Energetic proton irradiation history of the howardite parent body regolith and implications for ancient solar activity

Abstract— Previous studies have shown that the Kapoeta howardite, as well as several other meteorites, contains excess concentrations of cosmogenic Ne in the darkened, solar-irradiated phase compared to the light, non-irradiated phase. The two explanations offered for the nuclear production of these Ne excesses in the parent body regolith are either from galactic cosmic-ray proton (GCR) irradiation or from a greatly enhanced flux of energetic solar “cosmic-ray” protons (SCR), as compared to the recent solar flux. Combining new isotopic data we obtained on acid-etched, separated feldspar from Kapoeta light and dark phases with literature data, we show that the cosmogenic ²¹Ne/²²Ne ratio of light phase feldspar (0.80) is consistent with only GCR irradiation in space for ～3 Ma. However, the ²¹Ne/²²Ne ratio (0.68) derived for irradiation of dark phase feldspar in the Kapoeta regolith indicates that cosmogenic Ne was produced in roughly equal proportions from galactic and solar protons. Considering a simple model of an immature Kapoeta parent body regolith, the duration of this early galactic exposure was only ～3–6 Ma, which would be an upper limit to the solar exposure time of individual grains. Concentrations of cosmogenic ²¹Ne in pyroxene separates and of cosmogenic ¹²⁶Xe in both feldspar and pyroxene are consistent with this interpretation. The near-surface irradiation time of individual grains in the Kapoeta regolith probably varied considerably due to regolith mixing to an average GCR irradiation depth of ～10 cm. Because of the very different depth scales for production of solar ～Fe tracks, SCR Ne, and GCR Ne, the actual regolith exposure times for average grains probably differed correspondingly. However, both the SCR ²¹Ne and solar track ages appear to be longer because of enhanced production by early solar activity. The SCR/GCR production ratio of ²¹Ne inferred from the Kapoeta data is larger by a at least a factor of 10 and possibly as much as a factor of ～50 compared to recent solar particle fluxes. Thus, this study indicates that our early Sun was much more active and emitted a substantially higher flux of energetic (>10 MeV/nucleon) protons.     

Some things we can infer about the Moon from the composition of the Apollo 16 regolith

Abstract— Characteristics of the regolith of Cayley plains as sampled at the Apollo 16 lunar landing site are reviewed and new compositional data are presented for samples of <1 mm fines (“soils”) and 1–2 mm regolith particles. As a means of determining which of the many primary (igneous) and secondary (crystalline breccias) lithologic components that have been identified in the soil are volumetrically important and providing an estimate of their relative abundances, more than 3 × 10⁶ combinations of components representing nearly every lithology that has been observed in the Apollo 16 regolith were systematically tested to determine which combinations best account for the composition of the soils. Conclusions drawn from the modeling include the following. At the site, mature soil from the Cayley plains consists of 64.5% ± 2.7% components representing “prebasin” materials: anorthosites, feldspathic breccias, and a small amount (2.6% ± 1.5% of total soil) of nonmare, mafic plutonic rocks, mostly gabbronorites. On average, these components are highly feldspathic, with average concentrations of 31–32% Al₂O₃ and 2–3% FeO and a molar Mg/(Mg + Fe) ratio of 0.68. The remaining 36% of the regolith is syn- and postbasin material: 28.8% ± 2.4% mafic impact-melt breccias (MIMBs, i.e., “LKFM” and “VHA basalts”) created at the time of basin formation, 6.0% ± 1.4% mare-derived material (impact and volcanic glass, crystalline basalt) with an average TiO₂ concentration of 2.4%, and 1% postbasin meteoritic material. The MIMBs are the principal (80–90%) carrier of incompatible trace elements (rare earths, Th, etc.) and the carrier of about one-half of the siderophile elements and elements associated with mafic mineral phases (Fe, Mg, Mn, Cr, Sc). Most (71%) of the Fe in the present regolith derives from syn- and postbasin sources (MIMBs, mare-derived material, and meteorites). Thus, although the bulk composition of the Apollo 16 regolith is nominally that of noritic anorthosite, the noritic part (the MIMBs) and the anorthositic part (the prebasin components) are largely unrelated. There is compositional evidence that 3–4% of the soil is Th-rich material such as that occurring at the Apollo 14 site, and one fragment of this type was found among the small regolith particles studied here. If regolith such as that represented by the Apollo 16 ancient regolith breccias was a protolith of the present regolith, such regolith cannot exceed ～71% of the present regolith; the rest must be material added or redistributed since closure of the ancient regolith breccias. The postclosure material includes the mare-derived material and the Apollo-14-like component. Compositions of all mature surface soils from Apollo 16, even those collected 4 km apart on the Cayley plains, are very similar, which is in stark contrast to the wide compositional range of the lithologies of which the soil is composed. This uniformity indicates that the ratio of MIMBs to feldspathic prebasin components is not highly variable in the megaregolith over distances of a few kilometers, that there are no large, subsurface concentrations of “pure” mafic impact-melt breccia, and that the intimate mixing is inherent to the Cayley plains at a gross scale. Thus, the mixing of mafic impact-melt breccias and feldspathic prebasin components must have occurred during formation and deposition of the Cayley plains; such uniformity could not have been achieved by small postdeposition impacts into a stratified megaregolith. Using this conclusion as one constraint, and the known distribution of Th on the lunar surface as another, and the assumption that the Imbrium impact is primarily responsible for formation of the Cayley plains, arguments are presented that the Apollo 16 MIMBs derive from the Imbrium region, and, consequently, that one-fourth of the Apollo 16 regolith is primary Imbrium ejecta in the form of mafic impact-melt breccias.     

