Evidence of space weathering in regolith breccias II: Asteroidal regolith breccias

Abstract– Space weathering products, such as agglutinates and nanophase iron‐bearing rims are easily preserved through lithification in lunar regolith breccias, thus such products, if produced, should be preserved in asteroidal regolith breccias as well. A study of representative regolith breccia meteorites, Fayetteville (H4) and Kapoeta (howardite), was undertaken to search for physical evidence of space weathering on asteroids. Amorphous or npFe⁰‐bearing rims cannot be positively identified in Fayetteville, although possible glass rims were found. Extensive friction melt was discovered in the meteorite that is difficult to differentiate from weathered materials. Several melt products, including spherules and agglutinates, as well as one irradiated rim and one possible npFe⁰‐bearing rim were identified in Kapoeta. The existence of these products suggests that lunar‐like space weathering processes are, or have been, active on asteroids.     

Particle track measurements in lunar regolith breccias

Particle track measurements have been reported for 25 (5%) of the regolith breccias in the collection; they have been reported for 16 breccias (30%) in the reference suite. The most frequently reported measurement for these 25 breccias is the maximum surface exposure age of the compacted rock (48% of the published breccia measurements). Information on the nature of the precompaction regolith is given for 9 rocks (36%) and on the nature of the compaction event for 6 rocks (24%). Most of the breccias appear to have simple post-compaction surface exposure histories (89%). From the few track density frequency distributions (7) that are available and inferring from the low exposure ages of these rocks (75% < 10⁶ yr), it appears that most of these breccias are amenable to studies which separate the contemporary surface exposure age from information about the precompaction regolith. If the number of immature-submature precompaction soils (6 out of 10 of the breccias for which appropriate data are available) represents many regolith breccias, then we can infer that regolith breccias may sample the deeper, less reworked materials in the lunar soil and compliment the samples available from the returned cores.     

Composition and petrology of HED polymict breccias: The regolith of (4) Vesta

We have done petrologic and compositional studies on a suite of polymict eucrites and howardites to better understand regolith processes on their parent asteroid, which we accept is (4) Vesta. Taking into account noble gas results from companion studies, we interpret five howardites to represent breccias assembled from the true regolith: Elephant Moraine (EET) 87513, Grosvenor Mountains (GRO) 95535, GRO 95602, Lewis Cliff (LEW) 85313, and Meteorite Hills (MET) 00423. We suggest that EET 87503 is paired with EET 87513, and thus is also regolithic. Pecora Escarpment (PCA) 02066 is dominated by melt‐matrix clasts, which may have been formed from true regolith by impact melting. These meteorites display a range in eucrite:diogenite mixing ratio from 55:45 to 76:24. There is no correlation between degree of regolith character and Ni content. The Ni contents of howardite, eucrite, and diogenites (HEDs) are mostly controlled by the distribution of coarse chondritic clasts and metal grains, which in some cases resulted from individual, low‐velocity accretion events, rather than extensive regolith gardening. Trace element compositions indicate that the mafic component of HED polymict breccias is mostly basalt similar to main‐group eucrites; Stannern‐trend basaltic debris is less common. Pyroxene compositions show that some trace element‐rich howardites contain abundant debris from evolved basalts, and that cumulate gabbro debris is present in some breccias. The scale of heterogeneity varies considerably; regolithic howardite EET 87513 is more homogeneous than fragmental howardite Queen Alexandra Range (QUE) 97001. Individual samples of a given howardite can have different compositions even at roughly 5 g masses, indicating that obtaining representative meteorite compositions requires multiple or large samples.     

