hydrological analysis

The aim of this study is to improve classification results of multispectral satellite imagery for supporting flood risk assessment analysis in a catchment area in Cyprus. For this purpose, precipitation and ground spectroradiometric data have been collected and analyzed with innovative statistical analysis methods. Samples of regolith and construction material were in situ collected and examined in the spectroscopy laboratory for their spectral response under consecutive different conditions of humidity. Moreover, reflectance values were extracted from the same targets using Landsat TM/ETM+ images, for drought and humid time periods, using archived meteorological data. The comparison of the results showed that spectral responses for all the specimens were less correlated in cases of substantial humidity, both in laboratory and satellite images. These results were validated with the application of different classification algorithms (ISODATA, maximum likelihood, object based, maximum entropy) to satellite images acquired during time period when precipitation phenomena had been recorded.

Evaluation of Anthropogenic Impacts on Soiland Regolith Materials Based on BCR Sequential Extraction Analysis

This study was conducted in Assaluyeh region, southwest of Iran to show utilization and efficiency of sequential extraction analysis in environmental impact studies using soil and related regolith materials as sample media. In order to investigate distribution of heavy metals, optimized BCR (Community Bureau of Reference) sequential extraction analysis method was used. In this caseeight representative samples which state all types of different regolith materials in the area were investigated. Total concentration and chemical partitioning of a number of elements (Pb, Cu, Zn, Cr, Mn and Fe) were studied to cover all spectral from typical anthropogenic elements, lead and copper, elements of assumed mixed origin, chromium and zinc, to mostly lithogenic elements, iron and manganese. Results stated that the concentrations of each chemical phase extracted from samples are below the total concentration of each element.The results could be used to establish best media indicators for environmental studies with dependence on type of elements determined in regolith material. Using sequential extraction analysis, the association of particular elements with geochemical phases in soils and related materials was investigated as well asthe importance of particular phases in the scavenging of heavy metals. Results showed that the organic and sulphide phase was less important in scavenging mechanism –in spite of industrial and petrochemical activities in the study area– than exchangeable and carbonate fraction, which showed to be most important phase with respect to metal scavenging capacity.

OS PARÂMETROS DE RESISTÊNCIA AO CISALHAMENTO E A ESTABILIDADE DAS ENCOSTAS NO PLANALTO DE PARAIBUNA E SERRA DE CARAGUATATUBA

The central aim of this work is to confront spatial behavior of shear strength parameters of soil and regolith materials at Paraibuna Plateau, with those of Caraguatatuba Scarps. The behavior of slope stability and its correlated variables in the soil-regolith transition along slope profiles was evaluated, for granite, migmatite and micaschist parent materials. For this propose, we had making measures of shear strength for residual soils and regolith materials with soil “Cohron Sheargraph” apparatus and evaluated the shear stress tension behavior at soil-regolith boundary along slope profiles, in each environment and lithology. When we compeer the results for the two areas, a compensation mechanism appears. At Paraibuna plateau, the major part of the shear resistance is attributed to cohesion, when at Caraguatatuba scarps is the friction angle the major strength component. An important question here is the change of soil bulk-density variable along slope profile evolution, because the intense clay production, mainly Kaolinite and Gibbsite at B and C-horizons, in the plateau tropical environment. We detected too, at threshold conditions, that slight change in soil bulk-density may changes drastically the equilibrium of stress-strength tensions at potential rupture surface with its consequent loss of stability and landslide event production. At Caraguatatuba scarps, the extremely humid environment would dissolves Kaolinite and promotes the shut down of cohesion.