The regolith portion of the lunar meteorite Sayh al Uhaymir 169

Abstract— –Sayh al Uhaymir (SaU) 169 is a composite lunar meteorite from Oman that consists of polymict regolith breccia (8.44 ppm Th), adhering to impact‐melt breccia (IMB; 32.7 ppm Th). In this contribution we consider the regolith breccia portion of SaU 169, and demonstrate that it is composed of two generations representing two formation stages, labeled II and III. The regolith breccia also contains the following clasts: Ti‐poor to Ti‐rich basalts, gabbros to granulites, and incorporated regolith breccias. The average SaU 169 regolith breccia bulk composition lies within the range of Apollo 12 and 14 soil and regolith breccias, with the closest correspondence being with that of Apollo 14, but Sc contents indicate a higher portion of mare basalts. This is supported by relations between Sm‐Al₂O₃, FeO‐Cr₂O₃‐TiO₂, Sm/Eu and Th‐K₂O. The composition can best be modeled as a mixture of high‐K KREEP, mare basalt and norite/troctolite, consistent with the rareness of anorthositic rocks. The largest KREEP breccia clast in the regolith is identical in its chemical composition and total REE content to the incompatible trace‐element (ITE)‐ rich high‐K KREEP rocks of the Apollo 14 landing site, pointing to a similar source. In contrast to Apollo 14 soil, SaU 169 IMB and SaU 169 KREEP breccia clast, the SaU 169 regolith is not depleted in K/Th, indicating a low contribution of high‐Th IMB such as the SaU 169 main lithology in the regolith. The data presented here indicate the SaU 169 regolith breccia is from the lunar front side, and has a strong Procellarum KREEP Terrane signature.     

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.     

