The lunar crater chronology has been built by connecting radiometric ages of sampled terrains with the areal crater densities on those terrains, particularly the density at diameters (D) ≥ 1 km (aka ...N(>1)). In the past, very few crater chronologies have considered the effect of terrain properties on the crater densities and size-frequency distributions (SFDs) used to build them. This influence is especially important when N(>1) cannot be directly measured. Here we study this influence by using the Model Production Function (MPF) chronology, which incorporates terrain properties into computing N(>1) and absolute model ages (AMAs) through computing crater diameters using standard crater scaling laws. Furthermore, we also gain a better understanding of actual lunar terrain properties by adjusting the MPF to reproduce AMAs close to the radiometric ages of several Apollo sample sites. The properties examined are the consolidation state of the terrain, its effective cratering strength, and density. Overall, we find that the impact melt of Copernicus crater (∼0.8 Ga) is stronger and more consolidated than the mare and highland terrains investigated. The mare become less consolidated and weaker as they age (from 3.1 to 3.8 Ga) – likely due to fracturing and regolith formation by subsequent impacts. The highlands (∼3.8 Ga) are the weakest terrain. The analysis of terrain proprieties allows our MPF computations to reproduce the radiometric ages, and the impact melt and ejecta of Copernicus crater to have the same age, as expected. The new lunar terrain property constraints can be used with the MPF to derive more robust absolute model ages for unsampled terrains. The values presented in this work for impact melt, ejecta, mare, and highlands can serve as references.
•Model Production Function used to infer how terrain properties affect age derivation.•New constraints on strength terrain properties of Apollo sampling sites are given.•Crater model ages should consider terrain properties, particularly for D < 200 m.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Precise crystallisation ages have been determined for a range of Apollo basalts from Pb-Pb isochrons generated using Secondary Ion Mass Spectrometry (SIMS) analyses of multiple accessory phases ...including K-feldspar, K-rich glass and phosphates. The samples analysed in this study include five Apollo 11 high-Ti basalts, one Apollo 14 high-Al basalt, seven Apollo 15 low-Ti basalts, and five Apollo 17 high-Ti basalts. Together with the samples analysed in two previous similar studies, Pb-Pb isochron ages have been determined for all of the major basaltic suites sampled during the Apollo missions. The accuracy of these ages has been assessed as part of a thorough review of existing age determinations for Apollo basalts, which reveals a good agreement with previous studies of the same samples, as well as with average ages that have been calculated for the emplacement of the different basaltic suites at the Apollo landing sites. Furthermore, the precision of the new age determinations helps to resolve distinctions between the ages of different basaltic suites in more detail than was previously possible. The proposed ages for the basaltic surface flows at the Apollo landing sites have been reviewed in light of these new sample ages. Finally, the data presented here have also been used to constrain the initial Pb isotopic compositions of the mare basalts, which indicate a significant degree of heterogeneity in the lunar mantle source regions, even among the basalts collected at individual landing sites.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The Late Heavy Bombardment Bottke, William F; Norman, Marc D
Annual review of earth and planetary sciences,
08/2017, Volume:
45, Issue:
1
Journal Article
Peer reviewed
Heavily cratered surfaces on the Moon, Mars, and Mercury show that the terrestrial planets were battered by an intense bombardment during their first billion years or more, but the timing, sources, ...and dynamical implications of these impacts are controversial. The Late Heavy Bombardment refers to impact events that occurred after stabilization of the planetary lithospheres such that they could be preserved as craters and basins. Lunar melt rocks and meteorite shock ages point toward a discrete episode of elevated impact flux between ∼3.5 and ∼4.0-4.2 Ga, and a relative quiescence between ∼4.0-4.2 and ∼4.4 Ga. Evidence from Precambrian impact spherule layers suggests that a long-lived tail of terrestrial impactors lasted to ∼2.0-2.5 Ga. Dynamical models that include populations residual from primary accretion and destabilized by giant planet migration can potentially account for the available observations, although all have pros and cons. The most parsimonious solution to match constraints is a hybrid model with discrete early, post-accretion and later, planetary instability-driven populations of impactors.
