Nectaris Basin Research Articles

Crustal origin for olivine in the lunar Shioli crater ejecta boulders: Insights from the geological setting of Theophilus crater and Nectaris basin

On the Moon, impact craters and basins expose a wide range of crustal and mantle rocks that provide excellent opportunity for sampling them, understanding their origins and reconstructing spatial and temporal evolution of lunar interior. The previous studies detected olivine-bearing mantle rocks in and around large impact craters and basins. The Japanese SLIM mission landed on the ejecta of a ∼ 280-m-diameter Shioli crater that was emplaced on the ejecta blanket of ∼ 103-km-diameter Theophilus crater, for characterizing potential mantle-derived olivine in the Shioli crater ejecta boulders. To test this hypothesis, we studied the geological setting of Shioli crater, host Theophilus crater and Nectaris multi-ring basin using the orbiter data from Chandrayaan-1 and 2, Lunar Reconnaissance Orbiter, and Kaguya missions and the earth-based Arecibo radar observation. The asymmetrically distributed secondary craters and impact melt ponds around Theophilus crater suggests that a northeast-directed oblique impact produced this crater. Composition of Theophilus crater and surrounding region indicates that the crater excavated a heterogeneous target composed of a thin layer of high-Al olivine basalt (Mare Nectaris) underlain by anorthositic highland rocks possibly intruded by Mg-suite plutons; layers of Cyrillus crater ejecta blanket and Nectaris basin materials (both ejecta and impact melt sheets) were also present beneath the mare basalt flows. Hence, the Theophilus ejecta blanket is a mixture of all these materials. Our dating of Theophilus crater suggests that it is a ∼ 2 Ga Eratosthenian crater. Shioli is a fresh simple crater that was formed at ∼ 1 Ma on the uprange ejecta blanket of Theophilus, where the Arecibo radar data indicated the presence of abundant buried Theophilus ejecta boulders. An ESE-directed hypervelocity oblique impact event produced the elongated Shioli crater and its asymmetrically distributed bright ejecta. Shioli is a primary impact crater indicating the role of impact spallation processes associated with this hyper-velocity impact in producing thousands of ejecta (or spall) boulders around Shioli crater, displaying their asymmetric dispersal pattern and spatial variation of boulder sizes and shapes. The larger and elongated boulders are concentrated near the crater rim, while their size and axial ratio gradually decreases outward from the crater rim. The SLIM mission landed on a thin downrange ejecta of Shioli crater, where fewer large-size boulders are present. Our compositional study suggests that the Shioli ejecta boulders are composed of olivine basalt (Mare Nectaris) mixed with highland anorthositic fragments, including the reworked Cyrillus ejecta and Nectaris basin materials. The Shioli ejecta boulders were produced by complex impact fragmentation of already existing, buried Theophilus ejecta boulders. The regional crustal structure of Nectaris basin and its petrological composition suggest that both Nectaris basin and Theophilus crater did not excavate the lunar mantle. Therefore, the Shioli ejecta boulders are of crustal origin, including the olivine minerals present in them. Our results have important implications for the origin of olivine in the Shioli crater boulders being investigated by the SLIM mission.

Diverse Geological Evolution of Impact Basins on the Moon

Impact basins are the dominant landforms on the lunar surface, and their geological evolution varies. This research studied the diversity in the geological evolution of three impact basins: the Dirichlet–Jackson Basin, the Nectaris Basin, and the Orientale Basin. First, the regional topography and geomorphology of the three basins were studied using the SLDEM2015 digital elevation model (DEM). Clementine ultraviolet–visible (UVVIS) data and Moon Mineralogy Mapper (M3) data were used to study the chemical composition and mineralogical composition of the three basins. Additionally, the lunar crust thickness data have been used to study the subsurface structure of the three basins. The topographical analogies of the three basins indicate that the shapes of the basins are cavity-like. However, the shape of the Dirichlet–Jackson basin is not an obvious cavity compared with the other basins. The positions with minimum and maximum crustal thickness of the three basins are located at the center and the rim. The uplift of the crust-mantle interface of the Nectaris Basin and Orientale Basin is relatively larger than in the Dirichlet–Jackson Basin. Below the center of the maria of the Nectaris Basin and Orientale Basin, collapses occurred at the crust–mantle interface. The concentrations of FeO and TiO2 in the non-mare formation of the basin and maria show expected bimodal distributions. Moreover, we found exposures of olivine-rich materials in the Nectaris Basin and Orientale Basin which are located in the Rosse and Maunder craters, respectively. These exposures of olivine may be explained by the fact that the formation of the large impact basin, which might penetrate and blast away the upper lunar crust, excavating deep-seated material.

