South Pole-Aitken Research Articles

Potential sample sites for South Pole–Aitken basin impact melt within the Schrödinger basin

Determining the age of the South Pole–Aitken (SPA) basin ranks among the highest priorities in lunar science. This datum would constrain the timing of the oldest and largest basin-forming event on the Moon, information that is essential to any evaluation of the collisional evolution of the early Solar System. To locate material that preserves the age of SPA, a geochemical model of SPA impact melt is integrated with chemical and mineralogical analyses of the lunar surface determined from orbit. Results suggest the southern wall of Schrödinger basin contains material with the mineralogical and geochemical signatures of SPA melt and, thus, represents a candidate destination for sampling material that can constrain the age of the SPA impact.

Constraints on the depths of origin of peak rings on the Moon from Moon Mineralogy Mapper data

Important to understanding the process of basin formation on planetary bodies are constraints on the mineralogy and depths of origin of interior ring structures. We summarize previous analyses of the mineralogy of basin materials on the Moon and use hyperspectral image-cubes from the Chandrayaan-1’s Moon Mineralogy Mapper (M3) to determine the mineralogy of interior rings in lunar protobasins and peak-ring basins. Nearly all peak rings outside of South Pole-Aitken (SPA) basin have extensive outcrops of pure anorthosite (⩾99% plagioclase) on the order of several square kilometers in areal dimensions. No obvious mantle components were identified. Outcrops spectrally dominated by pyroxene occur within SPA and other areas of thinner crust, such as regions within large ancient impact basins. In addition, many outcrops of candidate shocked plagioclase are observed within the same peak rings containing crystalline plagioclase. These spectral observations strongly support a crustal origin for peak rings on the Moon. Recent analyses of the Orientale basin and other lunar basins show that the inner rings of multi-ring basins are also anorthosite-rich and therefore derived from the lunar crust. To further constrain the depths of origin of materials forming peak rings, we compare the pre-impact crustal thickness for each basin with calculated vertical reference points, including: (1) maximum depth of excavation, which is the deepest point at which the crater will excavate material, (2) maximum depth of melting, which is deeper than the maximum depth of excavation and represents the maximum extent of impact-induced melting beneath the sub-impact point, and (3) maximum depth of the transient cavity, which is deepest part of the growing transient cavity that is formed of both excavated and displaced target material. Taken together with the observed mineralogy, the origin of peak-ring lithologies is constrained to stratigraphic levels near the maximum depth of excavation and likely shallower than this if the lower crust is comprised of noritic materials. The maximum depth of melting for peak-ring basins extends far into the mantle and is therefore not a valid proxy for estimating the depth of origin of materials forming peak rings. We find that our estimates of the depths of origin of peak-ring materials are consistent with current models of peak-ring formation, including predictions by hydrocode simulations and conceptual models emphasizing the role of interior impact melting and centro-symmetric collapse of the walls of the transient cavity. Firmer constraints on the depths of origin of peak rings on the Moon await an improved understanding of the crustal compositional structure, particularly that of the lower crust, and improved model predictions on the sampling depths and shock pressures experienced by uplifted peak-ring materials.

Landing site selection for Luna-Glob mission in crater Boguslawsky

Boguslawsky crater (72.9°S, 43.3°E, ~100km in diameter) is a primary target for the Luna-Glob mission. The crater has a morphologically smooth (at the resolution of WAC images), flat, and horizontal floor, which is about 55–60km in diameter. Two ellipses were selected as specific candidate landing areas on the floor: the western ellipse is centered at 72.9°S, 41.3°E and the eastern ellipse is centered at 73.9°S, 43.9°E. Both ellipses represent areas from which Earth is visible during the entire year of 2016 and lack permanently shadowed areas. Boguslawsky crater is located on or near the rim of the South Pole–Aitken basin, which provides the unique possibility to sample some of the most ancient rocks on the Moon that probably pre-date the SPA impact event. The low depth/diameter ratio of Boguslawsky suggests that the crater has been partly filled after its formation. Although volcanic flooding of the crater cannot be ruled out, the more likely process of filling of Boguslawsky is the emplacement of ejecta from nearby and remote large craters/basins. Three morphologically distinctive units are the most abundant within the selected landing ellipses: rolling plains (rpc), flat plains (fp), and ejecta from crater Boguslawsky-D (ejf), which occurs on the eastern wall of Boguslawsky. The possible contribution of materials from unknown sources makes the flat and rolling plains less desirable targets for landing. In contrast, ejecta from Boguslawsky-D represents local materials re-distributed by the Boguslawsky-D impact from the wall onto the floor of Boguslawsky. Thus, this unit, which constitutes about 50% of the eastern landing ellipse, represents a target of clearer provenance and a higher scientific priority.