Lunar regolith breccia Dhofar 287B: A record of lunar volcanism

Abstract— Dhofar 287 (Dho 287), a recently found lunar meteorite, consists in large part (95%) of low‐Ti mare basalt (Dho 287A) and a minor, attached portion (?5%) of regolith breccia (Dho 287B). The present study is directed mainly at the breccia portion of this meteorite. This breccia consists of a variety of lithic clasts and mineral fragments set in a fine‐grained matrix and minor impact melt. The majority of clasts and minerals appear to have been mainly derived from the low‐Ti basalt suite, similar to that of Dho 287A. Very low‐Ti (VLT) basalts are a minor lithology of the breccia. These are significantly lower in Mg# and slightly higher in Ti compared to Luna 24 and Apollo 17 VLT basalts. Picritic glasses constitute another minor component of the breccia and are compositionally similar to Apollo 15 green glasses. Dho 287B also contains abundant fragments of Mg‐rich pyroxene and anorthite‐rich plagioclase grains that are absent in the lithic clasts. Such fragments appear to have been derived from a coarse‐grained, Mg#‐rich, Na‐poor lithology. A KREEP component is apparent in chemistry, but no highlands lithologies were identified. The Dho 287 basaltic lithologies cannot be explained by near‐surface fractionation of a single parental magma. Instead, magma compositions are represented by a picritic glass; a low‐Ti, Na‐poor glass; and a low‐Ti, Na‐enriched source (similar to the Dho 287A parental melt). Compositional differences among parent melts could reflect inhomogeneity of the lunar mantle. Alternatively, the low‐Ti, Na‐poor, and Dho 287A parent melts could be of hybrid compositions, resulting from assimilation of KREEP by picritic magma. Thus, the Dho 287B breccia contains lithologies from multiple magmatic eruptions, which differed in composition, formational conditions, and cooling histories. Based on this study, the Dho 287 is inferred to have been ejected from a region located distal to highlands terrains, possibly from the western limb of the lunar nearside, dominated by mare basalts and KREEP‐rich lithologies.     

Lunar meteorite regolith breccias: An in situ study of impact melt composition using LA‐ICP‐MS with implications for the composition of the lunar crust

Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104/88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104/88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA‐ICP‐MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg#_<70), have high Sc/Sm ratios (typically >10), and have low incompatible trace element (ITE) concentrations (i.e., typically <3.2 ppm Sm, <1.5 ppm Th). Feldspathic impact melt in DaG 400, PCA 02007, and MAC 88104/05 are similar in composition to that estimated composition for upper feldspathic lunar crust ( Korotev et al. 2003 ). However, these melt types are more mafic (i.e., less Eu, less Sr, more Sc) than feldspathic impact melts returned by the Apollo 16 mission (e.g., the group 3 and 4 varieties). Mafic impact melt clasts are common in MET 01210 and less common in PCA 02007 and MAC 88104/05. We show that unlike the Apollo mafic impact melt groups ( Jolliff 1998 ), these meteorite impact melts were not formed from melting large amounts of KREEP‐rich (typically >10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc/Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP‐poor and Sc‐rich, and represent mixing between feldspathic lithologies and low‐Ti or very low‐Ti (VLT) basalts. As PCA 02007 and MAC 88104/05 were likely sourced from the Outer‐Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low‐Ti basalt contribution.     

Evidence for asteroid space weathering from the Sloan Digital Sky Survey

By studying color variations between young and old asteroid families we find evidence for processes that modify colors of asteroids over time. We show that colors of aging surfaces of S-type asteroids become increasingly ‘redder’ and measure the rate of these spectral changes. We estimate that the mean spectral slope between 0.35 and 0.9 μm increases with time t (given in My) as ≈0.01 μm⁻¹×log₁₀t. This empirical fit is valid only for 2.5?t?3000 My (the time interval where we have data) and for the mean spectral slope determined from wide-wavelength filter photometry obtained by the Sloan Digital Sky Survey. We also find that Gy-old terrains of S-type asteroids reflect about 15% more light at ∼1-μm wavelengths than an ∼5-My-old S-type asteroid surface when the flux is normalized by the reflected light at 0.55 μm. We attribute these effects to space weathering. This result has important implications for asteroid geology and the origin of meteorites that reach the Earth. Our results also suggest that surfaces of C-type asteroids exhibit color alterations opposite to those of the S-type asteroids.     