Lunar soil evolution processes and Apollo 16 core 60013/60014

Abstract— Soils of the 62-cm deep Apollo 16 double drive tube 60013/14 are mature at the top and submature at the bottom. Modal analyses of 5529 grains from the 90–150 μm and the 500–1000 μm. fractions from 12 levels of the core show that, in general, agglutinate abundance increases somewhat monotonically to the top and mimics the I_s/FeO profile. There is a general decrease in the modal abundance of monomineralic fragments towards the top, suggesting that agglutinates were formed in part at the expense of monomineralic grains, especially feldspars, which are by far the most abundant mineral in these soils. In detail, the top 27 cm of the core differs from the bottom 21 cm, and the middle 14 cm is intermediate in its properties. In the upper segment, variations in the abundances of feldspars correspond with those of feldspathic fragmental breccias and cataclastic anorthosites; in the bottom segment, a similar but weak correspondence between feldspars and crystalline matrix breccias is observed. Mixing of the comminuted products of these three rock types likely produced the bulk of the core material. Many single feldspars in all size fractions are remarkably fresh, show no damage from shock, and are similar in appearance to the large feldspars in anorthosites and feldspathic fragmental breccias, which we consider to be the primary sources of single feldspars in this core. Major (Na, Al, Si, K, Ca) and minor (Fe, Ba) element analyses of 198 single feldspar grains indicate the presence of only one population of feldspars, which is consistent with our interpretation of feldspar provenance. Classification of 890 monomineralic feldspar, olivine, pyroxene, and glass spherules on the basis of the presence or absence of thin brownish coating—related to reworking at the surface—shows that coated grains are much more abundant in the top segment than in the bottom segment. A comparison with the mixing and maturation model (McKay et al., 1977) of soils in the core 60009/10, some 60 m away from 60013/14, shows that mixtures of an immature, nearly pure plagioclase soil (dominant in 60009/10) and another immature, crystalline breccia-rich soil (dominant in 60013/14) may have matured through in situ reworking to produce the soils under investigation. We conclude that the soils in this core are products of mixing along soil evolution Path 2 of McKay et al. (1974). Superimposed on that soil column is the reworking of the upper part, which has evolved more recently along Path 1. This core thus represents a consanguineous column of the lunar regolith with an upper reworked segment.     

Apollo 17 regolith, 71501,262: A record of impact events and mare volcanism in lunar glasses

Abstract— Thirteen glasses from Apollo 17 regolith 71501,262 have been chemically analyzed by electron microprobe and isotopically dated with the ⁴⁰Ar/³⁹Ar dating method. We report here the first isotopic age obtained for the Apollo 17 very low titanium (VLT) volcanic glasses, 3630 ± 40 Ma. Twelve impact glasses that span a wide compositional range have been found to record ages ranging from 102 ± 20 Ma to 3740 ± 50 Ma. The compositions of these impact glasses show that some have been produced by impact events within the Apollo 17 region, whereas others appear to be exotic to the landing site. As the data sets that include compositions and ages of lunar impact glasses increase, the impact history in the Earth‐Moon system will become better constrained.     

A primary analysis of microwave brightness temperature of lunar surface from Chang-E 1 multi-channel radiometer observation and inversion of regolith layer thickness

In China’s first lunar exploration project, Chang-E 1 (CE-1), a multi-channel microwave radiometer was aboard the satellite, with the purpose of measuring microwave brightness temperature (Tb) from lunar surface and surveying the global distribution of lunar regolith layer thickness. In this paper, the primary 621 tracks of swath data measured by CE-1 microwave radiometer from November 2007 to February 2008 are collected and analyzed. Using the nearest neighbor interpolation to collect the Tb data under the same Sun illumination, global distributions of microwave brightness temperature from lunar surface at lunar daytime and nighttime are constructed. Based on the three-layer media modeling (the top dust-soil, regolith and underlying rock media) for microwave thermal emission of lunar surface, the CE-1 measured Tb and its dependence upon latitude, frequency and FeO + TiO₂ content, etc. are discussed. The CE-1 Tb data at Apollo landing sites are especially chosen for validation and calibration on the basis of available ground measurements. Using the empirical dependence of physical temperature upon the latitude verified by the CE-1 multi-channel Tb data at Apollo landing sites, the global distribution of regolith layer thickness is further inverted from the CE-1 brightness temperature data at 3 GHz channel. Those inversions at Apollo landing sites and the characteristics of regolith layer thickness for lunar maria are well compared with the Apollo in situ measurements and the regolith thickness derived from the Earth-based radar data. Finally, the statistical distribution of regolith thickness is analyzed and discussed.     

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.     

