A key driver of future lunar scientific exploration is to deter-mine the source(s) of water ice and volatiles found on the Moon’s surface at the present day, and to understand past origins of water and volatiles in the Earth-Moon system. Potential caches of water and other volatiles at the lunar surface are being targeted by spacecraft missions for both scientific and exploration objectives. Such volatile reservoirs at the lunar surface are of key astrobiological interest and may also prove to be useful for in situresource utilisation for the next generation of lunar surface exploration. Over the past decade, laboratory measurements of hydroxyl and water locked in volcanic picritic glass beads and melt inclusions they host, in the phosphate mineral apatite, and in nominally anhydrous minerals have also allowed investigation of the diver-sity and budgets of volatile reservoirs in the lunar interior. Lunar surface volatile budgets have been constrained by spectral identifications of hydroxide ions bound in regolith soils, concentrations of volatiles within the regolith of permanently shadowed polar craters, and in Apollo regolith glasses and impact breccias. However, the origin of these dif-ferent reservoirs is poorly constrained: they are likely a mixture of both indigenous and exogenous sources. Unravelling the geochemical and isotopic records of volatiles at the lunar surface is complicated by (i) limited direct measurements of end-member compositions (i.e.impactor types, so-lar wind-derived volatiles, and interior sources), (ii) few com-plete volatile inventory analyses of lunar soil and regolith samples, and (iii) complications with calculating isotopic con-tributions from spallogenically-derived components.
To address the first of these challenges, Katherine Joy of the Department of Earth and Environmental Sciences of the University of Manchester, and the Center for Lunar Science and Exploration at the Lunar and Planetary Institute, Romain Tartèse, also of the Department of Earth and Environmental Sciences of the University of Manchester, Scott Messenger of the Robert M Walker Laboratory for Space Science at NASA's Johnson Space Center, Michael Zolensky, also of the Center for Lunar Science and Exploration at the Lunar and Planetary Institute, and of Astromaterials Research and Exploration Science at NASA's Johnson Space Center, Yves Marrocchi of the Université de Lorraine, David Frank, also of Astromaterials Research and Exploration Science at NASA's Johnson Space Center, and the Institute of Geophysics and Planetology at the University of Hawai’i at Manoa, and David Kring, again of Astromaterials Research and Exploration Science at NASA's Johnson Space Center, and of the NASA Solar System Exploration Research Virtual Institute, have determined the isotopic composition of the volatile elements hydrogen, carbon, nitrogen and oxygen in a carbonaceous chondrite meteorite fragment from the Moon. This study, published in the journal Earth and Planetary Science Letters on 22 April 2020, therefore, provides the first ground-truth for the composi-tion of asteroid-derived volatiles hosted by the lunar regolith.
The types of meteoritic material that are recovered today on the Earth might also be rep-resentative of the sources of material striking the Moon, contribut-ing to the addition of volatiles to the lunar regolith. There are, however, two significant knowledge gaps preventing us from mak-ing this assessment. Firstly, weakly consolidated, and likely the most volatile-rich, meteorites (and micrometeorites) are largely contaminated (especially volatile elements) or destroyed during descent through the Earth’s atmosphere and are poorly sampled, or unavailable in the Earth’s meteorite collection. Thus, knowledge of their con-tribution to the meteoritic volatile inventory is incomplete. Sec-ondly, the flux and nature of meteoritic debris maybe variable as the dynamical evolution of small bodies in the Solar System has changed through time. Thus, meteoritic fragments found in the lu-nar regolith may provide us with a complementary and often older record compared to meteorites found on Earth. To address both of those existing shortcomings we need to examine the Moon’s surficial regolith for volatile-rich meteoritic relics. Retention of such projectile material at the lunar surface depends upon impact velocity, impact angle, and impactor and target material properties including strength and porosity. Theoretical calculations indicate that aster-oids deliver the majority of lunar near-surface volatiles compared with cometary dust particles. To date, all impactor debris found on the Moon have been very small, rang-ing from a few microns to only a few mm in size, yet provide a vital ground-truth for characteri-sation of volatile resources by future in situmission analysis of the lunar regolith, and samples returned to the Earth.
Joy et al. characterised the petrography, mineralogy and geochemistry of a fragment of the Bench Crater carbonaceous chondrite meteorite. The meteorite was identified from a range of small (0.6 to 3 mm) rock particles ex-tracted from soil sample 12037, collected from the rim of a 75 m impact crater on the nearside of the Moon at the Apollo 12 landing site (3.01°S 23.4°W).