Origin and history of chondrite regolith,fragmental and impact-melt breccias from Spain

Abstract— Six ordinary chondrite breccias from the Museo Nacional de Ciencias Naturales, Madrid (Spain), are described and classified as follows: the solar gas-rich regolith breccia Oviedo (H5); the pre-metamorphic fragmental breccias Cabezo de Mayo (type 6, L-LL), and Sevilla (LL4); the fragmental breccias Cañellas (H4) and Gerona (H5); and the impact melt breccia, Madrid (L6). We confirm that chondrites with typical light-dark structures and petrographic properties typical of regolith breccias may (Oviedo) or may not (Cañellas) be solar gas-rich. Cabezo de Mayo and Sevilla show convincing evidence that they were assembled prior to peak metamorphism and were equilibrated during subsequent reheating. These meteorites contain small melt rock clasts that were incorporated into the host chondrite while still molten and/or plastic and cooled rapidly and, yet, are totally equilibrated with their hosts. Compositions of olivine and low-Ca pyroxene in host chondrite and breccia clasts in Cabezo de Mayo are transitional between groups L and LL. It is suggested, based on mineralogic and oxygen isotopic compositions of host and clasts, that the rock formed on the L parent body by mixing, prior to peak metamorphism. This was followed by partial equilibration of two different materials: the indigenous L chondrite host and exotic LL melt rock clasts.     

Lunar Prospector measurements of secondary electron emission from lunar regolith

We present the first in situ measurements of the secondary electron emission efficiency of lunar regolith, utilizing Lunar Prospector measurements of secondary electrons emitted from the negatively charged night side and accelerated upward by surface electric fields. By comparing measurements of secondary currents emitted from the surface and incident primary electron currents, we find that the secondary yield of lunar regolith is a factor of ∼3 lower than that measured for samples in the laboratory. This lower yield significantly affects current balance at the lunar surface and the resulting equilibrium surface potentials. This information must be folded into models of the near-surface plasma sheath, in order to predict the effects on dust and other components of the lunar environment, and ultimately determine the importance for surface exploration and scientific investigations on the Moon.     

On the origin of shocked and unshocked CM clasts in H‐chondrite regolith breccias

Abstract— CM chondrite clasts that have experienced different degrees of aqueous alteration occur in H‐chondrite and HED meteorite breccias. Many clasts are fragments of essentially unshocked CM projectiles that accreted at low relative velocities to the regoliths of these parent bodies. A few clasts were heated and dehydrated upon impact; these objects most likely accreted at higher relative velocities. We examined three clasts and explored alternative scenarios for their formation. In the first scenario, we assumed that the H and HED parent bodies had diameters of a few hundred kilometers, so that their high escape velocities would effectively prevent soft landings of small CM projectiles. This would imply that weakly shocked CM clasts formed on asteroidal fragments (family members) associated with the H and HED parent bodies. In the second scenario, we assumed that weakly shocked CM clasts were spall products ejected at low velocities from larger CM projectiles when they slammed into the H and HED parent bodies. In both cases, if most CM clasts turn out to have ancient ages (e.g., ?3.4‐4.1 Ga), a plausible source for their progenitors would be outer main belt objects, some which may have been dynamically implanted 3.9 Ga ago by the events described in the so‐called “Nice model.” On the other hand, if most CM clasts have recent ages (<200 Ma), a plausible source location for their parent body would be the inner main belt between 2.1–2.2 AU. In that case, the possible source of the CM‐clasts' progenitors' parent fragments would be the breakup ?160 Ma ago of the parent body 170 km in diameter of the Baptistina asteroid family (BAF).     

On the relationship between the Apollo 16 ancient regolith breccias and feldspathic fragmental breccias,and the composition of the prebasin crust in the Central Highlands of the Moon