An Effusive Lunar Dome Near Fracastorius Crater: Spectral and Morphometric Properties

We examine a dome within the boundary between Fracastorius crater and Mare Nectaris. The dome has a noticeable vent structure and appears to be perpendicular to wrinkle ridges in the southern Mare Nectaris basin. The spectral signature of this dome, derived from Clementine UVVIS and Chandrayaan-1 M3 reflectance data, revealed that Fracastorius has low TiO2 content and primarily basaltic material. Using altimeter data, we measured the dome diameter to be 28.6 km, with a dome height of 241.5 m, and a flank slope of 1°. Based on rheological modeling of the dome and a viscoelastic model of the presumed feeder dike, we obtained a magma viscosity of 3.1 × 105 Pa s, an effusion rate of 5.9 m3 s−1, a duration of multiple effusion processes of 4.15 years, and a magma rise speed of 2.1 × 10−4 m s−1. From these measurements, we estimate the feeder dike geometry to have a horizontal dike length of 234 km and a width of 11.8 m. A comparison of the Fracastorius dome with other noted lunar domes with similar morphometric properties reveal similar magma viscosities to domes found near craters Mee, Milichius and Petavius.

Characterization and interpretation of the global lunar impact basins based on remote sensing

Impact basins are primary geological structures on the Moon, and play key roles in revealing the lunar history. Due to the different identification standards currently used, the basin identification results are highly inconsistent. Except for the major basins (e.g., Orientale, Schrödinger, Imbrium, Crisium, Apollo, and Nectaris Basin), detailed sub-formation interpretations for most other basins are lacking, which hampers the construction of a complete (global) geological interpretation for the lunar impact basins. Based on multisource remote sensing data and previous works, we established a basin identification standard, and a new global lunar basin catalog containing 81 basins. According to the ring diameter ratios, the purest anorthosite (PAN) distribution, and basin radial textures, we divided the basin sub-formations into the central-peak, peak-ring, basin-floor, basin-wall, basin-rim, and basin-ejecta formations. We interpreted the ejecta formation and other basin sub-formations by combining the Focal Flow data with LROC WAC images, topographic data, gravity anomalies, and spectral data. Our new lunar geologic map shows more precise distribution of basin formations, covering nearly 70% of the lunar surface. Moreover, we obtained the origin of basin rings using basin sub-formations map. Additionally, the basin sub-formation map can contribute to the basin impact conditions, such as the discovered ring (concentric with the outermost ring) provides evidence for three impacts in the Mare Moscoviense, and the SPA sub-formation distribution indicates the impact direction of SPA is SE-NW. Furthermore, the sub-formation distribution can facilitate the geological characteristics and evolution study of the lunar exploration sites.

Ballistic Sedimentation of Impact Crater Ejecta: Implications for the Provenance of Lunar Samples and the Resurfacing Effect of Ejecta on the Lunar Surface