Small-scale lunar farside volcanism

We identified and mapped 28 mare basalt occurrences, between the Australe and South Pole-Aitken basins on the southern lunar farside, and determined their absolute model ages (AMAs) by performing crater size–frequency distribution (CSFD) measurements. Our study area can be subdivided into seven major mare basalt occurrences in and around Bolyai, Roche V, Rosseland, Pauli, and Roche craters, south of both Rosseland and Coblentz craters, as well as mare patches between Eötvös and Roche craters. The AMAs of the mare basalts in and around Bolyai crater range from 2.1Ga to 3.5Ga (two units), varying drastically within short distances. The mare patches south of Coblentz crater contain nine units that have AMAs ranging from 2.1Ga to 3.8Ga. The mare basalts in Roche V crater show an AMA of 2.2Ga. We mapped seven volcanic units in Rosseland crater and derived AMAs for five of these units. The mare basalts in Rosseland crater show the youngest AMAs found in our study area, ranging from 1.5Ga to 2.9Ga. The mare basalt occurrence south of Rosseland crater shows significantly older ages with an AMA of 3.3Ga. The mare basalts in Pauli and Roche craters show AMAs from 1.7Ga to 3.1Ga. The mare patches between Eötvös and Roche craters show a similar range of AMAs from 1.9Ga to 3.1Ga. The AMAs of the mare basalts in our study show that the lunar farside was volcanically active for nearly as long as the lunar nearside (1.2Ga ago), or at least longer than previously thought (2.5Ga ago) and predicted by models of the ascent and eruption of lunar basalts. In addition, we calculated thicknesses and volumes of the investigated mare basalts. With thicknesses between ∼21m and ∼172m and volumes from ∼0.1km3 to ∼379km3, the mare basalts in our study area show a wide range of dimensions, similar to other mare basalts of the lunar near- and farsides.

Trajectory Design for MoonRise: A Proposed Lunar South Pole Aitken Basin Sample Return Mission

This paper presents the mission design for the proposed MoonRise New Frontiers mission: a lunar far side lander and return vehicle, with an accompanying communication satellite. Both vehicles are launched together, but fly separate low-energy transfers to the Moon. The communication satellite enters lunar orbit immediately upon arrival at the Moon, whereas the lander enters a staging orbit about the lunar Lagrange points. The lander descends and touches down on the surface 17 days after the communication satellite enters orbit. The lander remains on the surface for nearly two weeks before lifting off and returning to Earth via a low-energy return.

Imaging the South Pole–Aitken basin in backscattered neutral hydrogen atoms

The lunar surface is very efficient in reflecting impinging solar wind ions as energetic neutral atoms (ENAs). A global analysis of lunar hydrogen ENAs showed that on average 16% of the solar wind protons are reflected, and that the reflected fraction can range from less than 8% to more than 24%, depending on location. It is established that magnetic anomalies reduce the flux of backscattered hydrogen ENAs by screening-off a fraction of the impinging solar wind. The effects of the surface properties, such as porosity, roughness, chemical composition, and extent of weathering, were not known.In this paper, we conduct an in-depth analysis of ENA observations of the South Pole–Aitken basin to determine which of the surface properties might be responsible for the observed variation in the integral ENA flux. The South Pole–Aitken basin with its highly variable surface properties is an ideal object for such studies. It is very deep, possesses strikingly elevated concentrations in iron and thorium, has a low albedo and coincides with a cluster of strong magnetic anomalies located on the northern rim of the basin. Our analysis shows that whereas, as expected, the magnetic anomalies can account well for the observed ENA depletion at the South Pole–Aitken basin, none of the other surface properties seem to influence the ENA reflection efficiency. Therefore, the integral flux of backscattered hydrogen ENAs is mainly determined by the impinging plasma flux and ENA imaging of backscattered hydrogen captures the electrodynamics of the plasma at the surface. We cannot exclude minor effects by surface features.