Single grain noble gas analysis of Antarctic micrometeorites by stepwise heating method with a newly constructed miniature furnace

Ten micrometeorites weighing 0.14–18.5 μg, each retrieved from surface snow near the Dome Fuji Station, Antarctica (snow-AMMs), were studied to elucidate their noble gases, mineralogy, morphology, and chemical compositions. Low densities in the range of 0.2–1.4 g/cm3 estimated for seven samples suggested a porous inner structure. Noble gases were extracted from each particle using stepwise heating with a laboratory manufactured miniature furnace. Isotopic ratios of He and Ne indicate that the light noble gases with high 4He concentrations ranging from 10−2 to 10−4 cm3 STP/g are mostly of solar origin. The higher concentrations of 4He observed for several samples are comparable with those of IDPs enriched in solar He, but exceed those reported for ice-AMMs. In contrast to He and Ne, heavy noble gases Ar, Kr, and Xe are primordial ones resembling Q-gas trapped in chondrites, although a small contribution of solar Ar is indicated for some samples with higher 36 Ar/132Xe ratios than that for the Q-gas. Three particles released appreciable amounts of He at temperatures lower than 800°C, suggesting heating temperatures lower than 700°C at the time of atmospheric entry. Other particles released at most 10% of total He at the temperatures up to 800°C. Based on their sizes, weights, and release profiles of 4He, initial speeds of less than 14 km/s at atmospheric entry were indicated for the particles. The slow entry speeds imply that all the snow-AMMs studied in this work were likely derived from asteroids. The present work demonstrates that the miniature furnace can be applicable to noble gas analysis of tiny grains from the Itokawa asteroidal regolith materials returned by the Hayabusa mission.

Irradiation History of Itokawa Regolith Material Deduced from Noble Gases in the Hayabusa Samples

Noble gas isotopes were measured in three rocky grains from asteroid Itokawa to elucidate a history of irradiation from cosmic rays and solar wind on its surface. Large amounts of solar helium (He), neon (Ne), and argon (Ar) trapped in various depths in the grains were observed, which can be explained by multiple implantations of solar wind particles into the grains, combined with preferential He loss caused by frictional wear of space-weathered rims on the grains. Short residence time of less than 8 million years was implied for the grains by an estimate on cosmic-ray-produced (21)Ne. Our results suggest that Itokawa is continuously losing its surface materials into space at a rate of tens of centimeters per million years. The lifetime of Itokawa should be much shorter than the age of our solar system.

Isotopic evidence for a Martian regolith component in shergottite meteorites

[1] The 80Kr excesses determined in the gas-rich impact-melt (GRIM) glasses in EET79001 and Shergotty correlate poorly with the 129XeM (Martian atmosphere) suggesting that the majority of the neutron-capture 80Krn was not shock-implanted along with 129XeM into these glasses during impact. This inference is consistent with the variations in δ80Krn excesses observed in these samples. The results reported here indicate that the 80Krn excesses in these glasses were produced in the same way as the 149Sm isotopic deficits, i.e., by thermal neutron (n) capture on Br and Sm occurring in the glass-precursor regolith materials on Mars. The thermal neutron fluences calculated from 80Krn excesses (∼0.3–1.0 × 1015 n/cm2) and from 149Sm deficits (1.0 ± 0.4 × 1015 n/cm2) agree with each other confirming that 80Krn was mostly produced in situ. In the Martian regolith, thermal (n) and fast (N) neutrons occur together. Also, in the GRIM glasses, the 83Kr/86Kr ratios correlate positively with 84Kr/86Kr indicating that the cosmogenic Kr contains a fast neutron-produced component in addition to Kr produced by galactic cosmic and solar cosmic irradiation. Using 83KrN and 84KrN excesses produced by fast neutron reactions on Rb and Sr targets in some of these glasses, we determine fast neutron fluences of ∼3–47 × 1015 N/cm2. The integrated fluences of thermal and fast neutrons in GRIM glasses suggest that the glass-precursor materials were irradiated at different depths in the top few meters of the water-ice-bearing regolith near the shergottite source regions on Mars.

