Solar Wind Gases Research Articles

Regolith and Megaregolith Formation of H-Chondrites: Thermal Constraints on the Parent Body

Spectral reflectivity data and its location near an orbital resonance suggest that Asteroid 6 Hebe may be the source body for H-chondrites, the second largest meteorite group. Recent spacecraft images of asteroids and theoretical modeling indicate that, contrary to previous ideas, asteroids can retain thick regoliths. We model the thermal evolution of a Hebe-sized object coated with a thick insulating regolith and heated by26Al and other long-lived radionuclides. The heat conduction equations for spherically symmetric objects were solved using finite-difference approximations. We assumed a three-layer structure with regolith and megaregolith overlying a rocky core. The three layers differed in bulk density, porosity, and thermal conductivity. Interior peak temperatures were set to match metamorphic temperatures of H6 chondrites.The regolith has a major influence on thermal history, and the results are very different from those for a simple rocky body published by various authors. Regolith insulation produces a uniform interior peak temperature of ∼1250 K and moves the petrographic type boundaries close to the surface of the parent body. Petrologic types 3–6 can be produced within 10 km of the asteroid's surface with only moderate (∼1 km) regolith thicknesses. The calculations indicate that H4–H6 formation would be consistent with the cooling rate estimates and Pb–Pb formation ages if the material originated in the near surface regions. We suggest that many if not all H-chondrites could have been formed in a megaregolith and thick regolith. Their observed properties are consistent with this environment, especially the abundance of regolith breccias and H-chondrites of all petrologic types with implanted solar wind gases.

Two ion fluid numerical investigations of solar wind gas releases

A two ion species system of fluid equations is used to investigate mechanisms responsible for transverse accelerations of plasma releases into high Mach number flows, such as that of the December 1984 Active Magnetospheric Particle Tracer Experiment (AMPTE) release of barium ions into the solar wind. We extend previous work in single ion species systems by developing a set of modified MHD equations which incorporates both the unmagnetized nature of the released ions and the interactions of the two ion fluids. An analysis of this system of equations suggests two mechanisms that could cause transverse shifts of the plasma release. One of these is based on a magnetic drift wave, the other on a hybrid cyclotron mode. Numerical simulations produce clearly observable shifts in the case of each mechanism. The modified two‐fluid MHD equations, simulation results, and physical mechanisms are discussed, and their relevance to the AMPTE barium release is examined.

A Review of3He Resources and Acquisition for Use as Fusion Fuel

A combination of man-made and natural resources on earth could provide sufficient 3He fuel for the technological development of D-3He fusion reactors. Helium exists in natural gas wells; however, at the current rate of natural gas usage, this resource would provide <5 kg/yr of 3He. The radioactive decay of 3H produced in fission production reactors could yield 110 kg of 3He by the year 2000 if it were retained. Apparently, a large amount of 3He exists within the earth’s mantle, but it is inaccessible. A significant quantity of 3He, which could be imported to supply a fusion power industry on earth for hundreds of years, exists on the moon. The solar wind has deposited >1 million tonnes of 3He in the fine regolith that covers the surface of the moon. The presence of this solar wind gas was confirmed by analyses of the lunar regolith samples brought to earth. A strong correlation is noted between the helium retained and the TiO2 content of the regolith; consequently, remote-sensing data showing high-titanium-bearing soils in the lunar maria areas have been used to locate potentially rich sites for helium extraction. Surface photographs of Mare Tranquillitatis have shown that nearly 50% of this mare may be minable and capable of supplying ∼7100 tonnes of 3He. A mobile mining vehicle is proposed f or use in the excavation of the soil and the release of the helium and other solar wind gases. The evolved gases would be purified by a combination of permeators and cryogenic techniques to provide a rich resource of H2, helium, CO2, H2O, and N2, followed by helium isotopic separation systems. The energy and financial payback from those operations are substantial when this fuel is utilized in a D-3He fusion reactor located on earth. The implementation of this mining operation would have minimal impact on the lunar environment. Several legal regimes ensure that such a lunar enterprise can be implemented without severely disrupting international order.