AbstractA clear understanding of ballistic sedimentation is critical for interpreting the evolution of the lunar surface and the provenance of lunar samples. Quantitative models have been established to estimate the characteristics of ejecta deposits, such as the abundance of distantly sourced ejecta compared to the amount of excavated local materials. Here, we revise a previously established ballistic sedimentation model by considering the shielding effect of ejecta deposits on the emplacement process of later‐arriving ejecta. Our model shows that, due to the shielding effect, the ratio of excavated local materials to ejecta in continuous ejecta deposits increases significantly with increasing distance from the center of their parent crater. Model‐derived results imply that basin formation resurfaces regions within continuous ejecta deposits and proximal light plains but only partially resurfaces distal areas; thus, the partial resurfacing effect should be considered when dating units older than the youngest basin, Orientale, or units located close to large craters. Applying our model to the Apollo 16 landing site, we found that 10% of the Apollo 16 regolith may be of Orientale origin, which is almost the same as those of Imbrium (9%) and Serenitatis ejecta (10%). The most abundant basin ejecta in Apollo 16 regolith is from the Nectaris basin (23%). These results provide important constraints for interpreting the provenance of the Apollo 16 samples.

Olivine-bearing lithologies on the Moon: Constraints on origins and transport mechanisms from M3 spectroscopy, radiative transfer modeling, and GRAIL crustal thickness

High-resolution hyperspectral data from Chandrayaan-1’s Moon Mineralogy Mapper (M3) allow detection of olivine on the lunar surface. Olivine exposed at the surface may originate as mantle material or igneous products (intrusive or extrusive). Potential transport mechanisms include excavation of the mantle or lower crustal material by impacts that form basins and complex craters, differentiation of impact melt sheets, or magmatic emplacement of lavas, cumulates, or xenoliths. A sample of the lunar mantle, which has not been conclusively identified in the lunar sample collection, would yield fundamental new insights into the composition, structure, and evolution of the lunar interior. Olivine identified in remote spectral data is generally accepted to originate from the primary mantle, because abundant olivine is expected to exist in the mantle and lower crust, yet have sparse occurrences in the upper crust. In this study, we identified 111 M3 single-pixel spectra with characteristic absorption features consistent with olivine at Crisium, Nectaris, and Humorum basins and near the craters Roche and Tsiolkovsky. In an effort to determine the origins and transport mechanisms that led to these individual exposures, we estimated mineral abundances using radiative transfer modeling and examined crustal thickness estimates, topography and slope maps, and images from the Lunar Reconnaissance Orbiter Camera (LROC). At Crisium basin, where crustal thickness is near 0 km (Wieczorek et al., 2013), mantle olivine may have been exposed by basin-forming impact and deposited on the rim. Picard crater, which is superposed on the floor of Crisium, also exhibits potential mantle olivine in its ejecta. Within Nectaris basin, olivine exposures are confined to the rims of small craters on the mare, which are inferred to excavate a layer of olivine-rich mare basalt. Olivine occurrences on the rim of Humorum basin, including those located on a graben, are likely to be cumulates of shallow intrusions that were transported magmatically to the surface. Near Roche crater, olivine may have originated in shallow dikes that reached the subsurface and were exposed by impacts. In addition to verifying both known and previously unidentified olivine exposures, our combined geophysical, spectral, and radiative transfer modeling investigation has allowed identification of both igneous and mantle-derived olivine.