Earth’s moon is a primary exploration target for space agencies around the world. The Moon records and preserves key information about fundamental processes that shape planetary crusts such as impact ...cratering. Understanding shock effects in lunar anorthite (Ca‐rich endmember of plagioclase feldspar), the principal component of anorthosite and the most common crustal mineral on the Moon, is key to the early evolution of the Moon and terrestrial planets in the solar system. However, there has not been a systematic study of shock effects in lunar anorthite using modern analytical techniques that could be used in future lunar surface exploration, such as Raman spectroscopy. This study examined 23 polished thin sections from all six Apollo missions using optical and Raman spectroscopy. We documented a variety of shock features recording low to moderate shock levels, including fractures, deformed twins, undulatory extinction, planar features, and partially isotropic plagioclase. A notable nonobservation was the absence of planar deformation features (PDFs) or completely isotropic (i.e., diaplectic glass) in this suite of samples. Raman spectroscopy results of the observed shock features show similar progressive changes in terrestrial samples: As shock level increases, band broadening, reduction of intensities, and peak loss were observed. Our Raman data are efficient in identifying shock levels and distinguishing planar features from PDFs and deformed twins, and differentiating amorphous areas from crystalline plagioclase, suggesting Raman spectroscopy as a useful tool for purposely selecting moderately to strongly shocked samples to return in future lunar missions. Our study can also help the interpretation of Raman data of impact materials from the past and future exploration missions and demonstrate the utility of Raman spectroscopy for documenting and selecting samples for future lunar missions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ages determined for magnesian and ferroan anorthosite crustal rock suites overlap, suggesting they formed contemporaneously about 4.3–4.5 Ga. A notable exception is the Sm-Nd age previously ...determined on Mg-suite gabbronorite 67667 which is at least 100 Ma younger than the youngest ferroan anorthosite. New chronologic measurements of 67667 presented here yield concordant Sm-Nd and Rb-Sr mineral isochron ages of 4349 ± 31 Ma and 4368 ± 67 Ma, suggesting the sample is older than previous estimates. Furthermore, a whole rock Sm-Nd isochron of Mg-suite rocks from the Apollo 14, 15, 16, and 17 landing sites yields an age of 4348 ± 25 Ma, indicating that Mg-suite magmatism was widespread and roughly contemporaneous on the lunar nearside. Analysis of Sm-Nd internal isochron ages confirms that Mg-suite magmatism was restricted to a period between about 4.33 and 4.35 Ga at the Apollo 14, 15, 16, and 17 landing sites and was synchronous with magmatism at the Apollo 16 site associated with the ferroan anorthosite suite between 4.35 and 4.37 Ga. Magnesian- and ferroan anorthosite suite rocks with ages younger than ∼4.33 Ga appear to have experienced slow cooling in the deep lunar interior, so that the ages record when the samples cooled below the closure temperature of the Sm-Nd isotopic system and not the time they crystallized.
The ages determined for Mg-suite and ferroan anorthosite suite rocks are concordant with the age determined for the formation of urKREEP of 4350 ± 34 Ma using the Sm-Nd isotopic systematics of 67667 and measurements completed on norite 78238, troctolite 76535, KREEP basalt 15386, and gabbronorite NWA 773. Crystallization ages of Mg-suite and FAS are also concordant with the average of 146Sm-142Nd ages previously determined for the formation of the mare basalt source region of 4333 ± 30 Ma. The similarity of ages for Mg-suite magmatism, ferroan anorthosite suite magmatism, urKREEP formation, and formation of the mare basalt source regions implies the processes that produced these rocks were petrogenetically linked. It also implies that both early-stage and late-stage lunar magma ocean cumulates formed over a relatively short duration of <40 Ma. Late and somewhat rapid solidification of a lunar magma ocean can account for the concordance of ferroan anorthosite suite rocks, urKREEP, and the mare basalt source regions. However, the major and trace element compositions of Mg-suite magmas preclude them from being a primary differentiation product of the lunar magma ocean. Instead, the Mg-suite could be produced as a result of mixing of magma ocean solidification products during density driven overturn occurring immediately after, or perhaps during, solidification of the lunar magma ocean. This scenario not only accounts for the chronology of the various rock suites, but is consistent with the petrogenesis of the Mg-suite that involves the interaction between pre-existing Mg-rich, plagioclase-rich, and urKREEP-rich cumulates of the magma ocean.