Abstract Two types of texturally and compositionally similar breccias that consist largely of fragmental debris from meteorite impacts occur at the Apollo 16 lunar site: Feldspathic fragmental breccias (FFBs) and ancient regolith breccias (ARBs). Both types of breccia are composed of a suite of mostly feldspathic components derived from the early crust of the Moon and mafic impact-melt breccias produced during the time of basin formation. The ARBs also contain components, such as agglutinates and glass spherules, indicating that the material of which they are composed occurred at the surface of the Moon as fine-grained regolith prior to lithification of the breccias. These components are absent from the FFBs, suggesting that the FFBs might be the protolith of the ARBs. However, several compositional differences exist between the two types of breccia, making any simple genetic relationship implausible. First, clasts of mafic impact-melt breccia occurring in the FFBs are of a different composition than those in the ARBs. Also the feldspathic “prebasin” components of the FFBs have a lower average Mg/Fe ratio than the corresponding components of the ARBs; the average composition of the plagioclase in the FFBs is more sodic than that of the ARBs; and there are differences in relative abundances of rare earth elements. The two breccia types also have different provenances: the FFBs occur primarily in ejecta from North Ray crater and presumably derive from the Descartes Formation, while the ARBs are restricted to the Cayley plains. Together these observations suggest that although some type of fragmental breccia may have been a precursor to the ARBs, the FFBs of North Ray crater are not a significant component of the ARBs and, by inference, the Cayley plains. The average compositions of the prebasin components of the two types of fragmental breccia are generally similar to the composition of the feldspathic lunar meteorites. With 30–31% Al₂O₃, however, they are slightly richer in plagioclase than the most feldspathic lunar meteorites (～29% Al₂O₃), implying that the crust of the early central nearside of the Moon contained a higher abundance of highly feldspathic anorthosite than typical lunar highlands, as inferred from the lunar meteorites. The ancient regolith breccias, as well as the current surface regolith of the Cayley plains, are more mafic than (1) prebasin regoliths in the Central Highlands and (2) regions of highlands presently distant from nearside basins because they contain a high abundance (～30%) of mafic impact-melt breccias produced during the time of basin formation that is absent from other regoliths.     

Collisional timing of asteroids space weathering: A first approach

The space weathering, i.e. the evolution of surface properties over time, due to the exposure to external factors, has been shown to affect the optical properties of the asteroids, usually causing reddening (an effect which is measured in terms of the spectral slope in the visible and near infrared range) and darkening over time. However, some problems remain open. In particular, the timescale for reddening, which we estimate from laboratory experiments, is shorter—maybe, by two or even more orders of magnitude—than the typical asteroidal ages. Thus we should expect a complete saturation of the reddening effects for most of the objects, which does not happen, instead of a general significant dependence of the slope on the age, as indeed we find.In this paper we discuss, with the aid of a simplified model, how the collisions may affect the timing of the reddening process. We show that the collisions might halt the reddening, unless a significant reaccumulation of the fragments created in the cratering collisions takes place. In this case the timing for the complete reddening is driven by the collisional events, thus providing a rationale for the observed slope-age and slope-exposure relations.     

Simulation of nanophase iron production in lunar space weathering

Nanophase iron (np-Fe⁰) particles produced by space weathering have been widely observed in lunar soil. Current research suggests that np-Fe⁰ could have important effects on the chemical, optical and magnetic properties of the lunar soil. To investigate the relationship between np-Fe⁰ and these properties of lunar soil, simulation of the production process of np-Fe⁰ by space weathering is necessary because of the scarcity of lunar samples for research purposes. New methods using microwave heating and magnetron sputtering techniques to simulate np-Fe⁰ production both in the glass phase and on the grain surfaces, respectively, are investigated in this study. Both the formation and occurrence of np-Fe⁰ are taken into account in the experiment. The X-ray Diffraction (XRD) spectra show that metallic iron has formed in the glass phase produced by microwave heating of ilmenite. Using scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), the size of np-Fe⁰ particles produced in a microwave heating experiment, which is held for 8 min at 1300 °C, is determined to be about 100–500 nm. Compared to the glass of lunar sample 10084, the major composition of the glass matrix is formed by microwave heating compares favorably. In magnetron sputtering experiment the size of np-Fe⁰ particles is about 20–30 nm, and appears on the grain surfaces. The characteristics of np-Fe⁰ produced in the simulations are consistent with those of lunar samples documented in the literature.     

Lunar highland meteorite Dhofar 026 and Apollo sample 15418: Two strongly shocked,partially melted,granulitic breccias