A large spectral survey of small lunar craters: Implications for the composition of the lunar mantle

A global spectral survey of 4506 immature craters with diameters <1 km was carried out using near-IR data from the Kaguya Spectral Profiler to characterize the composition of the lunar megaregolith. On the basis of band minima and radiative transfer mixing models, small crater spectra fall into three groups: (1) mare basalts with strong absorptions at relatively long wavelengths indicating high ratios of high- to low-Ca pyroxene; (2) norites containing about 50% plagioclase and with pyroxene assemblages dominated by low-Ca pyroxene that occur within the South Pole-Aitken Basin (SPA), near Apollo 14 and other locations near Imbrium Basin, and three major cryptomaria deposits; and (3) noritic anorthosites occurring within the Feldspathic Highlands Terrane containing about 20 wt% pyroxene with a pyroxene assemblage containing exclusively very low-Ca pyroxene. Very few pure anorthosites are present in this survey and there are no occurrences of pyroxene-poor olivine-rich assemblages. Models of the composition of basin ejecta incorporate large amounts of mantle material and the spectral results require that that the sampled mantle is orthopyroxenite. Basin depth-diameter ratios used in the models required to match the measured composition are consistent with prior estimates for the largest basins. The composition found in the SPA and Imbrium regions are consistent with mafic impact melt breccias or basaltic impact melts of basin origin. For SPA we model this composition and find it requires an extremely low impact angle. While this is consistent with prior work on an oblique impact for the SPA event, a more robust solution invokes the production of norite in impact melt seas.

Geological context of potential landing site of the Luna-Glob mission

The region planned for performing the Luna-Glob mission is located in the southern part of the swell surrounding the largest South Pole-Aitken (SPA) basin. The photogeological analysis of the surface topography of this region using the LRO-WAC (resolution of 100 Mpxl) photomaps made it possible to define the following groups of morphological units (area types): (1) related to the formation of relatively fresh impact craters; (2) associated with larger (>100 km across) degraded craters including (2a) external and (2b) inner facies; and (3) occupying intercrater spaces. The comparison of the geological map with the map illustrating the distribution of the epithermal neutron flow (Mitrofanov et al., 2012) shows no correlation between them. Consequently, one should not expect development of rock complexes, which would be characterized by elevated concentrations of water in the region chosen for the Luna-Glob mission and, thus, considered among the first-priority targets. The comparison of the neutron flow distribution with the map of circular polarization of the Mini-RF radar beam also shows no correlation. This means that high values of circular polarization reflect elevated concentrations of rock fragments rather than water accumulations. Even though ice fragments are present, their sizes should only slightly be less as compared with the radar wavelength (12.6 cm). The region planned for investigations in the scope of the Luna-Glob mission corresponds to the swell of the largest (and, likely, oldest) preserved basin and offers a potential opportunity to analyze ancient material of this planet and introduce important constraints into the spectrum of models proposed for explaining the Moon’s origin.

Surveying the South Pole-Aitken basin magnetic anomaly for remnant impactor metallic iron

The Moon has areas of magnetized crust (“magnetic anomalies”), the origins of which are poorly constrained. A magnetic anomaly near the northern rim of South Pole-Aitken (SPA) basin was recently postulated to originate from remnant metallic iron emplaced by the SPA basin-forming impactor. Here, we remotely examine the regolith of this SPA magnetic anomaly with a combination of Clementine and Lunar Prospector derived iron maps for any evidence of enhanced metallic iron content. We find that these data sets do not definitively detect the hypothesized remnant metallic iron within the upper tens of centimeters of the lunar regolith.

Petrography, geochronology and source terrain characteristics of lunar meteorites Dhofar 925, 961 and Sayh al Uhaymir 449

Dhofar (Dho) 925, 961 and Sayh al Uhaymir (SaU) 449 are brecciated lunar meteorites consisting of mineral fragments and clasts from a range of precursor lithologies including magnesian anorthositic gabbronorite granulites; crystalline impact melt breccias; clast-bearing glassy impact melt breccias; lithic (fragmental) breccias; mare basalts; and evolved (silica-rich) rocks. On the similarity of clast type and mineral chemistry the samples are likely grouped, and were part of the same parent meteorite. Phosphate Pb–Pb ages in impact melt breccias and matrix grains demonstrate that Dho 961 records geological events spanning ∼500Ma between 4.35 and 3.89Ga. These Pb–Pb ages are similar to the ages of ‘ancient’ intrusive magmatic samples and impact basin melt products collected on the lunar nearside by the Apollo missions. However, the samples’ bulk rock composition is chemically distinct from these types of samples, and it has been suggested that they may have originated from the farside South Pole-Aitken impact basin (i.e., Jolliff et al., 2008). We test this hypothesis, and conclude that although it is possible that the samples may be from the South Pole-Aitken basin, there are other regions on the Moon that may have also sourced these complex breccias.