Monte Carlo simulation of GCR neutron capture production of cosmogenic nuclides in stony meteorites and lunar surface

Abstract— A purely physical model based on a Monte Carlo simulation of galactic cosmic ray (GCR) particle interaction with meteoroids is used to investigate neutron interactions down to thermal energies. Experimental and/or evaluated excitation functions are used to calculate neutron capture production rates as a function of the size of the meteoroid and the depth below its surface. Presented are the depth profiles of cosmogenic radionuclides ³⁶Cl, ⁴¹Ca, ⁶⁰Co, ⁵⁹Ni, and ¹²⁹I for meteoroid radii from 10 cm up to 500 cm and a 2π irradiation. Effects of bulk chemical composition on n‐capture processes are studied and discussed for various chondritic and lunar compositions. The mean GCR particle flux over the last 300 ka was determined from the comparison of simulations with measured ⁴¹Ca activities in the Apollo 15 drill core. The determined value significantly differs from that obtained using equivalent models of spallation residue production.     

Preservation potential of implanted solar wind volatiles in lunar paleoregolith deposits buried by lava flows

The lunar surface is bathed in a variety of impacting particles originating from the solar wind, solar flares, and galactic cosmic rays. These particles can become embedded in the regolith and/or produce a range of other molecules as they pass through the target material. The Moon therefore contains a record of the variability of the solar and galactic particle fluxes through time. To obtain useful temporal snapshots of these processes, discrete regolith units must be shielded from continued bombardment that would rewrite the record over time. One mechanism for achieving this preservation is the burial of a regolith deposit by a later lava flow. The archival value of such deposits sandwiched between lava layers is enhanced by the fact that both the under- and over-lying lava can be dated by radiometric techniques, thereby precisely defining the age of the regolith layer and the geologic record contained therein. The implanted volatile species would be vulnerable to outgassing by the heat of the over-lying flow, at temperatures exceeding 300-700 °C. However, the insulating properties of the finely particulate regolith would restrict significant heating to shallow depths. We have therefore modeled the heat transfer between lunar mare basalt lavas and the regolith in order to establish the range of depths below which implanted volatiles would be preserved. We find that the full suite of solar wind volatiles, consisting predominantly of H and He, would survive at depths of ∼13-290 cm (for 1-10 m thick lava flows, respectively). A substantial amount of CO, CO₂, N₂ and Xe would be preserved at depths as shallow as 3.7 cm beneath meter-thick flows. Given typical regolith accumulation rates during mare volcanism, the optimal localities for collecting viable solar wind samples would involve stacks of thin mare lava flows emplaced a few tens to a few hundred Ma apart, in order for sufficient regolith to develop between burial events. Obtaining useful archives of Solar System processes would therefore require extraction of regolith deposits buried at quite shallow depths beneath radiometrically-dated mare lava flows. These results provide a basis for possible lunar exploration activities.     

Isotopic composition of trapped and cosmogenic noble gases in several Martian meteorites

Abstract— Isotopic abundances of the noble gases were measured in the following Martian meteorites: two shock glass inclusions from Elephant Moraine (EET) 79001, shock vein glass from Shergotty and Yamato (Y) 793605, and whole-rock samples of Allan Hills (ALH) 84001 and Queen Alexandra Range (QUE) 94201. These glass samples, when combined with literature data on a separate single glass inclusion from EET 79001 and a glass vein from Zagami, permit examination in greater detail of the isotopic composition of Ne, Ar, Kr, and Xe trapped from the Martian atmosphere. The isotopic composition of Martian Ne, if actually present in these glasses, remains poorly defined. The ⁴⁰Ar/³⁶Ar ratio of trapped Martian atmospheric Ar is probably considerably lower than the nominal ratio of 3000 measured by Viking, and data on impact glasses suggest a value of ～1900. The atmospheric ³⁶Ar/³⁸Ar ratio is ≤4.0. Martian atmospheric Kr may be enriched in lighter isotopes by ～0.5%/amu compared to both solar-wind Kr and to the Martian composition previously reported. The isotopic composition of Xe in these glasses agrees with that previously reported in the literature. The Martian atmospheric ³⁶Ar/¹³²Xe and ⁸⁴Kr/¹³²Xe elemental ratios are higher than those reported by Viking by factors of ～2.5–1.6 (depending on the ⁴⁰Ar/³⁶Ar ratio adopted) and ～1.8, respectively, and are discussed in a separate paper. Cosmogenic gases indicate space exposure ages of 2.7 ± 0.6 Ma for QUE 94201 and Shergotty and 14 ± 1 Ma for ALH 84001. Small amounts of ²¹Ne produced by energetic solar protons may be present in QUE 94201 but are not present in ALH 84001 or Y-793605. The space exposure age for Y-793605 is 4.9 ± 0.6 Ma and appears to be distinctly older than the ages for basaltic shergottites. However, uncertainties in cosmogenic production rates still makes somewhat uncertain the number of Martian impact events required to produce the exposure ages of Martian meteorites.     

