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Heavy Bombardment on the Earth at ~3.85 Ga: The Search for Petrographic and Geochemical Evidence
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The Moon experienced an interval of intense bombardment peaking at ~3.85 ± 0.05 Ga; subsequent mare plains as old as 3.7 or 3.8 Ga are preserved. It can be assumed that the early Earth must have been subjected to an even more intense impact flux resulting from its larger size and because of its proximity to the Moon. Siderophile-element analyses (e.g., Ir abundance) of the oldest sediments on Earth could be used to indicate past escalated influxes of extraterrestrial material. In addition, shocked minerals may also be present in the oldest extant rocks of sedimentary origin as detrital minerals, and remnants of impact ejecta might exist in early Archean formations. Searches for impact signatures have been initiated in the oldest sediments on the Earth, from the early Archean (>3.7 Ga) terrane of West Greenland; some of these rocks have been interpreted to be at least 3.8 Ga in age. So far, unequivocal evidence of a late heavy bombardment on the early Earth remains elusive. We conclude that either the sedimentation rate of the studied sediments was too fast and therefore too diluting to record an obvious signal, or the ancient bolide flux has been overestimated, or the bombardment declined so rapidly that the Greenland sediments, some even at ~3.85 Ga in age, do not overlap in time with it.
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... They do not form in uids. But large impacts produce disproportionately great quantities of melt-uids [11], [12], [13], [14], [15]. In consequence, diagnostic shock metamorphic features may have been immediately swamped by melt or eventually annealed by the heat retained in the molten mass, thermal metamorphism overwhelming shock metamorphism. ...
... This failure would be even more severe in estimating the amount of melt produced by large impacts that pierced the thin hot crust of the early Earth [15]. The disproportionately great amount of molten material produced by large impacts [11], [12], [13], [14], [15] includes melts produced during long-lived postimpact decompression-melting [24]. ...
... This 'Asian Circle' -which is not the same feature as the Himalayan arc -has a diameter of approximately 5350 km, a gure that should be compared to the 'approximately 5000 kilometres', twice independently calculated from the lunar cratering record for the diameter of the largest expected terrestrial LHB scar, once by Thomas Gold (personal communication, 1986), and once by Ryder et al. (2000) [14]. ...
Bombardment of the Earth at an early date established initial conditions that have affected all later regional geology. No part of the Earth's crust escaped. Activity at the brittle-ductile boundary has caused large-diameter scars to be sporadically regenerated upward throughout the four-billion years of our planet's history. Although these three-dimensional 'craterform' scars have evolved in many different manners, and many are covered over at any given time, many retain observable two-dimensional map-outlines with circular curvature. These inherited scar-features have been regenerated 'cold' from below and are fundamentally different from 'astroblemes', as presently defined, whose constituent rocks had been directly subjected to the high temperatures and pressures that accompany extra-terrestrial impacts. The varied present-day manifestations of these scars need to be described and categorized as has been done for faults, folds, rocks, and minerals by earlier workers in the earth sciences.
... Significant mass fluxes during a late heavy bombardment (LHB) of $5*10 21 to 10 23 g have been calculated for the inner solar system (Ryder, 2001(Ryder, , 2002Levison et al., 2001). Based on lunar cratering data from Ryder et al. (2000), a terminal LHB can be linked to fluxes at least $1000 times the present flux between 3.90 Ga to 3.85 Ga, and still a few hundred times enhanced fluxes from 3.85 Ga to 3.80 Ga (Koeberl, 2004(Koeberl, , 2006. In contrast, following the lunar cratering record of Hartmann (1999), the EMMAC scenario (Maurette et al., 2000;Maurette, 2006) considers two distinct time windows: a first one, termed sterilization episode, lasting $100 Ma at the end of the formation time interval of the Earth at 4.45 Ga characterized by a MM influx $2*10 6 times the present flux, and a second one, termed early life episode, exhibiting a $500 times greater MM flux from 4.2 to 3.9 Ga ago. ...
... Tera et al. (1974), Wetherill (1975) and Koeberl (2004Koeberl ( , 2006 used the term ''Late Heavy Bombardment" to denote the intense and cataclysmic spike in bombardment in the inner solar system around 3.85 Ga derived from lunar chronology. Based on the arguments of Ryder et al. (2000) for the ages of the large impact basins on the Moon, a $ 60 Ma period of considerable bombardment should have occurred on the Moon between 3.90 and $3.84 Ga. ...