Crystal accumulation in a 4.2 Ga lunar impact melt

The ages of lunar impact basins and the role of fractional crystallization in producing compositional heterogeneity of lunar melt sheets are long-standing problems with significant implications for solar system dynamics and the petrologic evolution of the lunar crust. Here we document the formation of a basin-scale impact on the Moon at 4.20±0.07Ga based on the 147Sm–143Nd isochron age of a magnesian, noritic anorthosite melt rock from lunar breccia 67955. Major element compositions of plagioclase and mafic silicates in the melt rock imply a substantial component of primary Mg-suite cumulates or related lithologies in the pre-impact crustal stratigraphy. Trace element compositions of the plagioclase, including diagnostic ratios such as Sr/Ba, are also mostly similar to those in plagioclase from Mg-suite cumulates, with a small number of grains trending toward compositions observed in ferroan anorthosites. Mineral-melt distribution coefficients applied to trace element compositions of the 67955 plagioclase and pyroxene predict parental melt compositions that contrast strongly with the bulk rock. Compared to the whole rock, parental melts calculated from the plagioclase are enriched in REE (ΣREELa–Yb=131–885, average 619ppm vs. 39.8ppm) and they have more fractionated REE patterns (La/Ybn=1.2–9.8, average 4.9 vs. 1.5) with deep negative Eu anomalies (Eu/Eu∗=0.09–0.40 vs. 1.36). Trace element data for the pyroxenes also imply incompatible-element enriched parental melts. Subsolidus equilibration between the plagioclase and the pyroxene apparently rotated the REE patterns, but the conclusion that the parental melt was highly enriched in REE relative to the whole rock appears robust. Quantitative modeling shows that fractional crystallization of the 67955 whole rock composition cannot reproduce the range of Ba, Sr, Ti, and La concentrations measured in the 67955 plagioclase. Rather, the models require an initial melt composition that was strongly enriched in these elements, and they suggest that fractional crystallization became less efficient as crystallization proceeded. The contrast between the inferred parental melt composition and the bulk rock implies formation of the 67955 noritic anorthosite as a crystal cumulate, and that the cogenetic residual melt was strongly enriched in incompatible elements. If so, this would be the first documented example of fractional crystallization in a lunar impact melt sheet. The petrological and geochemical characteristics of the 67955 noritic anorthosite suggest that it formed by an impact in the Procellarum-KREEP Terrane, and was transported to the Apollo 16 site as Imbrium ejecta. Inheritance of ejecta related to this pre-Imbrium basin may contribute to the common occurrence of ∼4.2Ga 40Ar–39Ar plateau ages in breccia clasts and regolith fragments from the rim of North Ray crater. In that case, those data may provide no constraints on the age of the Nectaris basin, despite its proximity to the Apollo 16 site.

Study of olivine-rich dark halo crater – Beaumont L in Mare Nectaris using high resolution remote sensing data

Study of dark-haloed craters (DHCs) can provide important information about the geology, mineralogy and evolution of certain hidden mare deposits known as cryptomare. Some DHCs have been identified in the Mare Nectaris region of the near side of the Moon. Beaumont L represents one such DHC situated on the western flank of the Nectaris basin. Moon Mineralogical Mapper (M3) images were used to investigate the composition of DHCs. Morphological investigations have been carried out using Terrain Mapping Camera (TMC) and Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera images. The morphological details captured by TMC and LROC Narrow Angle Camera (NAC) images provide evidence that Beaumont-L is of impact origin and do not show evidence of a volcanic origin. The compositional analysis using M3 data indicates the presence of an olivine rich cryptomare unit excavated due to the Beaumont L impact. Our study also confirms that the band I feature in the reflectance spectra of Beaumont L is completely attributable to olivine deposits without contribution from any type of glass/melt deposits. The presence of olivine in Beaumont L suggests either excavation of olivine-rich cryptomare or a subsurface mafic pluton.

Re-examining the main asteroid belt as the primary source of ancient lunar craters

It has been hypothesized that the impactors that created the majority of the observable craters on the ancient lunar highlands were derived from the main asteroid belt in such a way that preserved their size–frequency distribution (Strom, R.G., Malhotra, R., Ito, T., Yoshida, F., Kring, D.A. [2005]. Science 309, 1847–1850). A more limited version of this hypothesis, dubbed the E-belt hypothesis, postulates that a destabilized contiguous inner extension of the main asteroid belt produced a bombardment limited to those craters younger than Nectaris basin (Bottke, W.F., Vokrouhlický, D., Minton, D., Nesvorný, D., Morbidelli, A., Brasser, R., Simonson, B., Levison, H.F. [2012]. Nature 485, 78–81). We investigate these hypotheses with a Monte Carlo code called the Cratered Terrain Evolution Model (CTEM), which models the topography of a terrain that has experienced bombardment due to an input impactor population. We detail our effort to calibrate the code with a human crater counter. We also take advantage of recent advances in understanding the scaling relationships between impactor size (Di) and final crater size (Dc) for basin-sized impact craters (Dc>300km) in order to use large impact basins as a constraint on the ancient impactor population of the Moon. We find that matching the observed number of lunar highlands craters with Dc≃100km requires that the total number of impacting asteroids with Di>10km be no fewer than 4×10-6km-2. However, this required mass of impactors has <1% chance of producing only a single basin larger than the ∼1200km Imbrium basin; instead, these simulations are likely to produce more large basins than are observed on the Moon. This difficulty in reproducing the lunar highlands cratering record with a main asteroid belt SFD arises because the main belt is relatively abundant in the objects that produce these “megabasins” that are larger than Imbrium. We also find that the main asteroid belt SFD has <16% chance of producing Nectarian densities of Dc>64km craters while not producing a crater larger than Imbrium, as required by the E-belt hypothesis. These results suggest that the lunar highlands were unlikely to have been bombarded by a population whose size–frequency distribution resembles that of the currently observed main asteroid belt. We suggest that the population of impactors that cratered the lunar highlands had a somewhat similar size–frequency distribution as the modern main asteroid belt, reflecting a similar rocky composition and collisional history, but had a smaller ratio of objects capable of producing megabasins compared to objects capable of producing ∼100km craters. We estimate that the impactor population required to match the lunar highlands had N>5.5km/N>70km≃630, while the modern main asteroid belt ratio is N>5.5km/N>70km≃100.