Vertical-structure effects on planetary microwave brightness temperature measurements: Applications to the lunar regolith

The effects of vertical variations in density and dielectric constant on nadir-viewing microwave brightness temperatures are examined. Stratification models as well as models of a continuous increase in density with depth are analyzed. Specific applications address the vertical structure of the lunar frontside regolith, utilizing combined constraints from Apollo data, bistatic radar signatures, and Earth-based measurements of the lunar microwave brightness temperature.Results have been analyzed in terms of the effects on the zeroth and first harmonic of the lunar disk-center brightness temperature variation over a lunation, and their wavelength dependence. Lunation-mean brightness temperatures, which are diagnostic of emissivity and steady-state sub-surface temperatures, are sensitive to both near-surface soil density gradients and single high-impedance dielectric contrasts. Models of the rapid density increase in the upper 5–10 cm of the lunar regolith predict brightness temperature decreases of 2–10°K between λ₀ = 3 and 30 cm. The magnitude of this spectral variation depends upon the thickness of a postulated low-density surface coating layer, and the magnitude of the density gradient in the transition soil layer. Comparable decreases in brightness temperature can be produced by a stratified two-layer model of soil overlaying bedrock if the high-density substrate lies within 1–2 m of the surface. Multiple soil layering on a centimeter scale, such as is observed in the Apollo core samples, is not likely to induce spectral variations in mean brightness temperature due to rapid regional variations in layer depths and thicknesses.The fractional variation in disk-center brightness temperature over a lunation (first harmonic) can be altered by vertical-structure effects only for the case in which a larger and abrupt dielectric contrast exists within the upper surface layer where the significant diurnal variations in physical temperature occur. Soil density variations do not cause scattering effects sufficient to significantly alter the microwave emission weighting function within the diurnal layer. For the Moon, this layer consists of the upper 10 cm. Since no widespread rock substrate as shallow as 10 cm exists in the lunar frontside, only volume scattering effects, due to buried shallow rock fragments, can explain the apparent high electrical loss inferred from Earth-based measurements of the amplitude of lunation brightness temperature variations.Representative models of the lunar frontside vertical structure have also been examined for their effects of radar cross-section measurements and resultant inferences of bulk dielectric constant. Models of the near-surface density gradient predict a significant increase in the remotely inferred dielectric constant value from centimeter to meter wavelengths. Such a model is in general agreement with the dielectric constant spectrum inferred from Earth-based brightness temperature polarization measurements, but is difficult to reconcile with the Apollo bistatic radar results at λ₀ = 13 and 116 cm.     

The production of cosmogenic nuclides by galactic cosmic‐ray particles for 2π exposure geometries

Abstract— We present a purely physical model for the calculation of depth‐dependent production rates in 2π exposure geometries by galactic cosmic rays (GCR). Besides the spectra of primary and secondary particles and the excitation functions of the underlying nuclear reactions, the model is based on the integral number of GCR particles in the lunar orbit. We derived this value from adjusting modeled depth profiles for ¹⁰Be, ²⁶Al, and ⁵³Mn to measured data from the Apollo 15 drill core. The J_0,GCR value of 4.54 cm^?2 s^?1 and the solar modulation parameter of M = 490 MeV determined this way for 1 AU is in reasonable agreement with the J_0,GCR value derived recently for the meteoroid orbits (Leya et al., 2000b). We also show that the mean GCR proton spectrum in the lunar orbit has not changed substantially over about the last 10 Ma. For the major target elements we present depth‐dependent production rates for ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl and ⁵³Mn, as well as for the rare gas isotopes ^20,21,22Ne. In addition we present production rates for ^36,38Ar from Fe and Ni. The new results are consistent with the data for stony meteoroids presented recently by our group (Leya et al., 2000b), but for the rare gas isotopes the new production rates sometimes differ significantly from earlier estimates. The applicability of the ²²Ne/²¹Ne ratio as a shielding parameter is also discussed.     