The delicate balance between soil production and erosion, and its role on landscape evolution

► The uranium-series isotope composition of regolith material can be used to determine the soil residence time. ► Soil residence times up to 30 and 90 kyr are calculated for Frogs Hollow and Bisley, respectively. ► Production rates are relatively similar for granitic and shale lithologies, but much higher over volcanic parent rock. ► Soil production matches erosion in soil-mantled landscapes, demonstrating quantitatively that this type of landscape results from a balance between these two processes ► Soil production is up to two orders of magnitude slower than erosion in cultivated areas. The diversity in landscapes at the Earth’s surface is the result, amongst other things, of the balance (or imbalance) between soil production and erosion. While erosion rates are well constrained, it is only recently that we have been able to quantify rates of soil production. Uranium-series isotopes have been useful to provide such estimates independently of erosion rates. In this study, new U-series isotope are presented data from weathering profiles developed over andesitic parent rock in Puerto Rico, and granitic bedrock in southeastern Australia. The site in Australia is located on a highland plateau, neighbouring a retreating escarpment where soil production rates between 10 and 50 mm/kyr have been determined. The results show that production rates are invariant in these two regions of Australia with values between 15 and 25 mm/kyr for the new site. Andesitic soils show much faster rates, about 200 mm/kyr. Overall, soil production rates determined with U-series isotopes range between 10 and 200 mm/kyr. This is comparable to erosion rates in soil-mantled landscapes, but faster than erosion in cratonic areas and slower than in alpine regions and cultivated areas. This suggests that soil-mantled landscapes maintain soil because they can: there is a balance between production and erosion. Similarly, thick weathering profiles develop in cratonic areas because, despite slow erosion rates, soil production is still significant. Bare landscapes in Alpine regions are probably the result of the inability of soil production to catch up with fast erosion rates, although this needs testing by U-series isotope studies of these regions. Finally, the range of production rates is up to several orders of magnitude lower than erosion rates in cultivated areas, demonstrating quantitatively the fast depletion of soil resources with common agricultural practices.

Secondary chaotic terrain formation in the higher outflow channels of southern circum-Chryse, Mars

Higher outflow channel dissection in the martian region of southern circum-Chryse appears to have extended from the Late Hesperian to the Middle Amazonian Epoch. These outflow channels were excavated within the upper 1 km of the cryolithosphere, where no liquid water is expected to have existed during these geologic epochs. In accordance with previous work, our examination of outflow channel floor morphologies suggests the upper crust excavated by the studied outflow channels consisted of a thin (a few tens of meters) layer of dry geologic materials overlying an indurated zone that extends to the bases of the investigated outflow channels (1 km in depth). We find that the floors of these outflow channels contain widespread secondary chaotic terrains (i.e., chaotic terrains produced by the destruction of channel-floor materials). These chaotic terrains occur within the full range of outflow channel dissection and tend to form clusters. Our examination of the geology of these chaotic terrains suggests that their formation did not result in the generation of floods. Nevertheless, despite their much smaller dimensions, these chaotic terrains are comprised of the same basic morphologic elements (e.g., mesas, knobs, and smooth deposits within scarp-bound depressions) as those located in the initiation zones of the outflow channels, which suggests that their formation must have involved the release of ground volatiles. We propose that these chaotic terrains developed not catastrophically but gradually and during multiple episodes of nested surface collapse. In order to explain the formation of secondary chaotic terrains within zones of outflow channel dissection, we propose that the regional Martian cryolithosphere contained widespread lenses of volatiles in liquid form. In this model, channel floor collapse and secondary chaotic terrain formation would have taken place as a consequence of instabilities arising during their exhumation by outflow channel dissection. Within relatively warm upper crustal materials in volcanic settings, or within highly saline crustal materials where cryopegs developed, lenses of volatiles in liquid form within the cryolithosphere could have formed, and/or remained stable. In addition, our numerical simulations suggest that low thermal conductivity, dry fine-grained porous geologic materials just a few tens of meters in thickness (e.g., dunes, sand sheets, some types of regolith materials), could have produced high thermal anomalies resulting in subsurface melting. The existence of a global layer of dry geologic materials overlying the cryolithosphere would suggest that widespread lenses of fluids existed (and may still exist) at shallow depths wherever these materials are fine-grained and porous. The surface ages of the investigated outflow channels and chaotic terrains span a full 500 to 700 Myr. Chaotic terrains similar in dimensions and morphology to secondary chaotic terrains are not observed conspicuously throughout the surface of Mars, suggesting that intra-cryolithospheric fluid lenses may form relatively stable systems. The existence of widespread groundwater lenses at shallow depths of burial has tremendous implications for exobiological studies and future human exploration. We find that the clear geomorphologic anomaly that the chaotic terrains and outflow channels of southern Chryse form within the Martian landscape could have been a consequence of large-scale resurfacing resulting from anomalously extensive subsurface melt in this region of the planet produced by high concentrations of salts within the regional upper crust. Crater count statistics reveal that secondary chaotic terrains and the outflow channels within which they occur have overlapping ages, suggesting that the instabilities leading to their formation rapidly dissipated, perhaps as the thickness of the cryolithosphere was reset following the disruption of the upper crustal thermal structure produced during outflow channel excavation.