The Bench Crater meteorite. (a) Transmitted light optical image of Bench Crater meteorite showing a dark matrix and brown-coloured ferromagnesian silicate aggre-gates. (b) Backscattered electron microscope image; the white arrow indicates the location where some matrix was extracted for Nanoscale secondary ion mass spectrometry analysis. (c) False colour X-ray element composite map showing the mineralogical composition of the meteorite. Colour scheme indicated on the figure, where ferromagnesian silicate aggregates are cyan; matrix material is blue; apatite, calcite and dolomite are yellow; pyrrhotite, pentlandite and magnetite are red; ilmenite is pink; and the feldspathic agglutinate rim is white. (d) False colour X-ray element composite map. Colour scheme indicated on the figure, where apatite is cyan; calcite and dolomite are blue; pyrrhotite and pentlandite (Sulphur-bearing phases) are yellow; and magnetite is red. Joy et al. (2020).
The 12037 bulk soil is sub-mature, meaning that it underwent medium levels of solar wind exposure at the lunar surface com-pared with other Apollo soil samples. The dura-tion of exposure of this 12037 soil to the space environment has been estimated to be 10-20 million years based upon the density of cos-mic ray tracks and nitrogen spallation ages. his exposure age is younger than the proposed Apollo 12 landing site Bench Impact Crater formation episode, thought to be less than about 99 million years, and probably less than 55 million years. It may be that the Bench Crater me-teorite represents part of the impactor that made this crater, but it may also have been delivered after this event. In any case, the Bench Crater meteorite likely impacted the Moon’s surface in the last 100 million years.
Optical image of grain mount 12037,188. The Bench Crater meteorite is indicated with the arrow. Other small regolith rock fragments in the mount are a collection of impact melt breccias with varying textures. Joy et al. (2020).
Small pieces of the Bench Crater meteorite in section 12037,188 were removed with a needle. Microtomed sec-tions were prepared and analysed using the NASA Johnson Space Center JEOL 2200FS field emission gun transmission electron microscope coupled to an Oxford energy-dispersive X-ray spectroscopy system to check the presence of hydrated phases and determine the composition of matrix materials. Two 15 × 15 μm areas (Area 1 and Area 2) of fragments pressed into gold foil were spatially mapped for hydrogen, carbon, and nitrogen isotope compositions and two 10 × 10 μm areas (Area 3 and 4) were spatially mapped for their hydrogen isotope composition in separate sessions using the NASA Johnson Space Center Nanoscale secondary ion mass spectrometry 50L ion microprobe.
Collection context of the 12037 soil sample at the Apollo 12 landing site from the rim of Bench crater. Astronaut traverses and sample selection sites are overlain on LROC Narrow Angle Camera image. Image is courtesy of NASA/Goddard Spaceflight Center/Arizona State University and taken from the LROC website using their Apollo sites viewing webtool and overlay feature function. Joy et al. (2020).
After removing a small sub-split of the sample for nanoscale secondary ion mass spectrometry work, we also analysed the whole polished grain mount of Bench Crater using the NASA Johnson Space Center JEOL 7600f FEG scanning electron microscope to collect backscatter electron images and X-ray element maps. Normalised phase modal proportions were estimated by combining these element maps and using Adobe Photoshop to count the number of pixels in each assigned phase type. Further backscatter electron imaging was carried out using a FEI Quanta 650 FEG scanning electron microscope at the University of Manchester, and mineral compositions were acquired using a Cameca SX 100 electron microprobe at the University of Manchester.
Collection site location of soil sample 12037 on the edge of Bench Crater at the Apollo 12 landing site showing astronaut’s shadow. Image is courtesy of NASA (image catalogue AS12-48-7064). Joy et al. (2020).
Finally, Joy et al. measured the oxygen isotopic compositions of silicates, magnetite and carbonate in situ in the Bench Crater meteorite thin section using a CAMECA IMS 1270 E7 at the Centre de Recherches Pétrographiques et Géochimiques in Nancy, France. After the analyses, all the SIMS spots were carefully checked by back-scattered electron-scanning electron microscope to estimate poten-tial contribution by adjacent minerals.
Location of the roughly 15 μm SIMS spots within (a) silicate phases in the largest aqueously altered chondrule, and (b) within magnetite phases. Joy et al. (2020).