Abstract— Studies of lunar meteorite Dhofar 026, and comparison to Apollo sample 15418, indicate that Dhofar 026 is a strongly shocked granulitic breccia (or a fragmental breccia consisting almost entirely of granulitic breccia clasts) that experienced considerable post‐shock heating, probably as a result of diffusion of heat into the rock from an external, hotter source. The shock converted plagioclase to maskelynite, indicating that the shock pressure was between 30 and 45 GPa. The post‐shock heating raised the rock's temperature to about 1200 °C; as a result, the maskelynite devitrified, and extensive partial melting took place. The melting was concentrated in pyroxene‐rich areas; all pyroxene melted. As the rock cooled, the partial melts crystallized with fine‐grained, subophitic‐poikilitic textures. Sample 15418 is a strongly shocked granulitic breccia that had a similar history, but evidence for this history is better preserved than in Dhofar 026. The fact that Dhofar 026 was previously interpreted as an impact melt breccia underscores the importance of detailed petrographic study in interpretation of lunar rocks that have complex textures. The name “impact melt” has, in past studies, been applied only to rocks in which the melt fraction formed by shock‐induced total fusion. Recently, however, this name has also been applied to rocks containing melt formed by heating of the rocks by conductive heat transfer, assuming that impact is the ultimate source of the heat. We urge that the name “impact melt” be restricted to rocks in which the bulk of the melt formed by shock‐induced fusion to avoid confusion engendered by applying the same name to rocks melted by different processes.     

Properties of the Hermean regolith: I. Global regolith albedo variation at 200 km scale from multicolor CCD imaging

Multicolor imaging of Mercury has been performed with the 0.5 m Swedish Vacuum Solar Telescope (SVST) on La Palma at five elongations from 1995 to 1999, resulting in a global Minnaert normalized map of the surface at 200 km resolution. Short exposure CCD imaging has been performed in the optical and near-infrared with broad- and intermediate band filters at wavelengths from 550 to 940 nm. Positions for 86 and morphological parameters for 63 bright albedo features on the Hermean surface have been determined. The distribution of bright albedo features is shown to be spatially uniform on the well known (i.e., observed by Mariner 10) and poorly known hemispheres, as well as for the global surface. The number densities of bright albedo features on the two hemispheres are very similar. This indicates that the late evolutionary history of the Hermean regolith has not varied on regional to global scales in terms of impacts generating bright ray craters, constituting approximately 70% of the detected bright albedo features. The locations of bright albedo features correspond well to those determined from nominal resolution and smeared (to the approximate resolution of the SVST data) Mariner 10 maps. Feature parameters (radius, center intensity and intensity gradient) have been determined and correlated with the geologic nature of a subset of observed features imaged by the Mariner 10 Vidicon camera. No difference in feature properties is apparent between the poorly known and well known hemispheres. Based on a comparitive study of Mariner 10 image data, ray craters tend to have higher center intensities and smaller intensity gradients than bright albedo features which are not ray craters. It is however concluded that it is not possible to uniquely determine the geologic nature of features with a high statistical significance, based on their morphological parameters at 200 km resolution. We do not find any general correlation between the locations of radar-bright and optically bright or dark albedo features. The surface contrast decreases from 35% to 25% over the wavelength range 550–940 nm. The range of feature contrasts is similar for all surface regions, except for the ray crater Kuiper, whose contrast to the mean surrounding surface is 50% at a wavelength of 750 nm. Kuiper is an extreme albedo feature also in terms of its center intensity and slope. The mean value of the Minnaert slope parameter for the global surface is determined to 0.76±0.10. A measured constant value of the Minnaert slope with wavelength indicates that the spectral slope for typical Hermean regolith should be linear over the wavelength range 550–940 nm.     

The optical effects of small iron particles that darken but do not redden: Evidence of intense space weathering on Mercury

Submicroscopic iron particles larger than about 50 nm, infused throughout mineral grains or glasses, are abundant in planetary materials altered by their environment such as shocked meteorites and lunar agglutinate glasses. Such particles darken their host material but do not redden their spectra but to date there has been no theoretical treatment of their optical effects. Using Mie theory, we modify the Hapke (2001) radiative transfer model of the effects of space weathering to include these effects. Comparison with laboratory measurements shows that the new treatment reproduces the relationship between submicroscopic iron size, abundance and reflectance. We apply this new model to near-IR spectra of Mercury recently obtained by the MESSENGER spacecraft and find that submicroscopic iron is much more abundant on Mercury than in lunar soils, with typical total submicroscopic iron abundances near 3.5 wt.% compared to about 0.5 wt.% for lunar soils We also find that the ratio of iron particles that darken but do not redden to the abundance of very small iron particles that impart the red slope to space weathered material is much larger than lunar (6 vs. 2). Both the total submicroscopic iron abundance and ratio of particle size fractions are consistent with the higher production of melt and vapor in micrometeorite impact on Mercury relative to the Moon (Cintala, 1992) that enables more accumulation of space weathering products before sequestration by regolith overturn. The radiative transfer model cannot directly constrain the abundance of opaque minerals on Mercury because of ambiguities between the darkening effects of opaques and submicroscopic iron particles larger than 50 nm, but assuming the opaques are the ultimate source of the submicroscopic iron, our results place a lower limit of 4-20 wt.% on opaque abundance on Mercury depending on the composition of the opaque phase and whether titanium metal also contributes to the space weathering effect.     