Composition of solar wind noble gases released by surface oxidation of a metal separate from the Weston meteorite

Surface-sited noble gases were released from metal separates from the Weston meteorite by step-wise oxidation at temperatures below 400°C. Gas release from residual silicates in the separates was successfully suppressed by prior pyrolysis at somewhat higher temperatures than the combustions. Helium, neon and argon extracted from the metal show clear evidence of a solar wind signature. Xenon and nitrogen, on the other hand, were dominated by air, so that no solar component could be identified for these gases. Uncertainties on krypton isotopes were too large to yield useful conclusions. Helium, neon and argon elemental and isotopic ratios appear to depend on depth within the metal grains, in most cases becoming heavier with increasing oxidation of the metal. The cause of this dependence on amount of oxidation is not yet known. Ratios derived for the surface-sited component are therefore somewhat model dependent. Our best estimates are that, at the time of Weston's exposure, the solar wind 4He 36Ar ratio was around 3.6 × 10 4 , 20Ne 36Ar was close to 45, 4He 3He was about 2500, 20Ne 22Ne about 13.3–13.4, 21Ne 22Ne about 0.033–0.035, and 36Ar 38Ar about 5.5–5.7. These ratios indicate that Weston did not acquire its solar wind gases from a recent exposure to solar wind, but more probably at a time in the past similar to or even earlier than the exposure time of Apollo 17 breccias such as 79035. The 36Ar 38Ar ratio, in conjunction with other recent determinations of this value, suggests that it is no longer appropriate to assume that the solar and terrestrial values are equivalent.

On the origin of Venus' atmosphere: Possible contributions from simple component mixtures and fractionated solar wind

Three-element correlation plots are used to assess the possible origin of C, N, and the noble gas elements in the atmosphere of Venus from various two-component mixtures and from elemental fractionation of solar gases during solar wind implantation and during low temperature adsorption. An origin of Venus' atmosphere by elemental fractionation of implanted solar wind gases into pre-Venus materials is approximately consistent wit Ar, Kr, Xe, and N, but does not readily explain the higher abundance of C and the lower abundance of Ne. Some simple two-component mixtures seem consistent with most of the data, especially a mixture of a Earth-like component with a lunar-like component that represents fractionated solar wind gases. Simple two-component mixtures with one component resembling volatiles in C1 chondrites are not consistent with the data and suggest that a significan fraction of these volatiles on Venus could not have been contributed by a C1-like veneer. None of these models can readily explain the low Ne/Ar ratio for Venus. Even lower Ne/Ar ratios observed in some meteorites suggest, however, that some mechanism existed in the inner Solar System to produce large Ne/Ar fractionation relative to the solar value. Preferential adsorption of Xe on C- and N-rich phases forming at low temperatures from the solar nebula may have caused the observed larger concentrations of C, N, and Xe relative to Ar, Kr, and Ne in carbonaceous meteorites compared to the atmospheres of the terrestrial planets. If so, the apparent deficiency of Xe relative to other noble gases for the Earth, Mars, and Venus compared to carbonaceous meteorites would not be due to hidden Xe in each planet, as has been previously suggested.

Accretion, metamorphism, and brecciation of ordinary chondrites: Evidence from petrologic studies of meteorites from Roosevelt County, New Mexico

We have made petrologic studies of a suite of 29 newly discovered ordinary chondrites from Roosevelt County, New Mexico, including an H3 chondrite that is rich in solar wind gases and contains clasts of melt rock and graphite‐magnetite‐rich material, and an L5 chondrite with a unique igneous inclusion. Our data and published work show that mean CaO concentrations of low‐Ca pyroxene can be used to distinguish petrologic types in H, L, and LL groups. Mean CaO concentrations in olivine show significant variations among equilibrated chondrites, but these are not correlated with petrologic type; varied cooling rates could be responsible. The degree of heterogeneity of FeO concentrations in olivines of types 4–6 is also not correlated with degree of metamorphism; it may be controlled more by postmetamorphic brecciation and mixing of material than by metamorphism. Mean FeO concentrations of silicates show average increases of 3–5% from type 4 to type 6 in each group. This increase and the accompanying increase in bulk concentrations of siderophile elements appear to predate parent body formation. Correlation of nebula‐controlled bulk chemistry and planetary metamorphic effects could only have been produced under special conditions of accretion. One possible origin is runaway growth, which allowed parent bodies to form by accretion kilometer‐sized planetesimals to larger bodies, accompanied by differential movement of chemically distinct large and small bodies to produce radially zoned parent bodies that were subsequently metamorphosed by internal heating. Alternatively, ordinary chondrites may have been metamorphosed in a variety of planetesimals, each of which had a uniform chemical composition and metamorphic temperature. Correlations between composition and temperature may have arisen if both were correlated with another parameter, such as heliocentric distance.