Apollo 12 ropy glasses revisited

Abstract— We analyzed ropy glasses from Apollo 12 soils 12032 and 12033 by a variety of techniques including SEM/EDX, electron microprobe analysis, INAA, and ³⁹Ar-⁴⁰Ar age dating. The ropy glasses have KREEP-like compositions different from those of local Apollo 12 mare soils; it is likely that the ropy glasses are of exotic origin. Mixing calculations indicate that the ropy glasses formed from a liquid enriched in KREEP and that the ropy glass liquid also contained a significant amount of mare material. The presence of solar Ar and a trace of regolith-derived glass within the ropy glasses are evidence that the ropy glasses contain a small regolith component Anorthosite and crystalline breccia (KREEP) clasts occur in some ropy glasses. We also found within these glasses clasts of felsite (fine-grained granitic fragments) very similar in texture and composition to the larger Apollo 12 felsites, which have a ³⁹Ar-⁴⁰Ar degassing age of 800 ± 15 Ma (Bogard et al, 1992). Measurements of ³⁹Ar-⁴⁰Ar in 12032 ropy glass indicate that it was degassed at the same time as the large felsite although the ropy glass was not completely degassed. The ropy glasses and felsites, therefore, probably came from the same source. Most early investigators suggested that the Apollo 12 ropy glasses were part of the ejecta deposited at the Apollo 12 site from the Copernicus impact Our new data reinforce this model. If these ropy glasses are from Copernicus, they provide new clues to the nature of the target material at the Copernicus she, a part of the Moon that has not been sampled directly.     

The Monahans chondrite and halite: Argon‐39/argon‐40 age,solar gases,cosmic‐ray exposure ages,and parent body regolith neutron flux and thickness

Abstract— The Monahans H‐chondrite is a regolith breccia containing light and dark phases and the first reported presence of small grains of halite. We made detailed noble gas analyses of each of these phases. The ³⁹Ar‐⁴⁰Ar age of Monahans light is 4.533 ± 0.006 Ma. Monahans dark and halite samples show greater amounts of diffusive loss of ⁴⁰Ar and the maximum ages are 4.50 and 4.33 Ga, respectively. Monahans dark phase contains significant concentrations of He, Ne and Ar implanted by the solar wind when this material was extant in a parent body regolith. Monahans light contains no solar gases. From the cosmogenic ³He, ²¹Ne, and ³⁸Ar in Monahans light we calculate a probable cosmic‐ray, space exposure age of 6.0 ± 0.5 Ma. Monahans dark contains twice as much cosmogenic ²¹Ne and ³⁸Ar as does the light and indicates early near‐surface exposure of 13–18 Ma in a H‐chondrite regolith. The existence of fragile halite grains in H‐chondrites suggests that this regolith irradiation occurred very early. Large concentrations of ³⁶Ar in the halite were produced during regolith exposure by neutron capture on ³⁵Cl, followed by decay to ³⁶Ar. The thermal neutron fluence seen by the halite was (2–4) × 10¹⁴ n/cm². The thermal neutron flux during regolith exposure was ～0.4‐0.7 n/cm²/s. The Monahans neutron fluence is more than an order of magnitude less than that acquired during space exposure of several large meteorites and of lunar soils, but the neutron flux is lower by a factor of ≤5. Comparison of the ³⁶Ar_n/²¹Ne_cos ratio in Monahans halite and silicate with the theoretically calculated ratio as a function of shielding depth in an H‐chondrite regolith suggests that irradiation of Monahans dark occurred under low shielding in a regolith that may have been relatively shallow. Late addition of halite to the regolith can be ruled out. However, irradiation of halite and silicate for different times at different depths in an extensive regolith cannot be excluded.     