Lunar meteorite Queen Alexandra Range 93069: A lunar highland regolith breccia with very low abundances of mafic components

Abstract— Lunar meteorite QUE 93069 found in Antarctica is a mature, anorthitic regolith breccia with highland affinities that was ejected from the Moon <0.3 Ma ago. The frequency distribution of mineral and lithic clasts gives information about the nature of the regolith and subregolith basement near the ejection site as well as about the abundances of rock types shocked to different degrees prior to the breccia formation. Thin section QUE 93069,37 consists of 67.5 vol% fine-grained (<～130 μm) constituents and 32.5 vol% mineral and lithic clasts and an impact melt vein. The most abundant types of these clasts are intragranularly recrystallized anorthosites and plagioclases (together 26.3 vol%) and feldspathic fine-grained to microporphyritic crystalline melt breccias (21.9 vol%). Mafic crystalline melt breccias are extremely rare (1.3 vol%). Granulitic lithologies are 10.4 vol%, recrystallized feldspathic melt breccias are 15.0 vol%, and glasses are 3.5 vol%. The impact melt vein cutting across the entire thin section was probably formed subsequent to the lithification process of the bulk rock at pressures below 20 GPa, because the bulk rock never experienced a higher peak shock pressure. Lunar meteorite QUE 93069 has a higher abundance of clear glass, occurring within melt spherules, glassy fragments, and an impact melt vein than lunar meteorites ALHA81005, Y-791197, Y-82192/3, Y-86032, or MAC 88104/5. The high abundance of melt spherules indicates that this lunar meteorite contains the highest content of typical regolith components. Mafic crystalline melt breccias are much rarer in QUE 93069 than in all other lunar highland regolith breccias. The extremely low abundance of mafic components may constrain possible areas of the Moon, from which the breccia was derived. The source area of QUE 93069 must be a highland terrain lacking significant mafic impact melts or mare components.     

Lunar heat flow and regolith structure inferred from interferometric observations at a wavelength of 49.3 cm

We discuss observations of the Moon at a wavelength of 49.3 cm made with the Owens Valley Radio Observatory Interferometer. These observations have been fit to models in order to estimate the lunar dielectric constant, the equatorial subsurface temperature, the latitude dependence of the subsurface temperature, and the subsurface temperature gradient. The models are most consistent with a dielectric constant of 2.52 ± 0.01 (formal errors), an equatorial subsurface temperature of 249_?5⁺⁸K, and a change in the subsurface temperature with latitude (ψ), which is proportional to cos^0.38ψ. Since the temperature of the Moon has been measured by the Apollo Lunar Heat Flow Experiment, we have been able to use our determination of the equatorial temperature to estimate the error in the flux density calibration scale at 49.3cm (608 MHz). This results in a correction factor of 1.03 ± 0.04, which must be applied to the flux density scale. This factor is much different from 1.21 ± 0.09 estimated by Muhleman et al. (1973) from the brightness temperature of Venus and apparently indicates that the observed decrease in the brightness temperature of Venus at long wavelengths is a real effect.The estimates of the temperature gradient, which are based on the measurement of limb darkening, are small and negative (temperature decreases with depth) and may be insignificantly different from zero since they are only as large as their formal errors. We estimate that a temperature gradient in excess of 0.6K/m at 10m depth would have been observed. Thus, a temperature gradient like that measured in situ at the Apollo 15 and 17 landing sites in the upper 2m of the regolith is not typical of the entire lunar frontside at the 10m depths where the 49.3 cm wavelength emission originates. This result may indicate that the mean lunar heat flow is lower than that measured at the Apollo landing sites, that the thermal conductivity is greater at 10m depth than it is at 2m depth, or that the radio opacity is greater at 10m depth than at 2m depth. The negative estimates of the temperature gradient indicate that the Moon appeared limb bright and might be explained by scattering of the emission from boulders or an interface with solid rock. The presence of solid rock at 10m depths will probably cause heat flows like those measured by Apollo to be unobservable by our interferometric method at long wavelengths, since it will cause both the thermal conductivity and radio opacity of the regolith to increase. Thus, our data may be most consistent with a change in the physical properties of the regolith to those of solid rock or a mixture of rock and soil at depths of 7 to 16m. Our results show that future radio measurements for heat flow determinations must utilize wavelengths considerably shorter than 50 cm (25 cm or less) to avoid the rock regions below the regolith.     