The Bench Crater meteorite is 3.00 × 1.75 mm in size. Transmitted light optical microscope observations show that the meteorite appears very well preserved with a low-level petrofabric (preferred matrix cross-cutting fracture orienta-tion), sub-rounded magnetite grains, and fractured sulphides. It is composed of brown-coloured aggregates of ferromagnesian minerals that are possibly aqueously altered chondrules (olivine + pyroxene, roughly 10% by area) of the meteorite; embedded within a fine-grained matrix (roughly 68% by area). Many ferromagnesian phases in the matrix have been replaced by the hydrated mineral saponite that retained structural water even after delivery to the lunar surface. This is consistent with low electron-probe microanalysis analytical totals of 91-93 percent by weight obtained on matrix areas, since water/hydroxide is not measured by electron-probe microanalysis. Lath shaped sulphides (about 5.3% by area) comprise pyrrhotite, pentlandite, and rare chalcopyrite, including the large rectangular sulphide crystal in the middle of the sample. Manganese-rich ilmenite occurs at trace levels (roughly 0.3% by area). Low-temperature aqueous alteration minerals in-clude framboidal and plaquet magnetite (roughly 12.4% by area). The Bench Crater meteorite also contains calcium-rich minerals includ-ing calcite and dolomite (roughly2.4% by area) and apatite (roughly 2.3% by area). Apatite is typically found within the aqueously altered chondrules, is chlorine-poor, and is characterised by low fluorine abundance (0.8-1.1% by weight) compared to apatite in indigenous lunar samples, which typically contains at least 3% fluorine by weight.
Backscattered electron images of Bench Crater meteorite fabric showing a representative matrix region of the meteorite (a), and close up views of different ferro-magnesian aggregates (F-M Agg) (aqueously altered chondrule) comprised of intermediate phases (I-M phase), amorphous phases, ilmenite, apatite and pyrrhotite (Pyrr). Black linear features are fractures. Joy et al. (2020).
The meteorite fragment has a patchy thin (less than 100 μm) vesicular crust, coating about 10% of its exterior. This crust is more feldspathic (12.8% by weight aluminium oxide) and enriched in titanium dioxide (2.2% by weight titanium dioxide) compared to carbonaceous chondrite meteorites (less than 5% by weight aluminium oxide, and less than 0.3% % by weight titanium dioxide) and instead is more similar to Apollo 12 bulk regolith compositions (13.9% by weight aluminium oxide, 3.1% by weight titanium dioxide). As the Moon has no atmosphere, this crust is not a fusion crust (formed on Earth during breakup of bolides as they are frictionally heated during atmospheric entry), but is more akin to agglutinitic or splash coat crusts found on rocks in the lunar regolith formed from flash impact melting. The lunar-like composition of the vesicular rim suggests interaction between the impactor and melted local basaltic lunar regolith either during or after its delivery. Temperatures of about 1100°C are required to melt lunar regolith. Therefore, some areas of the outer 10-30 μm of Bench Crater have likely been affected by flash heating, provoking dehydration of the matrix to form fibrous material. How-ever, the occurrence of hydrated saponite-bearing matrix just over 50 μm away from the glassy coat suggests that heat was quickly dissipated, and that the internal part of the sample was not baked and extensively degassed during the agglutination process.
Backscattered electron images of Bench Crater meteorite. (a)–(d) Close up of the thin (less than 100μm) irregular vesicular glassy agglutinitic rim that partially coats about 10% of the exterior of Bench Crater meteorite. Chemical analysis of the coat shows it to be feldspathic. Abbreviations: Ap, apatite; Mag, magnetite; F-M Agg, ferro-magnesian aggregate; Pyrr, pyrrhotite. Joy et al. (2020).
To try to classify Bench Crater’s parent meteorite group, Joy et al. measured in situ the oxygen isotope composition of magnetite, carbonate and the groundmass of the aqueously altered chondrules (within both intermediate phase and amorphous material). In matrix areas, the proportions of oxygen¹⁸ and oxygen¹⁷ values range from about 9.9 to 16.8 parts per thousand and 5.2 to 8.3 parts per thousand, respectively, generally falling along the terrestrial fractionation line, between the oxygen isotope compositions of the matrices of CM and CI carbonaceous chondrite falls (i.e. members of the Mighei and Ivuna carbonaceous chondrite meteorite families). The magnetite proportions of oxygen¹⁸ and oxygen¹⁷ values range from about 5.7 to 8.5 parts per thousand and 4.8 to 6.1 parts per thousand, respectively, and fall above the terrestrial fractionation line, on the same mass dependent fractionation line than the one defined by oxygen isotope compositions of magnetite in CI and CM carbonaceous chondrite falls. The two carbonate analyses yielded oxygen isotope compositions roughly similar to those measured in the aqueously altered chondrules, plotting at the oxygen¹⁶-rich end of the trend defined by carbonate analysis in CM chondrite falls.