A sawtooth-like timeline for the first billion years of lunar bombardment

We revisit the early evolution of the Moon's bombardment. Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins, and the abundance of highly siderophile elements in the lunar crust and mantle. We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, but also there was no prominent and narrow impact spike ∼3.9Gy ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile, with an uptick in the impact flux near ∼4.1Gy ago. The impact flux at the beginning of this weaker cataclysm was 5–10 times higher than the immediately preceding period. The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since ∼4.1Gy ago declined slowly and adhered relatively close to classic crater chronology models (Neukum and Ivanov, 1994). Overall we expect that the sawtooth event accounted for about one-fourth of the total bombardment suffered by the Moon since its formation. Consequently, considering that ∼12–14 basins formed during the sawtooth event, we expect that the net number of basins formed on the Moon was ∼45–50. From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains has an age of 4.38–4.42Gyr. Moreover, the largest lunar basin, South Pole Aitken, is older than 4.3Gy, and therefore was not produced during the lunar cataclysm.

The onset of the lunar cataclysm as recorded in its ancient crater populations

The earliest bombardment history of the Moon potentially provides powerful constraints for solar system evolution models. A major uncertainty, however, is how much of this history is actually recorded in lunar craters. For example, some argue that most ancient lunar craters and basins were produced by a declining bombardment of leftover planetesimals produced by terrestrial planet formation processes. Others believe that most lunar craters and large basins were formed in a narrow time interval between 3.8 and 4.0Ga, the so-called lunar cataclysm. In the light of recent improvements in our understanding of early solar system evolution, it is possible that the contributions from both scenarios could be represented in the lunar crater record. If so, when did the declining bombardment end and the lunar cataclysm begin?Here we show, using new counts of 15–150km diameter craters on the most ancient lunar terrains, that the craters found on or near Nectaris basin appear to have been created by projectiles hitting twice as fast as those that made the oldest craters on various Pre-Nectarian-era terrains. This dramatic velocity increase is consistent with the existence of a lunar cataclysm and potentially with a late reconfiguration of giant planet orbits, which may have strongly modified the source of lunar impactors. This work also suggests that the lunar cataclysm may have started near the formation time of Nectaris basin. This possibility implies that South Pole-Aitken basin (SPA), the largest lunar basin and one of the oldest by superposition, was not created during the cataclysm. This view is strengthened by our interpretation that a substantial fraction of ancient craters on SPA were made by low velocity impactors. Finally, we believe these results shed new light on the impact history of the primordial Earth.