Laboratory studies of the diffusive transport of 137Cs and 60Co through potential waste repository soils

Tests using reconstituted samples have been performed to assess the diffusive transport of 137Cs and 60Co through natural regolith materials from a region in South Australia being considered for a radioactive waste repository. A double diffusion cell apparatus made of polycarbonate resin was developed to estimate the effective diffusion ( D e) and sorption coefficients ( K d) that allowed large withdrawals from the source and collector cells and has enabled tests with low concentrations of radioactivity. An alternative to porous stainless steel filter plates has also been used to reduce uncertainty in test interpretation. Analysis of the transient data used a staged method of the Laplace transform to take into consideration the volume of the samples withdrawn from the apparatus during testing. At test completion samples were cut into slices and analysed for radionuclide concentration. Data obtained from the sliced samples confirmed that both numerical and experimental data produced acceptable mass balance. The D e values obtained in this study were of the order of 10 −6 cm 2 s −1 for both species, higher than previously published data. The K d values from the diffusion and batch sorption tests were in reasonable agreement for 137Cs, but an order of magnitude different for 60Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.

Major advances in exploration geochemistry, 1998–2007

ABSTRACT Exploration geochemistry continues to be a major contributor to mineral exploration programs at all scales. The decade has spanned a complete economic cycle in the minerals industry, during which there has been a change in exploration emphasis from the dominance of Au and diamonds to the addition of a range of base metals and the recent revival of U exploration. Increasing metal prices and the development of new geochemical techniques have allowed exploration activity to expand in previously under-explored terrains, including those characterized by thick, transported regolith cover. There has been significant evolution and refinement of geochemical dispersion models for transported regolith, especially processes associated with oxidizing sulphide mineralization. These models have prompted further developments in sampling methods, analytical techniques and approaches to anomaly detection. The routine measurement of fundamental soil and groundwater properties, such as pH, has been demonstrated to assist in detecting geochemical and mineralogical effects of buried mineralization on regolith. New lithogeochemical techniques have been developed for terrane selection and early discrimination of potentially mineralized units. The production of regolith-landform maps, including 3-D models, and more detailed techniques for differentiating the origin and nature of regolith materials now underpins the design of exploration programs in a number of terrains. Various exploration techniques, such as biogeochemistry and soil gas, have been revitalized by the application of new technologies. Ongoing improvements to detection limits and the reliability of multi-element analytical methods have permitted development and routine application of a wide range of selective geochemical extraction methods aimed at detecting subtle geochemical haloes developed in transported regolith cover. The results from a plethora of selective extraction orientation studies have, however, indicated the reliability of such methods to be uncertain, especially in arid terrains. Low-density geochemical mapping of large regions has been undertaken by government agencies for a variety of resource, environmental and health purposes, including attracting mineral exploration. Such maps typically demonstrate the dominance of underlying geology on regional geochemical patterns in regolith, even in industrialized regions. Philosophical and numerical approaches to defining and detecting geochemical anomalies continue to be reviewed. There is a need to supplement classical parametric outlier-detection approaches, routinely applied to geochemical data, with statistical techniques that reflect a recognition that subtle, multivariate patterns related to the geochemical effects of mineralization may emanate from within the main data cloud and not outlying or distinct clusters.