Cosmogenic radionuclides and noble gases in Antarctic H chondrites with high and normal natural thermoluminescence levels

Abstract— We report noble gas data for 37 H chondrites collected from the Allan Hills by EUROMET in the 1988–1989 field season. Among these are 16 specimens with high levels (>100 krad) of natural thermoluminescence (NTL), originally interpreted as signaling their derivation from a single meteoroid with an orbit that became Earth‐crossin‐100 ka ago. One of these 16 is an H3 chondrite with a cosmic‐ray exposure age of ～33 Ma and clearly represents a separate fall. The other 15 H4–6 chondrites derive from three separate meteoroids, each of which is represented by a five or six member group. These groups have mean exposure ages of 3.7, 4.1, and 6.6 Ma: the middle‐group members all contain solar Ne. The two younger groups also seem to each include a few H chondrites with normal NTL levels. Measurements of cosmogenic ¹⁰Be (1.5 Ma), ²⁶AI (710 ka), and ³⁶CI (301 ka) in 14 of the high‐NTL chondrites indicate that all reflect a simple irradiation history. In contrast, many of a different (38 member) randomly selected suite of Antarctic H chondrites seem to have different cosmic‐ray irradiation histories. The 3.7 and 6.6 Ma groups from the 37 member Allan Hills suite come from about 5–30 and about 5–10 cm depths in 80–125 and 60–125 cm radius meteoroids, respectively.     

Lunar microwave brightness temperature observations reevaluated in the light of Apollo program findings

Remote observations of the lunar radiowave emission are reexamined in the light of physical property data accumulated through the Apollo program. It is found that thermal and electrical properties determined for a number of different landing sites yield theoretical results in good agreement with remote observations for millimeter and short centimeter wavelengths. Theoretical models incorporating reflecting layers of rock and physical property data from the Apollo program are compared to the longer wavelength (5–500 cm) observational data to estimate a disk average steady state heat flow and a mean depth of the lunar regolith. It is found that a high heat flow, comparable to the heat flows measured at the Apollo 15 and 17 sites, is required to fit the available 5–20 cm wavelength remote data, and that a lunar surface layer relatively free of large boulders within the upper 10–30 m best fits the observations of a decreasing brightness temperature with wavelength for wavelengths greater than ～ 50 cm.     

The early geological history of the Moon inferred from ancient lunar meteorite Miller Range 13317

Miller Range (MIL) 13317 is a heterogeneous basalt‐bearing lunar regolith breccia that provides insights into the early magmatic history of the Moon. MIL 13317 is formed from a mixture of material with clasts having an affinity to Apollo ferroan anorthosites and basaltic volcanic rocks. Noble gas data indicate that MIL 13317 was consolidated into a breccia between 2610 ± 780 Ma and 1570 ± 470 Ma where it experienced a complex near‐surface irradiation history for ~835 ± 84 Myr, at an average depth of ~30 cm. The fusion crust has an intermediate composition (Al₂O₃ 15.9 wt%; FeO 12.3 wt%) with an added incompatible trace element (Th 5.4 ppm) chemical component. Taking the fusion crust to be indicative of the bulk sample composition, this implies that MIL 13317 originated from a regolith that is associated with a mare‐highland boundary that is KREEP‐rich (i.e., K, rare earth elements, and P). A comparison of bulk chemical data from MIL 13317 with remote sensing data from the Lunar Prospector orbiter suggests that MIL 13317 likely originated from the northwest region of Oceanus Procellarum, east of Mare Nubium, or at the eastern edge of Mare Frigoris. All these potential source areas are on the near side of the Moon, indicating a close association with the Procellarum KREEP Terrane. Basalt clasts in MIL 13317 are from a very low‐Ti to low‐Ti (between 0.14 and 0.32 wt%) source region. The similar mineral fractionation trends of the different basalt clasts in the sample suggest they are comagmatic in origin. Zircon‐bearing phases and Ca‐phosphate grains in basalt clasts and matrix grains yield ²⁰⁷Pb/²⁰⁶Pb ages between 4344 ± 4 and 4333 ± 5 Ma. These ancient ²⁰⁷Pb/²⁰⁶Pb ages indicate that the meteorite has sampled a range of Pre‐Nectarian volcanic rocks that are poorly represented in the Apollo, Luna, and lunar meteorite collections. As such, MIL 13317 adds to the growing evidence that basaltic volcanic activity on the Moon started as early as ~4340 Ma, before the main period of lunar mare basalt volcanism at ~3850 Ma.     