Joy et al. removed a small portion of the matrix of the Bench Crater meteorite and analysed its hydrogen, carbon and nitrogen isotope compositions in situin two different 15 × 15 μm areas These two areas yielded comparative bulk deuterium proportion values of −36 and +200 parts per thousand, and a comparative bulk average proportion of −13 parts per thousand.
Nanoscale secondary ion mass spectrometry imaging did not show any micron-size hydrogen-, carbon-or nitrogen-isotopically anomalous regions, con-trary to what is observed in interplanetary dust particles, ultracarbonaceous Antarctic micrometeorites, and the most prim-itive chondrites like Murchison, in which hotspots with elevated isotope ratios are common. There was also no isotopic distinction between carbon-rich and carbon-poor areas of the Bench Crater samples.
Comparison of the deuterium/hydrogen distribution in Murchison (a) and Bench Crater (b) Murchison is characterised by a bulk matrix relative proportional value of -117 with deuterium-rich hotspots with relative proportional value ranging from 2300 to 4800 parta per thousand, which is consistent with organic globules observed previously in the matrix of other primitive carbonaceous chondrites. The Bench Crater matrix does not display these deuterium-rich hotspots that are common among the most primitive chondrites and interplanetary dust particles. Joy et al. (2020).
The Bench Crater meteorite provides the first direct evidence of surviving hydrated carbonaceous chondrite material delivered to the lunar regolith. It is, thus, currently the only direct ground-truth of the planetary end-member for all hydrogen, nitrogen and carbon light-element isotopic analysis of lunar soils. These findings pro-vide important benchmarks for models of volatile processing and transport across the lunar surface.
The Bench Crater meteorite is mineralogically and composition-ally unusual compared with other carbonaceous chondrite groups. It is mineralogically distinct from CV types (Vigarano carbonaceous chondrite meteorites), which are lacking in calcite and dolomite and have comparatively lower abundances of phyllosilicates (1.9–4.2%) and magnetite (6%). than Bench Crater. Affinities have previously been made with C-ungrouped types (carbonaceous chondrites not placed in any family) and CM2 carbonaceous chondrites (Type 2 Mighei carbonaceous chondrites). For instance, the high abundance of acicular sulphides and the presence of aqueously altered chondrules are similar to the CM1/2 Boriskino Meteorite, and thermally metamorphosed CI, CM and CY chondrites (e.g. Ikeda). However, the high modal abundance of magnetite bears more resemblance to CI chondrites and matrix-rich (altered) CR chondrites such as Al Rais and Grove Mountains (GRO) 95577. Notably the me-teorite matrix is magnesian and is compositionally more similar to CI-meteorite groups than CM or CR types, suggesting a higher state of parent body aqueous alter-ation. It should be noted that minor dehydration due thermal pro-cessing is unlikely to have changed the magnesium oxide/iron oxide ratio; this ratio being constant in CM chondrites having experienced dehydration. Bench Crater also share similarities with the Lonewolf Nunataks (LON) 94101 meteorite, which has been described as a CM chondrite with coarse acicular sulphides and a wide variety of CM-and magnetite-rich CI-like clasts. Bench Crater might be akin a clast-rich CM chondrite, but its limited size and hence representativeness precludes defini-tive assessment of this possibility. All these observations indicate that the Bench Crater meteorite cannot be directly related to any known carbonaceous chondrite groups present in our current me-teorite collections.