Earth’s mantle contains Ne resembling the solar wind implanted Ne-B component in meteorites (²⁰Ne/²²NeNe-B: ∼12.7). The atmosphere, instead, displays a “planetary” signature (²⁰Ne/²²NeAtm: 9.80). We explore the parameter space of a model that explains these isotopic variations by the contribution of late accreting volatile-rich material (e.g., carbonaceous chondrite-like) to Earth́s atmosphere, while Earth́s mantle incorporated solar wind type Ne that was previously implanted into part of the accreting material.
Analyses of the present-day terrestrial influx mass distributions show two major peaks at large bodies >1 km and small ∼200 µm dust particles. The latter dominate the influx of the surface implanted Ne-B component. Ne measurements of small particles define a maximum surface flux (neon reaching the terrestrial surface) peaking at 9 µm, while larger micrometeorites experience ablation losses and isotopic fractionation upon atmospheric entry. Using these data, we reconstruct the unfractionated Ne-B upper atmosphere flux which peaks at ∼75 µm. As the extraterrestrial influx mass distribution between larger bodies and debris dust is governed by equilibrium due to collisions and fragmentation, it is an approximation of the early solar system (after nebula dissipation), where the mass distribution was similar but total fluxes were higher.
Contributions of Ne-B by small dust and planetary Ne-A from larger bodies strongly depend on formation region. Originating around the 1 AU region, early accretionary fluxes were dominated by Ne-B as large bodies likely contained only negligible amounts of Ne-A. Ne-B will be ultimately delivered to the earliest protoatmosphere by impact or thermal degassing and a significant fraction of Ne-B can enter the Earth́s interior via dissolution into a magma ocean before the Moon-forming impact. After the Moon-forming impact, Ne-B reenters the atmosphere by mantle degassing and a later meteoritic contribution modified the atmospheric composition. This meteoritic component was likely dominated by Ne-A, as the only remaining planetesimals at that time were in the asteroid belt or beyond, leading to preferential contributions of carbonaceous chondrite-type material.
In our model we take into account possible variations of several parameters, e.g. the isotopic composition of the late accretion (i.e., ²⁰Ne/²²Ne: 5.2–9.2). For example, a ²⁰Ne/²²Ne ratio of 8.2 (Ne-A composition) would imply ∼2% mass increase of Earth from CC-type material after the Moon-forming impact, and would require that todaýs atmosphere (²⁰Ne/²²Ne = 9.8) formed by roughly equal mixing of late accreted Ne-A and mantle Ne-B. The amount of Ne-B added from the mantle implies a certain degree of mantle degassing (in this case 82–96%, depending on todaýs mantle neon inventory) and constrains two further parameters: the fraction of solar wind irradiated material delivered to Earth before the Moon-forming impact and the magma ocean depth. The latter determines the fraction of Ne-B dissolved from a protoatmosphere. For example, magma ocean depths between 500 and 2900 km allow 4–15% dissolution of the protoatmospheric Ne-B inventory, and would require only less than 10% of irradiated accreting material. Only unreasonable magma ocean depths lower than 200 km require several ten percent of irradiated material.
... Source craters for these spherule layers have not been identified and the magnitudes of these events are debatable ( Bottke et al., 2012;Johnson and Melosh, 2012), but at least some were probably similar in scale to the lunar basins. Tungsten isotopic compositions of w3.7 Ga metasediments from SW Greenland provide indirect evidence for late accretion ( Schoenberg et al., 2002;Willbold et al., 2011Willbold et al., , 2015Rizo et al., 2016a), but abundances of HSE in early sediments are equivocal ( Ryder et al., 2000;Anbar et al., 2001). Abbott et al. (2012) and Bell and Harrison (2013) suggest that some geochemical features of Hadean and Archean zircons from Western Australia might be linked to the LHB, but petrographic evidence for shock in these grains remains elusive ( Ryder et al., 2000;Koeberl et al., 2000;Cavosie et al., 2004). ...