Nitrogen isotopes in lunar highlands breccias

Some breccias from the lunar highlands have probably trapped solar wind gases at a very early epoch in the history of the moon, as implied by their high contents of parentless fissiogenic xenon and sometimes, of parentless radiogenic 129Xe. Four samples of this type, on which noble gas data already exist, have been selected for analysis of nitrogen contents and isotopic composition, by using step-wise heating techniques: 14047, 14055, 14307, 60255. Since uncertainties in the evolution of the solar wind 15N/ 14N ratio with time are due in part to uncertainties in the measurement of the epoch of exposure, those samples provided the opportunity to measure the isotopic composition of nitrogen which has been trapped in the remote past, avoiding the problems inherent in the use of spallogenic nuclides. Results show that, in the samples studied from the Apollo 14 landing site, nitrogen is not particularly light, and has not been acquired, as a whole, in very ancient times. The conflicting presence of both parentless xenon and nitrogen of relatively “recent” isotopic signature can be explained if the hypothetical light nitrogen is diluted by more abundant, heavier nitrogen. Accordingly, the very ancient soil components which are implied in these objects by the presence of excess fission xenon have been re-exposed at a much later epoch, or mixed with some younger soil components, before the compaction event. The present data do not question the hypothesis of a secular isotopic variation of lunar trapped nitrogen, but cannot prove that very light nitrogen was trapped together with parentless fission xenon in the soil components of the highlands soil breccias. The very unusual release pattern of nitrogen in breccia 60255 can result from nitrogen isotopic homogenization with gas loss.

Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy

21Ne cosmic-ray exposure ages have been calculated from literature data for 201 H, 203 L and 38 LL chondrites, corrected for shielding differences when possible. The distributions of exposure ages again show the familiar peaks at 4.5 and 20 Myr for the H's, but no outstanding events for the L's and LL's. If the L-chondrite distribution is interpreted as a series of discrete events, then at least 6 peaks between 1 and 35 Myr are needed to model it. The observations, that every petrologic type occurs in every large peak and that even the higher petrologic types contain solar wind gases, suggest that the parent bodies have been fragmented and reassembled into a megabreccia. For the H chondrites, both large and small peaks contain about 15% solar-gas bearing meteorites, which could mean that surface material has been mixed to depths represented by the largest event, on the order of a kilometer. In contrast, only 3% of the L's contain solar wind, which may be related to breakup of their parent planet. Those L's with especially low radiogenic He (U,Th-He ages < 1 AE) tend to have low exposure ages: their distribution may be biased by a subgroup that had orbits coupling short capture lifetimes with significant solar heating.

Metallic minerals, thermal histories and parent bodies of some xenolithic, ordinary chondrite meteorites

We have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes, using reflected-light microscopy and electron-probe analysis. In Weston and Fayetteville, which both contain solar-flare tracks and solar-wind gases, kamacite, taenite and tetrataenite (ordered FeNi) and troilite show a variety of textures. On a Wood plot of central Ni content vs dimension, taenite analyses scatter as if metal grains cooled at rates of 10–1000 and 1–100 K/Myr respectively through 700 K, although metal in an H6 clast in Fayetteville plots coherently with a cooling rate of 50 K/Myr. We propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted, and were not subsequently heated above 650 K. We predict a similar history for all gas-rich ordinary chondrites. By contrast, metallic minerals throughout Bhola and Mezö-Madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to ~600 K of 0.1 and 1 K/Myr, respectively. We conclude that host and xenoliths in both chondrites were slowly cooled after compaction. Thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 K in different environments. According to the conventional onion-shell model for H, L or LL chondrite parent bodies, material of petrologic types 3–5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately. But if peak metamorphic temperatures were reached during, not after accretion, as seems plausible, maximum metamorphism may have occurred in planetesimals <10 km in radius. Cooling through 700 K may then have occurred in larger bodies that accreted from these planetesimals. Iron meteorites, mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies.