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The study of basaltic samples returned half a century ago through the Apollo missions has provided unparalleled insights into the magmatic processes associated with volcanism on planetary bodies. ...Traditional study of these samples has been conducted in two dimensions (2D) via interrogation of thin sections and subsequent in-situ chemical analyses, yet returned samples are three-dimensional (3D) objects and therefore preserve evidence of lunar volcanic processes in 3D. Here, X-ray computed tomography (XCT) was used for the first time to evaluate lava emplacement mechanisms on the lunar surface. A total of six samples from the Apollo 11, 12, 15, and 17 missions were studied. From volumetric mineralogies, textures, and petrofabrics, lunar lava cooling histories were inferred. Collectively, these physical characteristics were then correlated with pāhoehoe lava flow lobe stratigraphy. Samples 10057 and 15556 are inferred to have crystallized in the lobe crust of their respective lava flow lobes. Samples 12038, 12043, and 70017 may mark the transition between the vesiculated, fine-grained lobe crust, and the dense, coarse-grained lobe core in their respective flows. Finally, coarse-grained, and non-vesiculated sample 15085 is inferred to have crystallized in a lobe core. No statistically significant petrofabric is preserved in any of the samples, indicating that the basalt samples studied here, and the lava flows they originated from, experienced minimal strain during emplacement and solidification on the lunar surface. This is consistent with the low viscosities attributed to lunar mare lavas. Future in-situ sampling of extraterrestrial basaltic products should focus on detailed documentation of, and collection from, stratigraphically well-characterized lava flows to further evaluate the interpretations presented here.
•Lunar basalts texturally correlate to terrestrial pāhoehoe lava stratigraphy.•Lunar lavas generally experienced minimal strain during solidification.•Excel spreadsheet developed to assess true 3D component size distributions from XCT.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Lunar regolith is the most critical material for the in-situ resource utilization in the crewed Moon exploration missions. This natural material can be utilized for the additive manufacturing of ...concrete or ceramic parts on the Moon's surface to support permanent human presence on the surface of Earth's natural satellite. Due to the scarcity of regolith on Earth, its simulants are used in lab research to prepare the technology for Moon missions. The present study is devoted to the characterization of lunar regolith simulant material, recently developed by the University of Central Florida, that is considered as a suitable material for regolith-focused additive manufacturing technologies. This paper describes the characterization of the LHS-1 and LMS-1 simulants using XRF, XRD, SEM, EDX, DTA, TGA, UV/Vis/NIR spectroscopy, and Laser diffractometry methods to provide data on their mineral, chemical, and fractional composition, as well as, on their morphology and optical properties. The results were compared to the data of the previously developed simulants and the original lunar samples delivered by Apollo and Luna missions. It was found that LHS-1 and LMS-1 simulants well mimic the primary properties of the original lunar regolith and can be potentially used for ISRU research tasks.
•New mineral-based simulants for lunar highlands and mare regoliths simulants are described.•Simulants mimic mineral, chemical, and fractional composition of highlands and mare lunar soils.•The physical and chemical properties of the simulants were compared to the ones of genuine lunar samples.•LHS-1 and LMS-1 simulants can be used for ISRU testing, additive manufacturing studies, and more.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Numerous paleomagnetic studies attribute the magnetization preserved within Apollo samples to an ancient dynamo. However, other works propose that lunar rocks were instead magnetized by either ...transient impact‐related magnetic fields on the Moon or by the return spacecraft. To test whether lunar samples could have been magnetized during return to Earth, sample handling, or transport, we exposed lunar rocks to 5–10 mT fields for varying durations. We then determined how easily these magnetic overprints could be removed and how paleointensity estimates are affected by the overprints and their removal. We found that magnetic overprints were cleaned by alternating field (AF) demagnetization to ∼10–30 mT for nearly all samples and that acceptable paleointensities may be obtained from higher AF levels. Therefore, high coercivity (>30 mT) magnetizations observed within lunar rocks are generally not magnetic contamination and were initially acquired on the Moon.