Elastic collisions in ion irradiation experiments: A mechanism for space weathering of silicates

Ion irradiation experiments have been performed on silicates (bulk samples) rich of olivine, pyroxene, and serpentine to simulate the effects of space weathering induced on asteroids by solar wind ions. We have used different ions (H⁺, He⁺, Ar⁺, Ar²⁺) having different energies (from 60 to 400 keV) to weather the samples, probed by Raman spectroscopy and UV-vis-NIR reflectance spectroscopy. All the irradiated materials have shown reddening and darkening of reflectance spectra in the 0.25-2.7 μm spectral range. We have found that the increase of the spectral slope of the continuum across the 1-μm band is strongly related with the number of displacements caused by colliding ions because of elastic collisions with the target nuclei. The spectral slopes have been compared, at increasing ion fluence, with those from irradiated Epinal meteorite. We show that formation of nuclear displacements by solar wind ion irradiation is a physical mechanism that reddens the asteroidal surfaces on a time-scale lower than 10⁶ years.     

The petrology,geochemistry, and age of lunar regolith breccias Miller Range 090036 and 090070: Insights into the crustal history of the Moon

Meteorites ejected from the surface of the Moon as a result of impact events are an important source of lunar material in addition to Apollo and Luna samples. Here, we report bulk element composition, mineral chemistry, age, and petrography of Miller Range (MIL) 090036 and 090070 lunar meteorites. MIL 090036 and 090070 are both anorthositic regolith breccias consisting of mineral fragments and lithic clasts in a glassy matrix. They are not paired and represent sampling of two distinct regions of the lunar crust that have protoliths similar to ferroan anorthosites. ⁴⁰Ar‐³⁹Ar chronology performed on two subsplits of MIL 090070,33 (a pale clast impact melt and a dark glassy melt component) shows that the sample underwent two main degassing events, one at ~3.88 Ga and another at ~3.65 Ga. The cosmic ray exposure data obtained from MIL 090070 are consistent with a short (~8–9 Ma) exposure close to the lunar surface. Bulk‐rock FeO, TiO₂, and Th concentrations in both samples were compared with 2‐degree Lunar Prospector Gamma Ray Spectrometer (LP‐GRS) data sets to determine areas of the lunar surface where the regolith matches the abundances observed on the sample. We find that MIL 090036 bulk rock is compositionally most similar to regolith surrounding the Procellarum KREEP Terrane, whereas MIL 090070 best matches regolith in the feldspathic highlands terrane on the lunar farside. Our results suggest that some areas of the lunar farside crust are composed of ferroan anorthosite, and that the samples shed light on the evolution and impact bombardment history of the ancient lunar highlands.     

Identification of magnetite in lunar regolith breccia 60016: Evidence for oxidized conditions at the lunar surface

Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe‐oxide phase with disseminated domains of troilite. Fragments of plagioclase (An_95‐97), pyroxene (En_74‐75, Fs_21‐22,Wo_3‐4), and olivine (Fo_66‐67) are distributed in and adjacent to the Fe‐oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X‐ray absorption near edge spectroscopy (XANES) and X‐ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe³⁺/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe‐magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization‐driven mineral replacement of Fe‐metal or desulphurisation from Fe‐sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H₂O steam/liquid or CO₂ gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet impact.