... Tungsten isotopic compositions of w3.7 Ga metasediments from SW Greenland provide indirect evidence for late accretion ( Schoenberg et al., 2002;Willbold et al., 2011Willbold et al., , 2015Rizo et al., 2016a), but abundances of HSE in early sediments are equivocal ( Ryder et al., 2000;Anbar et al., 2001). Abbott et al. (2012) and Bell and Harrison (2013) suggest that some geochemical features of Hadean and Archean zircons from Western Australia might be linked to the LHB, but petrographic evidence for shock in these grains remains elusive ( Ryder et al., 2000;Koeberl et al., 2000;Cavosie et al., 2004). ...
... Significant mass fluxes during a late heavy bombardment (LHB) of ~5*10 21 to 10 23 g have been calculated for the inner Solar System (Ryder, 2001(Ryder, , 2002Levison et al., 2001). Based on lunar cratering data from Ryder et al. (2000), a terminal LHB can be linked to fluxes at least ~1000 times the present flux between 3.90 Ga to 3.85 Ga and still a few hundred times enhanced fluxes from 3.85 Ga to 3.80 Ga (Koeberl, 2004(Koeberl, , 2006. In contrast, following the lunar cratering record of Hartmann (1999), the EMMAC scenario (Maurette et al., 2000;Maurette, 2006) considers two distinct time windows: a first one, termed sterilization episode, lasting ~100 ...
... Ga derived from lunar chronology. Based on the arguments of Ryder et al. (2000) for the ages of the large impact basins on the Moon, a ~60 Ma period of considerable bombardment should have occurred on the Moon between 3.90 and ~3.84 Ga. ...
Studying the origin and evolution of cosmo- and geochemical reservoirs particularly requires knowledge about the composition and occurrence of the inert noble gases (He, Ne, Ar, Kr, Xe). Earth's atmosphere is characterized by a "planetary" noble gas signature, i.e., depleted from solar element abundances more intensively in lighter than in heavier gases, whereas Earth's interior hosts light noble gases (He and Ne) with a distinct "solar" composition. In particular, Ne isotopic ratios of both the convecting and more primitive mantle, the latter sampled by oceanic island basalts (OIBs), resemble the solar wind (SW) implanted Ne-B component in meteorites with 20Ne/22NeNe-B ~12.7. The atmosphere, instead, displays a lower 20Ne/22Ne ratio of 9.80. The reservoir of the primitive noble gas signatures, traditionally assumed to be isolated in the deep mantle, is not precisely located and some models speculate about Earth’s core as possible source. High resolution release experiments on interior samples of the iron meteorite Washington County (WC) were carried out in this study to identify volume correlated trapped noble gases and to investigate the possibility of noble gas partitioning into metal upon core segregation. Consisting of a mixture of predominantly cosmogenic and solar components, with only minor atmospheric additions, gases are released from schreibersite ((Fe,Ni)3P) at ~1100 °C and kamacite-taenite (Fe,Ni) at ≳1400 °C. The solar signatures are distinct in Ne and He/Ne isotopic ratios with clear 4He excess. Ar, Kr and Xe isotopic ratios are either dominated by spallation or are overprinted by air contamination. Measured 20Ne concentrations of ~4*10-8 cm³STP/g imply that solar wind-implantation into terrestrial precursors and incorporation of <1% core material that resembled Washington County metal would have been sufficient to provide solar type Ne in the core that satisfies observed mantle fluxes. This would be consistent with the core as potential source region. The actual acquisition of the light solar noble gases on Earth can be either explained by solar nebula gas dissolution into a magma ocean or accretion of solar wind irradiated material. The solar wind implantation model is assessed by applying constraints for the present terrestrial influx of particles ranging from 10-16–1025 g, and the size-specific Ne inventory of extraterrestrial matter. Present-day Ne contributions to Earth’s surface peak at interplanetary dust particle sizes of ~9 µm which contain a mean 20Ne/22Ne ratio of 12.61±0.41. This value represents Ne-B in unablated solar wind saturated particle surfaces and dominates the inventory of irradiated, though volatile-poor, matter that accreted to form Earth in the inner Solar system. This is opposed to volatile-rich objects from the outer Solar system containing planetary Ne-A with 20Ne/22Ne ~8.20. The data compilations allow determining the mass and size dependent upper atmosphere Ne flux and infer the contribution during early Earth formation of a) surface correlated Ne-B, dominated by ~75 µm particles with high surface/volume ratio and b) volume correlated Ne-A, dominated by larger bodies. The Ne-acquisition scenario considers delivery of solar wind implanted Ne-B shortly after dissipation of disk gas and Ne incorporation into Earth with 20Ne/22Ne: 12.61±0.41 by dissolution into a magma ocean before the Moon-forming impact. The late veneer contribution of Ne-A to degassed mantle Ne-B establishes the atmospheric inventory with 20Ne/22Ne: 9.80. The model calculations show that, because dominated by implanted components in cosmic dust, only a fraction of a few % of irradiated precursor material is sufficient to account for the solar Ne budget of Earth, thus, demonstrating the significance of dust accretion for the origin of volatiles.