Further studies on cosmic spherules from deep-sea sediments

We presume that cosmic spherules are ‘sparks’ formed by collisions in the asteroidal belt. With other debris they spiral to earth under the Poynting-Robertson effect. During a 106year residence in space, they become saturated with solar-wind gas. These 100 pm diameter spherules will survive a grazing atmospheric flight but on a more plunging flight they may melt. If this happens, the solar-wind gas develops sufficient pressure to explode the spherule. We support these proposals with (i) a size distribution analysis, which shows that spherules are destroyed if they become molten and also suggests that spherules make up 10 % of the zodiacal cloud, (ii) an examination of the spherules with the scanning electron microscope (s.e.m.), which strongly suggests that they have remained solid during atmospheric flight, (iii) an investigation of grindwheel sparks, which shows that forms resembling iron cosmic spherules can be reproduced, if sparks are quenched close to the wheel. X-ray diffraction analysis and s.e.m. photography yield convincing evidence that stony cosmic spherules are not ablation droplets from the crust of stony meteorites and, by implication, are not derived from meteors. The magnetite lattice parameter for the spherules is less than that for the meteorite crust. The fayalite fraction of the olivine m the spherules is close to that occurring in the interior of the meteorite and quite distinct from that occurring in its crust. The spherules are coarsely crystalline, whereas the meteorite crust appears ‘glassy’.

Model Calculations of Solar Wind Expansion Including an Enhanced Fraction of Ionizing Electrons

Collective interactions of the solar wind and newly ionized interstellar gas cause turbulent electron heating to ionizing energies analogous to laboratory experiments on the critical ionization velocity effect. Implications for solar wind and interstellar gas dynamics are calculated by simultaneously solving continuity equations for solar wind protons, interstellar hydrogen atoms, and energetic electrons. Electron impact ionization is shown to be practically as important as photoionization, giving rise to a stronger deceleration and heating of the distant solar wind, a weaker terminating shock, a smaller stand-off distance of the helio pause, and implying higher densities of the outer solar wind and the interstellar neutral gas.

Electronic absorption spectra of lunar minerals

Transition metals play an important rôle in the lunar rock-forming silicates, occurring particularly in pigeonite, augite, olivine and pyroxferroite. Measurements of the absorption spectra of lunar silicates can provide information on the oxidation states and co-ordinations of the transition metal ions. Such measurements lead-us to conclude that Ti3+ as well as Fe2+ and Ti4+ ions are present in lunar silicates. Fe3+ ions do not occur in concentrations greater than a few parts per million, and the spectral evidence for Cr2+ ions in olivines requires further substantiation - whereas occurrence of Cr3+ is very likely. The occurrence of low oxidation states in the transition elements of the lunar rocks suggests that low oxygen fugacities prevailed during the formation of the lunar crust. However, other reduction mechanisms postulated include release of solar-wind gas, and pressure-induced reduction on meteorite impact or during magma genesis at depth. Correlations of laboratory measurements with remote telescopic measurements confirm that major features in remote reflectance profiles arise from absorption by pyroxenes, enabling average pyroxene compositions to be determined for areas of the lunar surface. Intense absorption towards the ultraviolet-blue end of the spectrum corresponds to high titanium contents, as confirmed by molecular orbital calculations which support the use of remote spectral measurements to map titanium-rich areas of the Moon.

On the interaction of interplanetary shock waves with cometary atmospheres

The interaction of interplanetary shock waves with cometary atmospheres is described briefly in terms of ‘shock-bow shock’ and ‘shock-tangential discontinuity’ interactions. The tangential discontinuity is used to describe the boundary between the cometary and solar wind gases. Instability of the tangential discontinuity and its temporary breakup following impact by an interplanetary shock wave would, it is suggested, be followed by mixing between the solar wind and cometary plasma. It is also suggested that this physical process, part of which has already been studied extensively, is a principal factor responsible for cometary brightness variations following solar flare and/or stream-stream activity.