The Radiometric Map of Australia

Geoscience Australia and the Australian State and Territory Geological Surveys have systematically surveyed most of the Australian continent over the past 40 years using airborne gamma-ray spectrometry to map potassium, uranium and thorium elemental concentrations at the Earth’s surface. However, the individual surveys that comprise the national gamma-ray spectrometric radioelement database are not all registered to the same datum. This limits the usefulness of the database as it is not possible to easily combine surveys into regional compilations or make accurate comparisons between radiometric signatures in different survey areas. To solve these problems, Geoscience Australia has undertaken an Australia-Wide Airborne Geophysical Survey (AWAGS), funded under the Australian Government’s Onshore Energy Security Program, to serve as a radioelement baseline for all current and future airborne gamma-ray spectrometric surveys in Australia.The AWAGS survey has been back-calibrated to the International Atomic Energy Agency’s (IAEA) radioelement datum. We have used the AWAGS data to level the national radioelement database by estimating survey correction factors that, once applied, minimise both the differences in radioelement estimates between surveys (where these surveys overlap) and the differences between the surveys and the AWAGS traverses. The database is thus effectively levelled to the IAEA datum. The levelled database has been used to produce the first ‘Radiometric Map of Australia’ – levelled and merged composite potassium (% K), uranium (ppm eU) and thorium (ppm eTh) grids over Australia at 100 m resolution.Interpreters can use the map to reliably compare the radiometric signatures observed over different parts of Australia. This enables the assessment of key mineralogical and geochemical properties of bedrock and regolith materials from different geological provinces and regions with contrasting landscape histories.

The effects of low ambient temperatures on optically stimulated luminescence (OSL) processes: Relevance to OSL dating of martian sediments

Advances in luminescence dosimetry have made geochronological dating of materials from extreme environments possible through the use of optically stimulated luminescence (OSL) single-aliquot techniques. However, these environments present challenges not always encountered in routine OSL dating, such as sediments that have been stored at low, and possibly highly variable, ambient temperatures. In recent years OSL has been proposed as a method for dating recent depositional events on surfaces of other planets, specifically, Mars. As a result it has become necessary to examine the constraints that may be imposed on the OSL method by the extreme environments of extraterrestrial planetary bodies. In this paper we report on investigations of the possible effects a low storage and/or a low OSL measurement temperature could have on the OSL process and the subsequent results. Pertinent OSL properties include the stability of electron traps, the overall luminescence efficiency, and possible thermal assistance processes. The particular focus of the work is on the potential application of the OSL technique for dating surface sediments on Mars. We report the results of OSL experiments on martian simulant materials, and of generalized computer simulations of potential OSL behavior. It is concluded that the stimulation and irradiation/calibration temperatures need to be maintained fixed throughout the experiment – i.e. the dose estimation process – and that the temperature during OSL stimulation needs to be appreciably higher than the highest temperature experienced during natural irradiation. The consequences of these findings for establishing an OSL protocol and instrument package for dating martian regolith material are discussed.

Periglacial climate at the 2.5 Ma onset of Northern Hemisphere glaciation inferred from the Whippoorwill Formation, northern Missouri, USA

The Whippoorwill Formation is a gleyed diamicton that is present locally within bedrock depressions beneath the oldest glacial till in northern Missouri, USA. Stratigraphy, paleomagnetism, and cosmogenic–nuclide burial ages show that it was deposited between the Matuyama–Gauss magnetostratigraphic boundary at 2.58 Ma and the first advance of the Laurentide ice sheet into Missouri at 2.47 ± 0.19 Ma. High cosmogenic–nuclide concentrations also show that the constituents of the Whippoorwill Formation experienced long exposure at a stable landscape surface with erosion rates of 1–2 m/Ma. However, cosmogenic–nuclide concentrations are invariant with depth below the Whippoorwill Formation surface, indicating active mixing of the soil profile shortly before burial by till. The Whippoorwill Formation retains numerous features indicative of cryoturbation. Therefore, we interpret it as a buried Gelisol, a soil formed under periglacial conditions in the presence of permafrost. At the onset of Northern Hemisphere glaciation, climate cooling established permafrost conditions and accelerated erosion by inducing landscape instability. Thus, weathered regolith materials were mobilized and redeposited by gelifluction shortly before the ice sheet overrode the landscape.