... (4) This value (~0.1 kg/km2/year) for the modern Earth impactor flux [Equation (4)] is modelled to have been 1,000 times greater during the Late Heavy Bombardment (LHB), ~3.9-3.8 Ga (Frey 1980;Ryder et al. 2000;Ryder 2002), therefore roughly 100 kg/km2/year. This flux represents the absolute maximum for extraterrestrial input to Earth at any point during its history, perhaps equalled solely in the initial stages of planetary accretion (Koeberl 2006). ...
The implicit objective of this thesis was to bridge the wide gap that exists between the various domains involved in origins of life studies, and bring them together under one larger domain of stochastic systems chemistry. The introductory chapter of the thesis appreciated and accentuated this inherent interdisciplinarity, with relevant examples, essential to origins of life studies. Furthermore, the chapter contextualised the experimental facets that have contributed to the framework of the thesis. The sources of the starting materials required for the origins of life were expanded upon, including the endogenous and the exogenous materials. The contemporary models of origins of life in the literature were discussed from the context of experiments, theory and computer models, and the limitations of the models involving the lack of early Earth-representative environments have been evaluated. Furthermore, compelling arguments were put forth to encourage experimentation that is inclusive of realistic geological settings. The subsequent chapters discussed the settings for prebiotic chemistry within this geological framework. Adhering to the proposed new paradigm, three distinct yet innately linked experiments were proposed and undertaken – the Photochemistry on the International Space Station experiment (PSS), the inorganic hydrogel environment experiment and studies involving mineral-influenced formose reaction.
... An even more remarkable outcome of the Apollo missions relates to the ages of the impact events, showing that most of the collisional episodes are concentrated between 3.75 and 3.95 Ga . This aspect lead to the formulation of the LHB hypothesis, stating that the Moon experienced a cataclysm (i.e., a spike in the ux of impactors) which resulted in the production of the majority of the lunar basins (Tera et al., 1974;Ryder et al., 2000). However, this hypothesis has been challenged by Schaeer and Schaeer (1977), who pointed out the complications related to the scarce sample population, which is indeed representative of only a small fraction of the total area of the Moon's surface (Warren and Taylor, 2014). ...