Distinguishing between basalts produced by endogenic volcanism and impact processes: A non-destructive method using quantitative petrography of lunar basaltic samples

Impact processes play an important role in shaping and reshaping the surfaces of airless planetary bodies. Such processes produce regoliths and generate melts that crystallize and record the homogenization of the geology at the impact site. If the volume of melt is substantial, the resultant crystallized product has an igneous texture and may be free of xenolithic clasts making it difficult to distinguish from melts produced by endogenic magmatic processes. This has been clearly demonstrated during the return of the Apollo samples from the Moon, where Apollo 14 basalt 14310 was initially described as a mare basalt and was only subsequently reclassified as an impact melt following detailed and time consuming crystallization experiments. Another way of distinguishing lunar impact melts from endogenically-derived mare basalts is through the quantification of the highly siderophile elements (HSE: Pd, Rh, Ru, Ir, Pt, Os), which have relatively low abundances in pristine lunar samples but are high in meteorites and, therefore, may be enriched in impact melts. However, these analyses consume relatively large quantities of valuable sample and because of mass constraints cannot be performed on many lunar samples. In this paper we present a quantitative petrographic method that has the potential to distinguish lunar impact melts from endogenically-derived mare basalts using plagioclase and olivine crystal size distributions (CSDs). The slopes and intercepts of these CSDs are used to show that olivine from impact melts displays a steeper CSD relative to olivine from mare basalts. For plagioclase, generally impacts melts display CSDs with shallower gradients than those from endogenous mare basalts and, as for olivines, plot in a distinct field on a CSD slope vs. CSD intercept plot. Using just a thin section to distinguish impact melts from mare basalts enables the goals of future robotic sample return missions to determine the age of the South Pole-Aitken basin in the Moon, because such missions will potentially only return small (2–4mm) “rocklets” for analysis, obviating HSE analyses for impact melt identification.

GRAIL gravity constraints on the vertical and lateral density structure of the lunar crust

AbstractWe analyzed data from the Gravity Recovery and Interior Laboratory (GRAIL) mission using a localized admittance approach to map out spatial variations in the vertical density structure of the lunar crust. Mare regions are characterized by a distinct decrease in density with depth, while the farside is characterized by an increase in density with depth at an average gradient of ∼35 kg m−3 km−1 and typical surface porosities of at least 20%. The Apollo 12 and 14 landing site region has a similar density structure to the farside, permitting a comparison with seismic velocity profiles. The interior of the South Pole‐Aitken (SP‐A) impact basin appears distinct with a near‐surface low‐density (porous) layer 2–3 times thinner than the rest of the farside. This result suggests that redistribution of material during the large SP‐A impact likely played a major role in sculpting the lunar crust.

Volcanism on farside of the Moon: New evidence from Antoniadi in South Pole Aitken basin

The lunar surface is characterized by asymmetric distribution of its volcanic deposits. The nearside contains about 90% of the mare basalts, while there are a few on the farside with some are in the South Pole Aitken (SPA) basin, which is the largest and oldest impact basin on the Moon. Although the mare volcanism on the nearside of the Moon has been very well characterized, our understanding of the farside is relatively poor. Our geological mapping using Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 Moon Mineralogy Mapper (M3) data of 143km Antoniadi impact basin provides new insights into volcanic processes in the southern SPA. Antoniadi has excavated the SPA floor as deep as 9km. Distribution of secondary craters around Antoniadi shows that the basin was formed by oblique impact with the direction of 50° counter-clockwise from 178°E longitude. Our geological mapping has found, for the first time, several volcanic cones (0.2–2.5km diameter) in association with the mare basalt flows. As per morphological classification, these are C, E, N and P type volcanic cones with abundant volcanic fragments in their summit areas. The base and summit diameters of these cones are similar to those of Marius Hills on the nearside of the Moon. The cumulative size frequency distributions of the summit fragments provide a range of power index values between −3 and −9, suggesting the involvement of dynamic fragmentation of the volcanic materials during the eruptive processes. The grabens in the basin floor show evidence for fissure eruption of the mare basalts. The graben near the central peak exposes about 140m thick basalt sequence containing a minimum of five individual lava layers. The crater size–frequency distributions of the mare deposits suggest an age of 1.6Ga for the flows in the central part of the basin, while 2.6Ga for those on the outer floor near the basin wall, indicating the episodic volcanism in the basin. The young volcanism of 1.6Ga is reported for the first time from SPA. The M3 data reveal that the basin materials contain abundant low-Ca pyroxene, while the mare basalts with high-Ca pyroxene. We calculated the liquidus density of mare basalts based on FeO and TiO2 content, which suggest a range of density from 2.7 to 2.8g/cm3, which would be lower than a gross lunar crust. This would imply the emplacement of basaltic lavas through dikes by simple magma buoyancy or over-pressurization of magma chambers, if they present at the crust-mantle boundary when the magma density is greater than the host crust. The 1.6Ga old volcanism in Antoniadi suggests that the mantle beneath the SPA has been conducive for young magmatism, possibly due to presence of KREEP like material in the mantle.