New Cassini RADAR results for Saturn’s icy satellites

Cassini radar tracks on Saturn’s icy satellites through the end of the Prime Mission in 2008 have increased the number of radar albedo estimates from 10 ( Ostro et al., 2006) to 73. The measurements sample diverse subradar locations (and for Dione, Rhea, and Iapetus almost always use beamwidths less than half the target angular diameters), thereby constraining the satellites’ global radar albedo distributions. The echoes result predominantly from volume scattering, and their strength is thus strongly sensitive to ice purity and regolith maturity. The combination of the Cassini data set and Arecibo 13-cm observations of Enceladus, Tethys, Dione, Rhea ( Black et al., 2007), and Iapetus ( Black et al., 2004) discloses an unexpectedly complex pattern of 13-to-2-cm wavelength dependence. The 13-cm albedos are generally smaller than 2-cm albedos and lack the correlation seen between 2-cm and optical geometric albedos. Enceladus and Iapetus are the most interesting cases. We infer from hemispheric albedo variations that the E-ring has a prominent effect on the 13-cm radar “lightcurve”. The uppermost trailing-side regolith is too fresh for meteoroid bombardment to have developed larger-scale heterogeneities that would be necessary to elevate the 13-cm radar albedo, whereas all of Enceladus is clean and mature enough for the 2-cm albedo to be uniformly high. For, Iapetus, the 2-cm albedo is strongly correlated with optical albedo: low for the optically dark, leading-side material and high for the optically bright, trailing-side material. However, Iapetus’ 13-cm albedo values show no significant albedo dichotomy and are several times lower than 2-cm values, being indistinguishable from the weighted mean of 13-cm albedos for main-belt asteroids, 0.15 ± 0.10. The leading side’s optically dark contaminant must be present to depths of at least one to several decimeters, so 2-cm albedos can mimic the optical dichotomy; however, it does not have to extend any deeper than that. The fact that both hemispheres of Iapetus look Asteroid-like at 13 cm means that coherent backscattering itself is not nearly as effective as it is at 2 cm. Since Iapetus’ entire surface is mature regolith, the wavelength dependence must involve composition, not structure. Either the composition is a function of depth everywhere (with electrical loss much greater at depths greater than a decimeter or two), or the intrinsic electrical loss of some pervasive constituent is much higher at 13 cm than at 2 cm. Ammonia is a candidate for such a contaminant. If ammonia’s electrical properties do not depend on frequency, and if ammonia is globally much less abundant within the upper one or two decimeters than at greater depths, then coherent backscattering would effectively be shut down at 13 cm, explaining the Asteroid-like 13-cm albedo.

In situ multi-frequency measurements of magnetic susceptibility as an indicator of planetary regolith maturity

Space weathering is now generally accepted to modify the optical and magnetic properties of airless planetary regoliths such as those on the Moon and Mercury. Under micrometeorite and ion bombardment, ferrous iron in such surfaces is reduced to metallic iron spheres, found in amorphous coatings on almost all exposed regolith grains. The size and number distribution of these particles and their location in the regolith all determine the nature and extent of the optical and magnetic changes. These parameters in turn reflect the formation mechanisms, temperatures, and durations involved in the evolution of the regolith. Studying them in situ is of intrinsic value to understanding the weathering process, and useful for determining the maturity of the regolith and providing supporting data for interpreting remotely sensed mineralogy. Fine-grained metallic iron has a number of properties that make it amenable to magnetic techniques, of which magnetic susceptibility is the simplest and most robust. The magnetic properties of the lunar regolith and laboratory regolith analogues are therefore reviewed and the theoretical basis for the frequency dependence of magnetic susceptibility presented. Proposed here is then an instrument concept using multi-frequency measurements of magnetic susceptibility to confirm the presence of fine-grained magnetic material and attempt to infer its quantity and size distribution. Such an instrument would be invaluable on a future mission to an asteroid, the Moon, Mercury or other airless rocky Solar System body.