The detection and study of high pressure minerals either remotely through seismology or in natural specimens can provide important constraints on physical and chemical properties occurring at normally inaccessible conditions, such as during planetary impact events or deep inside planets. For four and a half billion years, countless impact events have shattered the Moon's surface, leaving a unique record of impact craters. Understanding the nature, and estimating the ages of the largest lunar craters was among the main goals of the Apollo missions. However, despite the large number of samples collected, the ages of the largest craters are still debated. 40Ar/39Ar ages constrained in lunar samples may be biased by subsequent thermal events, hampering our current understanding of the Moon's collisional history. A viable way to evaluate this possibility is to evaluate the behaviour of lunar regolith under shock compression. In this thesis, scanning and transmission electron microscope techniques are used to constrain shock conditions recorded in a regolith breccia, by a detailed description of shockinduced microtextures and mineralogical assemblages. I present the first observation of natural ferropericlase in a lunar rock. My observations suggest that the lunar ferropericlase formed as a result of shock-induced incongruent melting of olivine, a phenomenon found previously only in experiments. Furthermore, I estimated the pressure – temperature evolution of the shock event. Our results indicate that because of its porous nature, the lunar regolith can experience elevated temperatures even during low magnitude impacts. Based on these ndings, we suggest that a more accurate estimate of the ages of the main collisional episodes of the Moon's surface requires a reevaluation of the current 40Ar/39Ar constrains. Subduction of altered oceanic slabs and hydrous sediments control the input of water into the deep Earth's interior. During subduction, hydrous materials are exposed to increasing pressures and temperatures, which causes a chain of prograde metamorphic reactions to occur. Previous experimental investigations indicate that water, bound as hydroxyl groups, can be passed between hydrous phases and consequently delivered by subduction to the deepest portions of the Earth's mantle. Seismological surveys provide information on the seismic structures that characterize subducting scenarios, however, an accurate interpretation of the hydration state is achievable only through experimental constraints on the possible seismic signatures of these hydrous phases. In this thesis, I conducted two projects with the aim of characterizing the single-crystal elasticity of phase E and -(Al,Fe)OOH, two hydrous phases relevant for the delivery and stabilization of water in the Earth's deep interior. In the case of phase E, experimental methodologies were used for the synthesis of single crystals, and an accurate chemical characterization was achieved with state-of-the-art analytical techniques. Brillouin spectroscopy and X-ray diffraction analysis were employed to determine the full elastic tensor and unit-cell parameters, respectively. I found that phase E has very low aggregate velocities, signi cantly lower than those of other minerals expected to be stable at the same pressure and temperature conditions. By combining my findings with previous experimental investigations, aggregate velocities of subducted rocks were evaluated assuming different hydration states. These results imply that if present, phase E is capable of significantly lowering seismic wave velocities, raising the possibility that this hydrous phase could be detected remotely allowing hydrated regions of the deep mantle to be mapped. By performing Brillouin spectroscopy and X-ray diffraction measurements in a diamondanvil cell, the structure and elastic properties of -(Al,Fe)OOH have been examined up to pressures where a second order phase transformation occurs from the P21nm space group to Pnnm. The elastic tensors of both the P21nm and Pnnm structures were constrained experimentally. In addition, by tracking the intensity attenuation of selected reflections we were able to tightly constrain the transition pressure. Our findings are in agreement with previous investigations on the aluminium end member, suggesting that the incorporation of Fe3+ has a limited effect on the P21nm to Pnnm phase transition. Both X-ray diffraction and Brillouin spectroscopy results show that, prior to the transition into the Pnnm phase, the P21nm -(Al,Fe)OOH phase experiences an elastic softening. This softening is associated with a change in the hydrogen bond configuration from asymmetric (P21nm) to disordered (Pnnm). Similar changes can be expected in other hydroxide minerals, suggesting that the elastic softening may be a common precursor of hydrogen bond symmetrization.
... There is a general agreement that the primary crust formed during magma ocean solidification was basaltic in composition (Taylor, 1989(Taylor, , 1993Arndt and Chauvel, 1991) ; although felsic rocks were essential components of the crust already in the Hadean (Harrison et al., 2005(Harrison et al., , 2008Kemp et al., 2010;Wang and Wilde, 2018;Bell et al., 2011Bell et al., , 2014Blichert-Toft and Albarède, 2008;Iizuka et al., 2006;Russell and Arndt, 2005). Asteroid bombardment, especially the Late Heavy Bombardment (LHB) that probably occurred at ca. 3.85 Ga (Ryder et al., 2000;Koeberl, 2006), played a significant role in the resurfacing of our planet and destroyed the primary crust. Only a small volume of pre-LHB rocks survived asteroid resurfacing, recycling into the mantle, and erosion; hence virtually all rocks known on Earth were formed after the LHB. ...
We present the results of a study of an Eoarchean rock assemblage in the Dniester-Bouh Domain of the Ukrainian Shield. This comprises granulite-facies granitoids intercalated with mafic and ultramafic granulites. Zircon U-Pb geochronology indicates enderbite crystallisation at 3786 ± 32 Ma, followed by a subsequent event at ca. 3500 Ma. Several events can be tentatively identified that affected these rocks between ca. 3000 and 2700 Ma. The last zircon growth event took place in response to granulite facies metamorphism and included two separate episodes at ca. 2000 and ca. 1900 Ma. The oldest two zircon populations in enderbites have εHf values around 0, indicating their crystallisation from a protolith with a short crustal residence time. Zircons that crystallised during the 3000–2700 Ma event(s) vary in Hf isotope systematics from εHf ~ 1 at ca. 3000 Ma to εHf ~ -14 at c. 2700 Ma. Paleoproterozoic zircons reveal even more significant variations in εHf value from +6 to –22. Such variations are indicative of juvenile input and mixing with old non-radiogenic Hf.