Compositional diversity at Theophilus Crater: Understanding the geological context of Mg-spinel bearing central peaks

[1] Analysis of high resolution Moon Mineralogy Mapper (M3) data reveals the presence of a prominent Mg-spinel-rich lithology in the central peaks of Theophilus crater on the lunar nearside. Other peak-associated lithologies are comprised of plagioclase, olivine, and pyroxene-bearing materials. A consistent spatial association of Mg-spinel with mafic-free anorthosite is recognized. Documentation of Theophilus central peaks brings the global inventory of Mg-spinel-rich lithology to two widely separated occurrences, namely Theophilus on the lunar nearside and Moscoviense basin on the farside. The Theophilus crater target region lies on one of the inner rings of the Nectaris basin, indicating that the Mg-spinel-bearing lithology source was deep in the lunar crust.

Imbrium provenance for the Apollo 16 Descartes terrain: Argon ages and geochemistry of lunar breccias 67016 and 67455

In order to improve our understanding of impact history and surface geology on the Moon, we obtained 40Ar– 39Ar incremental heating age data and major + trace element compositions of anorthositic and melt breccia clasts from Apollo 16 feldspathic fragmental breccias 67016 and 67455. These breccias represent the Descartes terrain, a regional unit often proposed to be ejecta from the nearby Nectaris basin. The goal of this work is to better constrain the emplacement age and provenance of the Descartes breccias. Four anorthositic clasts from 67016 yielded well-defined 40Ar– 39Ar plateau ages ranging from 3842 ± 19 to 3875 ± 20 Ma. Replicate analyses of these clasts all agree within measurement error, with only slight evidence for either inheritance or younger disturbance. In contrast, fragment-laden melt breccia clasts from 67016 yielded apparent plateau ages of 4.0–4.2 Ga with indications of even older material (to 4.5 Ga) in the high-T fractions. Argon release spectra of the 67455 clasts are more variable with evidence for reheating at 2.0-2.5 Ga. We obtained plateau ages of 3801 ± 29 to 4012 ± 21 Ma for three anorthositic clasts, and 3987 ± 21 Ma for one melt breccia clast. The anorthositic clasts from these breccias and fragments extracted from North Ray crater regolith ( Maurer et al., 1978) define a combined age of 3866 ± 9 Ma, which we interpret as the assembly age of the feldspathic fragmental breccia unit sampled at North Ray crater. Systematic variations in diagnostic trace element ratios (Sr/Ba, Ti/Sm, Sc/Sm) with incompatible element abundances show that ferroan anorthositic rocks and KREEP-bearing lithologies contributed to the clast population. The Descartes breccias likely were deposited as a coherent lithologic unit in a single event. Their regional distribution suggests emplacement as basin ejecta. An assembly age of 3866 ± 9 Ma would be identical with the accepted age of the Imbrium basin, and trace element compositions are consistent with a provenance in the Procellarum-KREEP Terrane. The combination of age and provenance constraints points toward deposition of the Descartes breccias as ejecta from the Imbrium basin rather than Nectaris. Diffusion modeling shows that the older apparent plateau ages of the melt brecia clasts plausibly result from incomplete degassing of ancient crust during emplacement of the Descartes breccias. Heating steps in the melt breccia clasts that approach the primary crystallization ages of lunar anorthosites show that earlier impact events did not completely outgas the upper crust.

Mass flux in the ancient Earth‐Moon system and benign implications for the origin of life on Earth