Using lunar craters to date the South Pole–Aitken basin

The South Pole–Aitken (SPA) basin is known as the oldest and largest ancient impact basin on the Moon, but scientists are still unsure of its exact age. Previous efforts have determined that the impact occurred before 4 billion years ago, during the heavy bombardment period, also known as the lunar cataclysm.

Differentiation of the South Pole–Aitken basin impact melt sheet: Implications for lunar exploration

AbstractWe modeled the differentiation of the South Pole–Aitken (SPA) impact melt sheet to determine whether noritic lithologies observed within SPA formed as a result of the impact. Results indicate differentiation of SPA impact melt can produce noritic layers that may accommodate observed surface compositions but only in specific scenarios. One of nine modeled impact melt compositions yielded layers of noritic materials that account for observations of noritic lithologies at depths of ~6 km. In this scenario, impact occurred before a hypothesized lunar magma ocean cumulate overturn. The 50 km deep melt sheet would have formed an insulating quenched layer at the surface before differentiating. The uppermost differentiated layers in this scenario have FeO and TiO2contents consistent with orbital observations if they were subsequently mixed with the uppermost quenched melt layer and with less FeO‐ and TiO2‐enriched materials such as ejecta emplaced during younger impacts. These results verify that noritic lithologies observed within SPA could have formed as a direct result of the impact. Therefore, locations within SPA that contain noritic materials represent potential destinations for collecting samples that can be analyzed to determine the age of the SPA impact. Potential destinations include central peaks of Bhabha, Bose, Finsen, and Antoniadi craters, as well as walls of Leibnitz and Schrödinger basins. Additionally, potential remnants of the uppermost quenched melt may be preserved in gabbroic material exposed in “Mafic Mound.” Exploring and sampling these locations can constrain the absolute age of SPA, a task that ranks among the highest priorities in lunar science.

Constraints on the formation age and evolution of the Moon from 142Nd–143Nd systematics of Apollo 12 basalts

The Moon likely formed as a result of a giant impact between proto-Earth and another large body. The timing of this event and the subsequent lunar differentiation timescales are actively debated. New high-precision Nd isotope data of Apollo mare basalts are used to evaluate the Low-Ti, High-Ti and KREEP mantle source reservoirs within the context of lunar formation and evolution. The resulting models are assessed using both reported 146Sm half-lives (68 and 103 Myr). The linear relationship defined by 142Nd–143Nd systematics does not represent multi-component mixing and is interpreted as an isochron recording a mantle closure age for the Sm–Nd system in the Moon. Using a chondritic source model with present day μ142Nd of −7.3, the mare basalt mantle source reservoirs closed at 4.45−09+10 Ga (t1/2Sm146=68 Myr) or 4.39−14+16 Ga (t1/2Sm146=103 Myr). In a superchondritic, 2-stage evolution model with present day μNd142 of 0, mantle source closure ages are constrained to 4.41−08+10 (t1/2Sm146=68 Myr) or 4.34−14+15 Ga (t1/2Sm146=103 Myr).The lunar mantle source reservoir closure ages <4.5 Ga may be reconciled by 3 potential scenarios. First, the Moon formed later than currently favored models indicate, such that the lunar mantle closure age is near or at the time of lunar formation. Second, the Moon formed ca. 4.55 to 4.47 Ga and small amounts of residual melts were sustained within a crystallizing lunar magma ocean (LMO) for up to ca. 200 Myr from tidal heating or asymmetric LMO evolution. Third, the LMO crystallized rapidly after early Moon formation. Thus the Sm–Nd mantle closure age represents a later resetting of isotope systematics. This may have resulted from a global wide remelting event. While current Earth-Moon formation constraints cannot exclusively advocate or dismiss any of these models, the fact that U–Pb ages and Hf isotopes for Jack Hills zircons from Australia are best explained by an Earth that re-equilibrated at 4.4 Ga or earlier following the Moon-forming impact, does not favor a later forming Moon. If magma oceans crystallize in a few million years as currently advocated, then a global resetting, possibly by a large impact at 4.40 to 4.34 Ga, such as that which formed the South Pole Aitken Basin, best explains the late mantle closure age for the coupled Sm–Nd isotope systematics presented here.