Primordial neon, helium, and hydrogen in oceanic basalts

A primordial neon component in neon from Kilauea Volcano and deep-sea tholeiite glass has been identified by the presence of excess 20Ne; relative to atmospheric neon the 20Ne enrichments are 5.4% in Kilauea neon and about 2.5% in the basalts. The 20Ne anomalies are associated with high 3He/ 4He ratios; the ratio in Kilauea helium is 15 times the atmospheric ratio, while mid-ocean ridge basalts from the Atlantic, Pacific, and Red Sea have uniform ratios about 10 times atmospheric. Mantle neon and helium are quite different in isotopic composition from crustal gases, which are highly enriched in radiogenic 21Ne and 4He. The 21Ne/ 4He ratios in crustal gases are consistent with calculated values based on G. Wetherill's 18O (α,n) reaction; the lack of 20Ne enrichment in these gases shows that the mantle 20Ne anomalies are not radiogenic. 21Ne enrichments in Kilauea neon and “high- 3He” Pacific tholeiites are much less than in crustal neon, about 2 ± 2% vs. present atmospheric neon, as expected from the much lower 4He/Ne ratios. Neon concentrations in two Atlantic tholeiites were found to be only 1–2% of the values obtained by Dymond and Hogan; helium concentrations are slightly greater and our He/Ne ratios are greater by a factor of 150. The large Ne excess relative to solar wind and meteoritic gases is thus not confirmed. Pacific and Atlantic basalts appear to be quite different in He/Ne ratios however, and He and Ne may be inversely correlated. He concentration variations due to diffusive loss can be distinguished from variations due to two-phase partitioning or mantle heterogeneity by the effects on 3He/ 4He ratios. The He isotopic and concentration measurements on “low- 3He” basalts are consistent with diffusive loss and dilution of the 3/4 ratio by in-situ radiogenic 4He, and may provide a method for dating basalt glasses. Deuterium/hydrogen ratios in Atlantic and Pacific tholeiite glasses are 77% lower than the ratio in seawater. The inverse correlation between deuterium and water content observed by Friedman in erupting Kilauea basalts is consistent with a Rayleigh separation process in which magmatic water is separated from an initial melt with the same D/H ratio as observed in deep-sea tholeiites. The consistency of the D/H ratios in tholeiites containing primordial He and Ne components indicates that these ratios are probably characteristic of primordial or juvenile hydrogen in the mantle.

On the model of the solar wind-interstellar medium interaction with two shock waves

The model of the solar wind interaction with interstellar medium suggested by Baranovet al. (1970) is developed. In this model (TSM) the presence of two shock waves is assumed, through which the solar wind and interstellar gas pass, the latter moving relative to the Sun at supersonic speed (20 km s−1). The distance between shocks was considered earlier (Baranovet al., 1970; Baranov and Krasnobaev, 1971) to be small compared with their distance from the Sun, due to the hypersonic character of the flow. The structure of the subsonic flow portion may not be taken into account. In the present paper the distribution of the gas parameters in the region between shocks is calculated which, in particular, allows us to estimate the possibility of its experimental detection, observing radio-scintillation on interstellar irregularities (Baranovet al., 1975). The possible influence on the model of galactic hydrogen neutral atoms penetrating into interplanetary medium is estimated.

Formation of the lunar atmosphere

Measurements of40Ar and helium made by the Apollo 17 lunar surface mass-spectrometer are used in the synthesis of atmospheric supply and loss mechanisms. The argon data indicate that about 8% of the40Ar produced in the Moon due to decay of40K is released to the atmosphere and subsequently lost. Variability of the atmospheric abundance of argon requires that the source be localized, probably in an unfractionated, partially molten core. If so, the radiogenic helium released with the argon amounts to 10% of the atmospheric helium supply. The total rate of helium escape from the Moon accounts for only 60% of the solar windα particle influx. This seems to require a nonthermal escape mechanism for trapped solar-wind gases, probably involving weathering of exposed soil grain surfaces by solar wind protons.

Do stony meteorites come from comets?

The place of origin of stony meteorites can be determined from their trapped solar-wind gases. “Gas-rich” meteorites have only 10 −3−10 −4 the solar noble gas content and ⩽10 −2−10 −4 the surface exposure age of lunar soils. These differences suggest that the gas implantation took place between 1 and 8 AU from the Sun, in a region where the cratering rate was 10 2−10 3 times higher than at 1 AU. Both characteristics point to the asteroid belt. The predicted Ne 20 content a gas-rich meteorite formed at 2.5 AU is 1.2 × 10 −5 cc STP g −1, compared to an observed mean for H-chondrites of 0.5 × 10 −5 cc STP g −1. The observed prevalence of gas-rich meteorites (40–100% among carbonaceous chondrites, 2–33% among other classes) requires that the parent body remained long enough in the asteroid belt to develop a substantial regolith. This condition can be met by asteroids (∼ 10% of mass converted to regolith.in 4.5 × 10 9 yr), but not by short period comets (∼0.04% converted in 10 7 yr). It appears that a cometary origin can be ruled out for all stony meteorite clases that have gas-rich members. This includes carbonaceous chondrites.