Plain Language Summary
Scientists study magnetism preserved within rocks to understand the histories of magnetic fields on planets. However, exposure to strong magnetic fields such as those produced by spacecraft electronics or magnets can partially remagnetize rocks. Therefore, it is important to rule out magnetic contamination in samples prior to making inferences about the histories of natural magnetic fields generated by planets or by meteorite impacts. We conducted a series of experiments wherein we intentionally exposed various rocks from the Moon to strong magnetic fields to (a) see how easily magnetic contamination could be removed and (b) understand how the contamination and secondary effects related to its removal could skew our interpretation of ancient lunar magnetic field intensities. We found that, in nearly all cases, techniques commonly used by rock magnetism specialists were able to successfully remove magnetic contamination without sacrificing accuracy in determining the intensity of ancient lunar magnetic fields from magnetically uncontaminated portions of rocks. Therefore, we conclude that much of the magnetization observed in lunar rocks is natural in origin and was acquired on the Moon.
Key Points
Magnetic fields associated with spacecraft transit or certain laboratory procedures may impart lunar samples with magnetic overprints
We find such magnetic contamination can be successfully cleaned from nearly all samples, enabling reliable paleointensity determinations
The modern lunar paleointensity record reflects magnetization acquired on the Moon from ancient dynamo fields or impact‐related fields
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
•Statistics of 269 K–Ar datings of lunar highland returned samples are reviewed.•These are compared against 94 datings of meteoritic highland rocks from 21 localities.•Cumulative effect of impact ...gardening on presence of melt at surface is modelled.•Combined modelling and analysis contradicts idea of a short terminal cataclysm.
This work revisits the hypothesis of the so-called ‘lunar terminal cataclysm’ suggested by Tera et al. (1973, 1974) as a strong peak in the meteorite bombardment of the Moon around 3.9 Ga ago. According to the hypothesis, most of the impact craters observed on the lunar highlands formed during this short time period and thus formed the majority of the lunar highland impact breccias and melts. The hypothesis arose from the observation that the ages of highland samples from all the lunar missions are mostly grouped around 3.9–4.0 Ga. Since those missions, however, radiometric dating techniques have progressed and many samples, both old and new, have been re-analyzed. Nevertheless, the debate over whether there was a terminal cataclysm persists. To progress in this problem we summarized results of 269 K–Ar datings (mostly made using the 40Ar–39Ar technique) of highland rocks represented by the Apollo 14, 15, 16, 17 and Luna 20 samples and 94 datings of clasts of the highland rocks from 23 lunar meteorites representing 21 localities on the lunar surface, and considered them jointly with the results of our modelling of the cumulative effect of the impact gardening process on the presence of impact melt of different ages at the near-surface of the Moon.
The considered results of K–Ar dating of the Apollo-Luna samples of lunar highland rocks confirmed a presence of strong peak centered at 3.87 Ga. But since the time when the hypothesis of terminal cataclysm was suggested, it has become clear that this peak could be a result of sampling bias: it is the only prominent feature at the sites with an apparent domination of Imbrium basin ejecta (Apollo 14 and 15) and the age pattern is more complicated for the sites influenced not only by Imbrium ejecta but also that of other basins (Nectaris at the Apollo 16 site and Serenitatis at the Apollo 17 site). Our modelling shows that the cataclysm, if it occurred, should produce a strong peak in the measured age values but we see in the considered histograms and relative probability plots not only the 3.87 Ga peak (due to Imbrium basin), but also several secondary peaks caused by the formation of other basins distributed between 3.87 and 4.25 Ga.
The lunar terminal cataclysm hypothesis is in disagreement with the distribution of K–Ar ages for the highland rocks of the lunar meteorites. The population of lunar meteorites representing localities randomly distributed over the lunar surface, and thus free from the mentioned sampling bias, shows no ∼3.9 Ga peak as it should, if the cataclysm did occur.
We conclude that the statistics of sample ages contradict the terminal cataclysm scenario in the bombardment of the Moon. We also see evidence for the formation of several impact basins between 3.87 and 4.25 Ga which is likewise incompatible with the hypothesis of a short interval cataclysm. There remain other basins, including the largest South Pole – Aitken, the ages of which should be determined in future studies to further clarify the impact history. Sample-return missions targeted to date several key basins need to be planned, and the continued study of lunar meteorites may also bring new details to the general view of the impact bombardment of the Moon.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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