Conceptual Design of a Lunar Regolith Clustered-Reactor System

It is proposed that a fast-fission, heatpipe-cooled, lunar-surface power reactor system be divided into subcritical units that could be launched safely without the incorporation of additional spectral shift absorbers or other complex means of control. The reactor subunits are to be emplaced directly into the lunar regolith, utilizing the regolith not just for shielding but also as the reflector material to increase the neutron economy of the system. While a single subunit cannot achieve criticality by itself, coordinated placement of additional subunits will provide a critical reactor system for lunar surface power generation. A lunar regolith clustered-reactor system promotes reliability, safety, and ease of manufacture and testing at the cost of a slight increase in launch mass per rated power level and an overall reduction in neutron economy when compared to a single-reactor system. Additional subunits may be launched with future missions to increase the cluster size and power according to desired lunar-base power demand and lifetime. The results address the potential uncertainties associated with the lunar regolith material and emplacement of the subunit systems. Physical distance between subunits within the clustered emplacement exhibits the most significant feedback regarding changes in overall system reactivity. Narrow, deep holes will be the most effective in reducing axial neutron leakage from the core. The variation in iron concentration in the lunar regolith can directly influence the overall system reactivity, although its effects are less than the more dominant factors of subunit emplacement.

Effects of ejecta accumulation on the crater population of asteroid 433 Eros

The crater population of asteroid 433 Eros exhibits a deficit in small crater diameters that has been suggested to result from impact‐induced seismic shaking initiating downslope movements of regolith material, covering these small craters. As in lunar maria, saturation equilibrium was expected to characterize the crater population of Eros, but was surprisingly not shown by the data set. The surface of Eros displays evidence of burial by regolith especially for boulders, suggesting that ejecta coverage erases the craters in addition to seismic shaking erasure. In this work we investigate the production and erasure of craters by impact ejecta and compare derived crater size distributions with those measured for Eros. We simulate a bombardment of Eros by an impactor population derived from the Main Asteroid Belt and estimate the crater and ejecta characteristics with a scaling law, allowing ejecta to progressively create a regolith blanket. Assuming the contribution of the ejecta blanketing process only, we find a good agreement between the simulated and the observed population of 250 m to 4 km diameter craters for exposure times of 600 Ma and 400 Ma. This suggests a major impact or breakup that occurred about 500 Ma ago, inducing a surface reset. A mismatch for craters with a diameter smaller than ∼100 m remains, indicating that seismic shaking (or another erasure process) is still necessary to explain their low number. Our simulations emphasize the importance of an accurate modeling of both processes to fully understand and interpret the small body size‐frequency crater curves.

Multispectral images of Mercury from the first MESSENGER flyby: Analysis of global and regional color trends

We have analyzed MESSENGER multispectral images of Mercury in order to gain information on the nature of materials present on the planet's surface. The quantitative analysis tools of Lucey, which are built on the Hapke conceptual framework for the reflectance of a silicate regolith, allow us to derive spectral parameter images that are sensitive to the abundance of ferrous iron in silicates, the abundance of opaque phases, and the state of maturity of the surface. Consistent with previous estimates, we conclude that the silicate portion of Mercury's surface is low in ferrous iron, with a small range of variation. The spectral changes that accompany regolith maturation on Mercury are consistent with lunar-style space weathering via the accumulation of nanophase iron. The spectral character of Mercury's major surface units (high-reflectance plains, intermediate terrain, and low-reflectance material) can be attributed to variations in the abundance of an opaque phase(s). A class of minor color unit, the red spots, occurs in at least three morphological varieties, two of which are associated with deposits interpreted to be of pyroclastic origin. The bright crater floor deposits, a second type of minor unit, have extremely shallow (“blue”) spectral slopes that are unrelated to the presence of immature material.