Electron Microscopy of Irradiation Effects in Space

The paper discusses several discoveries made in the study of lunar material. In the examination of the effects of solar wind implantations the topics covered include (1) solar wind radiation damage parameters and their aging characteristics, (2) the theory of the ancient solar wind, (3) the solar wind sputtering erosion rate, (4) the physicochemical properties of amorphous coatings, (5) maturity indexes and the macroscopic properties of the lunar regolith, (6) solar wind gas bubbles, (7) the composition of very heavy nuclei in the contemporary solar wind, and (8) track aging processes. Conclusions are drawn from the results about other extraterrestrial features such as the parent bodies of the meteorites, early solar nebulas, and interstellar clouds.

Solar wind and cosmogenic rare gases in Luna 16 and 20 soils and their correlation with cosmic ray produced fossil tracks in lunar samples

The elemental and isotopic composition of Ne, Ar, Kr and Xe were measured in the bulk soil samples obtained from Luna 16 and 20 missions. The average cosmic ray exposure ages for the Luna 16 soils is higher than that of Luna 20 soils. The trapped solar wind composition is deduced by correcting for the cosmogenic component and is found to be in good agreement with the values published by others. The correlation between the solar wind gases and cosmic ray tracks in lunar samples is quantitatively studied and some systematic features regarding lunar regolith dynamics are tentatively established.

Trapped solar and cosmogenic noble gas abundances in Apollo 15 and 16 deep drill samples

Abundances and isotopic compositions of all the stable noble gases have been measured in 19 different depths of the Apollo 15 deep drill core, 7 different depths of the Apollo 16 deep drill core, and in several surface fines and breccias. All samples analyzed from both drill cores contain large concentrations of solar wind implanted gases, which demonstrates that even the deepest layers of both cores have experienced a lunar surface history. For the Apollo 15 core samples, trapped 4He concentrations are constant to within a factor of two; elemental ratios show even greater similarities with mean values of 4He/ 22Ne= 683±44 , 22Ne/ 36Ar= 0.439±0.057 , 36Ar/ 84Kr= 1.60±0.11·10 3 , and 84Kr/ 132Xe= 5.92±0.74 . Apollo 16 core samples show distinctly lower 4He contents, 4He/ 22Ne(567±74) , and 22Ne/ 36Ar(0.229±0.024) , but their heavy-element ratios are essentially identical to Apollo 15 core samples. Apollo 16 surface fines also show lower values of 4He/ 22Ne and 22Ne/ 36Ar. This phenomenon is attributed to greater fractionation during gas loss because of the higher plagioclase contents of Apollo 16 fines. Of these four elemental ratios as measured in both cores, only the 22Ne/ 36Ar for the Apollo 15 core shows an apparent depth dependance. No unambiguous evidence was seen in these core materials of appreciable variations in the composition of the solar wind. Calculated concentrations of cosmic ray-produced 21Ne, 80Kr, and 126Xe for the Apollo 15 core showed nearly flat (within a factor of two) depth profiles, but with smaller random concentration variations over depths of a few cm. These data are not consistent with a short-term core accretion model from non-irradiated regolith. The Apollo 15 core data are consistent with a combined accretion plus static time of a few hundred million years, and also indicate variable pre-accretion irradiation of core material. The lack of large variations in solar wind gas contents across core layers is also consistent with appreciable pre-accretion irradiation. Depth profiles of cosmogenic gases in the Apollo 16 core show considerably larger concentrations of cosmogenic gases below ∼65 cm depth than above. This pattern may be interpreted either as an accretionary process, or by a more recent deposition of regolith to the upper ∼70 cm of the core. Cosmogenic gas concentrations of several Apollo 16 fines and breccias are consistent with ages of North Ray Crater and South Ray Crater of ∼50·10 6 and ∼2·10 6 yr, respectively.