Acfer 217-A new member of the Rumuruti chondrite group (R)

Abstract— Previously, three meteorites from Australia and Antarctica were described as a new chondritic “grouplet” (Carlisle Lakes, Allan Hills (ALH) 85151, Yamato (Y) ?75302; Rubin and Kallemeyn, 1989). This grouplet was classified as the “Carlisle Lakes-type” chondrites (Weisberg et al., 1991). Recently, one Saharan sample and four more Antarctic meteorites were identified to belong to this group (Acfer 217, Y-793575, Y-82002, PCA91002, PCA91241). The latter two are probably paired. With the meteorite Rumuruti, the first fall of this type of chondrite is known (Schulze et al., 1994). We report here on the Saharan meteorite Acfer 217 which has chemical and mineralogical properties very similar to Rumuruti and Carlisle Lakes. All eight members of this group, Rumuruti, Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, and the paired samples PCA91002 and PCA91241 justify the introduction of a new group of chondritic meteorites, the Rumuruti meteorites (R). Acfer 217 is a regolith breccia consisting of up to cm-sized clasts (～33 vol%) embedded in a fine-grained, well-lithified clastic matrix. The most abundant mineral is olivine (～72 vol%), which has a high Fa-content of 37–39 mol%. The major minerals (olivine, low-Ca pyroxene, Ca-pyroxene, and plagioclase) show some compositional variability indicating a slightly unequilibrated nature of the meteorite. Considering the mean olivine composition of Fa_{37.8 ± 5.7}, a classification of Acfer 217 as a R3.8 chondrite would result; however, Acfer 217 is a regolith breccia consisting of clasts of various petrologic types. Therefore, we suggest to classify Acfer 217 as a R3–5 chondrite regolith breccia. The bulk meteorite is very weakly shocked (S2). The bulk composition of Acfer 217 and other R-meteorites show that the R-meteorites are basically chondritic in composition. The pattern of moderately volatile elements is unique in R chondrites; Na and Mn are essentially undepleted, similar to ordinary chondrites, while Zn and Se contents are similar to concentrations in CM chondrites. The oxygen isotopic composition in Acfer 217 is similar to that of Rumuruti, Carlisle Lakes, ALH 85151, and Y-75302. In a δ¹⁷O vs. δ¹⁸O-diagram, the R-meteorites form a group well resolved from other chondrite groups. Acfer 217 was a meteoroid of common size with a radius between 15–65 cm and with a single stage exposure history. Based on ²¹Ne, an exposure age of about 35 Ma was calculated.     

Constraints on the depth and variability of the lunar regolith

Abstract— Knowledge of regolith depth structure is important for a variety of studies of the Moon and other bodies such as Mercury and asteroids. Lunar regolith depths have been estimated using morphological techniques (i.e., Quaide and Oberbeck 1968; Shoemaker and Morris 1969), crater counting techniques (Shoemaker et al. 1969), and seismic studies (i.e., Watkins and Kovach 1973; Cooper et al. 1974). These diverse methods provide good first order estimates of regolith depths across large distances (tens to hundreds of kilometers), but may not clearly elucidate the variability of regolith depth locally (100 m to km scale). In order to better constrain the regional average depth and local variability of the regolith, we investigate several techniques. First, we find that the apparent equilibrium diameter of a crater population increases with an increasing solar incidence angle, and this affects the inferred regolith depth by increasing the range of predicted depths (from ～7–15 m depth at 100 m equilibrium diameter to ～8–40 m at 300 m equilibrium diameter). Second, we examine the frequency and distribution of blocky craters in selected lunar mare areas and find a range of regolith depths (8–31 m) that compares favorably with results from the equilibrium diameter method (8–33 m) for areas of similar age (～2.5 billion years). Finally, we examine the utility of using Clementine optical maturity parameter images (Lucey et al. 2000) to determine regolith depth. The resolution of Clementine images (100 m/pixel) prohibits determination of absolute depths, but this method has the potential to give relative depths, and if higher resolution spectral data were available could yield absolute depths.