Geologic structure generated by large‐impact basin formation observed at the South Pole‐Aitken basin on the Moon

AbstractThe South Pole‐Aitken (SPA) basin is the largest clearly recognized basin on the lunar surface. Determining the composition and structure of the SPA basin interior provides critical constraints on the deep crustal and/or mantle composition of the Moon and improves our understanding of large‐basin‐forming impact processes. Here we present a new mineralogical map of the SPA basin interior, based on high‐spatial‐resolution reflectance spectra using the SELENE (Kaguya) multiband imager, which is combined with topographic data in order to interpret the geologic context. The derived mineralogical map suggests extensive distribution of ejected low‐Ca pyroxene‐dominant mantle material with the presence of purest anorthosite crustal materials surrounding a possible melt pool of 0.26 to 0.33 of the basin diameter near the basin center, which is significantly smaller than that suggested by the crater‐scaling law. The absence of clear evidence of lower crustal material is consistent with recent impact simulation results.

Lunar mare volcanism: lateral heterogeneities in volcanic activity and relationship with crustal structure

Abstract Lunar mare basalts are spatially unevenly distributed, and their abundances differ between the nearside and farside of the Moon. Although mare asymmetry has been attributed to thickness variations in the low-density anorthositic crust, the eruptive mechanism of lunar magma remains unknown. In this study, we investigate the relationship between mare distribution and crustal thickness using geological and geophysical data obtained by the SELENE (Kaguya) and the Gravity Recovery and Interior Laboratory spacecraft, and quantitatively re-evaluate the influence of the anorthositic crust on magma eruption. We identify a lateral heterogeneity in the upper limit of crustal thickness that allows magma extrusion to the surface. In the Procellarum KREEP Terrane, where the surface abundances of heat-producing elements are extremely high, magmas can erupt in regions of crustal thickness below about 30 km. In contrast, magma eruptions are limited to regions of crustal thickness below about 20 km in other nearside regions, around 10 km in the South Pole–Aitken Basin and approximately 5 km in the farside Felspathic Highland Terrane. Such heterogeneity may result from lateral variations in magma production in the lunar mantle and/or crustal density.

Kaguya observation of the ion acceleration around a lunar crustal magnetic anomaly

We investigate Kaguya observation of ion acceleration around a lunar crustal magnetic anomaly located in the South Pole-Aitken basin at an altitude of 100km. The accelerated ions in the 230eV to 1.5keV energy range were identified by a characteristic dispersion signature in the energy-time spectrogram that appeared repeatedly upon Kaguya׳s approach to the magnetic anomaly. The interplanetary magnetic field was almost parallel to the solar wind velocity and thus the electric field was very small. The results of our analysis show that ions with energies below 230eV were accelerated up to 1.5keV by an electric field produced by the interaction between the solar wind and the magnetic anomaly. We argue that the low-energy ions mainly originated from the solar wind ions with energies of 450eV that were backscattered on the lunar surface.

Impact melt differentiation in the South Pole-Aitken basin: Some observations and speculations

The stratigraphy of the South-Pole Aitken basin (SPA) interior is consistent with that of a massive impact melt sheet that differentiated to form cumulates. Spectroscopic and geophysical constraints on the stratigraphy of SPA suggest a ~12.5km thick layer of norite above ultramafic pyroxenite and dunite layers. A similar stratigraphy is produced from differentiation by crystal settling of a ~50km thick impact melt sheet (lunar impact melt sheets >10km thick likely undergo differentiation by crystal settling) formed by an oblique impact (and thus containing ~20vol. % crustal material). We propose that impact melt differentiation can account for geophysical (nonzero crustal thickness) and geochemical (~2ppm Th) anomalies in SPA.