The origin of life on Earth is commonly considered to have been negatively affected by intense impacting in the Hadean, with the potential for the repeated evaporation and sterilization of any ocean. The impact flux is based on scaling from the lunar crater density record, but that record has no tie to any absolute age determination for any identified stratigraphic unit older than ∼3.9 Ga (Nectaris basin). The flux can be described in terms of mass accretion [Hartmann, 1980], and various independent means can be used to estimate the mass flux in different intervals. The critical interval is that between the end of essential crustal formation (∼4.4 Ga) and the oldest mare times (∼3.8 Ga). The masses of the basin‐forming projectiles during Nectarian and early Imbrian times, when the last 15 of the ∼45 identified impact basins formed, can be reasonably estimated as minima. These in sum provide a minimum of 2 × 1021 g for the mass flux to the Moon during those times. If the interval was 80 million years (Nectaris 3.90 Ga, Orientale 3.82 Ga), then the flux was ∼2 × 1013 g/yr over this period. This is higher by more than an order of magnitude than a flux curve that declines continuously and uniformly from lunar accretion to the rate inferred for the older mare plains. This rate cannot be extrapolated back increasingly into pre‐Nectarian times, because the Moon would have added masses far in excess of itself in post‐crust‐formation time. Thus this episode was a distinct and cataclysmic set of events. There are ∼30 pre‐Nectarian basins, and they were probably part of the same cataclysm (starting at ∼4.0 Ga?) because the crust is fairly intact, the meteoritic contamination of the pre‐Nectarian crust is very low, impact melt rocks older than 3.92 Ga are virtually unknown, and ancient volcanic and plutonic rocks have survived this interval. The accretionary flux from ∼4.4 to ∼4.0 Ga was comparatively benign. When scaled to Earth, even the late cataclysm does not produce ocean‐evaporating, globally sterilizing events. The rooted concept that such events took place is based on the extrapolation of a nonexistent lunar record to the Hadean. The Earth from ∼4.4 to ∼3.8 Ga was comparatively peaceful, and the impacting itself could have been thermally and hydrothermally beneficial. The origin of life could have taken place at any time between 4.4 and 3.85 Ga, given the current impact constraints, and there is no justification for the claim that life originated (or reoriginated) as late as 3.85 Ga in response to the end of hostile impact conditions.

Isotope analysis of crystalline impact melt rocks from Apollo 16 stations 11 and 13, North Ray Crater

Rubidium‐strontium and Sm‐Nd isotopic data were obtained for lunar impact melt rocks from Apollo 16 stations 11 and 13 as part of the North Ray Crater Target Rock Consortium study. Rubidium‐strontium whole‐rock data define an isochron age, T, = 3.69 ± 0.16 b.y. [λ = 0.0142 (b.y.)−1] and initial 87Sr/86Sr, ISr, = 0.69918±5 for feldspathic microporphyritic (FM) samples. Internal Rb‐Sr isochrons of T = 3.86 ± 0.05 b.y., ISr = 0.69952 ± 10 and T = 3.76 ± 0.04 b.y., ISr = 0.69916 ± 4 were determined for very high alumina (VHA) melt rock 67747 and anorthositic noritic melt rock (ANMR) 67559, respectively. (T,ISr) for these and other lunar melt rocks were examined to infer the nature of the protolith for the rocks. ISr values for the AN MR and FM rocks are consistent with an anorthositic protolith, probably a mixture of nonpristine “anorthositic gabbros” and pristine anorthosites, anorthositic norites, norites, troctolites, and possibly dunites. The source protolith of the VHA rocks apparently had a significant KREEP component and the protolith for Apollo 16 KREEP must have had nearly the same Rb/Sr ratio as that of Apollo 14 KREEP. Two‐stage model calculations using the (T,ISr) parameters suggest that some of the melt rocks were open Rb‐Sr systems during impact melting. Rubidium‐strontium model ages for the FM and VHA rocks show a smoothly monotonic decrease with increasing Rb content. Model ages of the ANMR are uniformly low relative to 4.56 b.y. It is suggested that these effects are due to formation of these rocks by impact partial melting. Rubidium‐strontium model ages of the KREEP rocks do not vary with Rb content, suggesting that they were closed Rb‐Sr systems during impact melting. Samarium‐neodymium data for mineral separates of 67747 showed only a small variation in Sm/Nd ratio and yield an imprecisely defined isochron of 3.6 ± 0.4 b.y. Plagioclase separates of 67559 appear not to have reached Nd isotopic equilibrium during impact melting; consequently, the internal isochron age is also imprecisely defined at 4.0 ± 0.4 b.y. ϵNd values for the melt rocks are about ‐1 to ‐2 at their Rb‐Sr or 40Ar‐39Ar ages, are consistent with the protolith compositions inferred from the Rb‐Sr data, and show that lunar crustal Sm‐Nd evolution departed from chondritic evolution early in lunar history. The average chondritic uniform reservoir (CHUR) model age of those ANMR and FM rocks that apparently remained closed Rb‐Sr and Sm‐Nd systems during impact melting is 4.37 ± 0.10 b.y., consistent with earlier suggestions that the lunar crust formed at about that time. The average CHUR model age of two VHA rocks is 4.10 ± 0.15 b.y., suggesting that these rocks were open Sm‐Nd systems as well as open Rb‐Sr systems during impact (partial?) melting. Current hypotheses for the site geology, stratigraphy, and radiometric age data lead us to suggest that the Nectaris basin and the Descartes formation were formed about 4.1 b.y. ago, and the Imbrium basin and the Cayley formation were formed about 3.81–3.86 b.y. ago. The ages of some individual melt rocks probably date “local” cratering events, however.