Bench Crater meteorite also shows oxygen isotope character-istics that connect it to the hydrated CI and CM carbonaceous chondrites. For example, the Bench Crater magnetite grains have oxygen isotope compositions that are on the same mass fractionation line as magnetite in CI and CM chondrite groups. Furthermore, Bench Crater carbonates are characterised by oxygen isotope compositions plotting at the oxygen¹⁶-rich end of the trend defined by carbonates from CM2 chondrite falls and some CM1 chondrites from the Antarctic meteorite collection (Allan Hills 83100, Allan Hills 84034 and Meteorite Hills 01070). The oxygen isotope com-position of silicates in Bench Crater aqueously altered chondrules plots along the terrestrial fractionation line, similarly to the oxygen isotope compositions measured in bulk CI chondrites and in their matrix and most olivine grains. The oxygen isotope compositions of silicates in aqueously altered chondrules yield an average relative proportion of oxygen¹⁷ of −0.12 parts per thousand . Such a oxygen¹⁷ proportion value is, within errors, intermediate between the oxygen¹⁷ proportion of CI chondrite matrix (0.37 parts per thousand) and to that of CM chondrite matrix ( oxygen¹⁷ proportion −1.76 parts per thousand ). Finally, the relative oxygen¹⁷ and oxygen¹⁸ isotope ratios in Bench Crater component are significantly lower than the oxygen¹⁷ and oxygen¹⁸ isotope ratio values of bulk CY chondrites.
The evolution of the relative oxygen¹⁷ values in Bench Crater meteorite components implies that the oxygen isotope compositions of sec-ondary phases (magnetite, carbonates) did not result from fluid circulation along a temperature gradient, which would produce a trend with a slope of 0.52. Instead, variations of relative oxygen¹⁷ values imply that the main process controlling the oxygen isotope compo-sitions of the Bench Crater secondary phases is related to iso-topic equilibrium between oxygen¹⁶-rich anhydrous silicates and oxygen¹⁷-and oxygen¹⁸-rich fluids as commonly reported in CM chondrites. In this sce-nario, magnetite would correspond to early precipitates whereas carbonates formed after significant hydrothermal alteration. Fur-thermore, the carbonate oxygen isotope compositions show similarity with those reported in CM chondrites whose precipitation temper-atures were estimated to be less than 200°C. This suggests that Bench Crater did not experience significant heating. and dehydration processes, although the lack of tochilinite would suggest a temperature higher than 320°C. However, we note that the stability field of tochilinite is strongly affected by magnesium-iron substitutions and is not well constrained.
Taken together, the oxygen isotope composition of Bench Crater mineral phases, along with its modal mineralogy and matrix composition, do not exactly match that of a known carbonaceous chondrite group. It might represent a rare type of carbonaceous asteroid parent body that is currently unsam-pled in the Earth’s meteorite collection. However, Bench Crater has important similarities with carbon-rich hydrated CI and CM chon-drites; the differences could be related to greater amounts of water ice grains originally accreted by the Bench Crater parent body. Such a hypothesis could be tested thanks to several assumptions, and following the methodology applied to CM, CO, CR and CV carbona-ceous chondrites. Bench Crater carbonates show relative oxygen¹⁷ and oxygen¹⁸ values plotting on the CM calcium-carbonate trend, suggesting they precipitated from water with a similar oxygen isotope evolution than that of CM chondrites. Assuming that the bulk oxygen isotope composition of Bench Crater is similar to the most altered CM chondrites (i.e. relative oxygen¹⁸ values of ten parts per thousand), and considering the oxygen isotope composition of anhydrous CM sil-icates as plausible Bench Crater protolith (i.e. relative oxygen¹⁸ values of -4.2 parts per thousand), Joy et al. calculated the water/rock ratio at different temperature ranging from 25 to 150°C. This gives a water/rock ratio for Bench Crater in the range 0.35 to 0.55, which is slightly higher than for most CM chondrites that are characterised by a narrow range of water/rock ratio of 0.3-0.4 irrespective of their petrological type. Although speculative, these results suggest thatthe Bench Crater asteroidal parent-body was more water-rich than the source location(s) of other CM chondrites. In addition, the physio-chemical conditions of hydrothermal alteration in the Bench Crater parent-body might have been different than in other CM chondrite original parent body(ies), as indicated by the higher abundance of magnetite in Bench Crater compared to most CM chondrites (i.e. 12.4% by area vs. 1.1-2.4 % by volume), and the lack of tochilinite.
The agglutinitic rim of Bench Crater indicates that it has been affected by flash heating on the lunar surface, but that such effects only affected the outer about 10-30 μm of the fragment. Matrix mineralogy and chemical composition also allows assessing the thermal history of the Bench Crater fragment as a whole. Low analytical totals obtained through electron microprobe analyses in the matrix of Bench Crater (91-93 %; by weight) may be related to the presence of micro-porosity, and/or species not accounted for during