Apollo 16 site geology and impact melts: Implications for the geologic history of the lunar highlands

The Apollo 16 mission provided direct samples of two lunar highlands geological units: the Descartes and Cayley Formations. The Descartes Formation is interpreted as hummocky Nectaris basin ejecta, reworked by secondary impacts from the Imbrium basin. The Cayley Formation was emplaced as a consequence of the Imbrium impact and incorporated large quantities of the local substrate, which in this region consists mostly of Nectaris ejecta. The composition of the upper crust in this area is primarily anorthositic gabbro; local craters near the site would produce an impact melt of this composition. I estimate the total volume of pure melt within the Apollo 16 site deposits to be about 10% or less, significantly smaller than previous estimates.Two chemically defined melt groups found at Apollo 16 are of broadly low‐K Fra Mauro (LKFM) basalt composition and could not be produced by local craters in the region. The volumetically most important melt group at Apollo 16 is of aluminous LKFM composition and appears to have formed in one event at 3.92 aeons; those rocks probably represent Nectaris basin impact melt. Imbrium basin impact melt may also be present, in quantities of less than 1%. The model presented here suggests that the Descartes Formation (Nectaris ejecta) is a two‐component mixture of anorthositic clastic debris (&gt;90% by volume) laden with aluminous LKFM Nectaris basin impact melt (&gt;10% by volume). If the age of the Nectaris basin is 3.92 aeons as suggested, then there is probably no in situ primordial surface dating from the time of crustal formation, and the total thickness of the highlands “megaregolith” may be greater than a factor of ten higher than the currently quoted value(1–2 km).

Stratigraphy of the descartes region (Apollo 16): implications for the origin of samples

Analysis of terrain in the Apollo 16 Descartes landing region shows a series of features that form a stratigraphic sequence which dominates the history and petrogenesis at the site. An ancient 150 km diam crater centered on the Apollo 16 site is one of the earliest recognizable major structures. Nectaris ejecta was concentrated in a regional low at the base of the back slope of the Nectaris basin to form the Descartes Mountains. Subsequently, a 60 km diam crater formed in the Descartes Mountains centered about 25 km to the west of the site. This crater dominates the geology and petrogenetic history of the site. Stone and Smoky Mountains represent the degraded terraced crater walls, and the dark matrix breccias and metaclastic rocks derived from North and South Ray craters represent floor fallback breccias from this cratering event. Subsequent major cratering occurred in the region (Dollond B, etc.) prior to the Imbrium and Orientale basin-forming events but had minor effect on the site. Based on this interpretation, contributions from Imbrium at the Apollo 16 site are minor and those from Orientale negligible. The petrology of the Apollo 16 rocks supports this stratigraphic and process model of a local crater-dominated history for this region.

Volatile Elements in Apollo 16 Samples: Possible Evidence for Outgassing of the Moon

Several Apollo 16 breccias, including one containing goethite, are strikingly enriched in volatile elements such as bromine, cadmium, germanium, antimony, thallium, and zinc. Similar but smaller enrichments are found in all highland soils. It appears that volcanic processes took place in the lunar highlands, involving the release of volatiles including water. The lunar thallium/uranium ratio is 2 x 10-(4) of the cosmic ratio, which suggests that the moon's original water content could not have exceeded the equivalent of a layer 22 meters deep. The cataclastic anorthosites at the Apollo 16 site may represent deep ejecta from the Nectaris basin.