All Eoarchean rocks are depleted in incompatible trace elements and have negative Ta-Nb, P, and Ti anomalies. Compared to the typical TTG associations, enderbites record depletion in felsic components (SiO2, Na2O, K2O, Rb, Th), and enrichment in mafic ones (TiO2, MgO, CaO, V), allowing them to be defined as “mafic” or “depleted” TTG.
Geochemical data indicate that mafic and ultramafic rocks of the Dniester-Bouh Domain formed by shallow high-degree melting of the mantle, with the absence of garnet in their source, and the presence of residual Ti-bearing minerals and/or amphibole. In contrast, enderbites were formed from a mixed garnet-bearing amphibolite – eclogite source, i.e. melting over a range of pressures/depths. Our preferred model for the formation of the Eoarchean rock association involves the shallow melting of mantle and formation of basalts and accompanying ultramafic cumulates at a spreading centre, with subsequent underthrusting of one segment of oceanic crust beneath the other, and partial melting of hydrated metamorphosed (eclogitized) mafic rocks in the underthrust plate, leading to the formation of the TTG melts.
... Maher and Stevenson 1988). The LHB hypothesis came into existence as a consequence of a 'bottle-neck' in the ages measured for lunar basalts returned to Earth by the Apollo astronauts: there seemed to be a cut-off point prior to ∼4.1 Ga with the suggestion being that older lunar crust (and by corollary Hadean terrestrial crust) had been destroyed by an increase in the flux of asteroid impacts (Ryder et al. 2000;Bottke et al. 2012), modelled to be related to perturbations in the orbits of the giant outer planets (Kemp et al. 2010;Marchi et al. 2014). However, currently the 'bottleneck' lunar crustal ages appear to be the effect of sampling only mare basalts. ...
The aim of this article is to provide the reader with an overview of the different possible scenarios for the emergence of life, to critically assess them and, according to the conclusions we reach, to analyze whether similar processes could have been conducive to independent origins of life on the several icy moons of the Solar System. Instead of directly proposing a concrete and unequivocal cradle of life on Earth, we focus on describing the different requirements that are arguably needed for the transition between non-life to life. We approach this topic from geological, biological, and chemical perspectives with the aim of providing answers in an integrative manner. We reflect upon the most prominent origins hypotheses and assess whether they match the aforementioned abiogenic requirements. Based on the conclusions extracted, we address whether the conditions for abiogenesis are/were met in any of the oceanic icy moons.
Lunar samples collected during Apollo missions are typically impact-related breccias or regolith that contain amalgamations of rocks and minerals with various origins (e.g., products of igneous differentiation, mantle melting, and/or impact events). The largest intact pre-Nectarian (∼≥3.92 Ga) fragments of igneous rock contained within the breccia and regolith rarely exceed 1 cm in size, and they often show evidence for impact recrystallization. This widespread mixing of disparate materials makes unraveling the magmatic history of pre-Nectarian period fraught with challenges. To address this issue, we combine U-Pb geochronology of Apollo 14 zircons (²⁰⁷Pb-²⁰⁶Pb ages from 3.93 to 4.36 Ga) with zircon trace element chemistry and thermodynamic models. Zircon crystallization temperatures are calculated with Ti-in-zircon thermometry after presenting new titania and silica activity models for lunar melts. We also present rare earth element (REE), P, actinide, and Mg+Fe+Al concentrations. While REE patterns and P yield little information about the parent melt origins of these out-of-context grains, U and Th concentrations are highly variable among pre-4.2 Ga zircons when compared to younger grains. Thus, the distribution of heat-producing radioactive elements in melt sources pervading the early lunar crust was heterogenous. Melt composition variation is confirmed by zircon Al concentrations and thermodynamic modeling that reveal at least two dominant magma signatures in the pre-4.0 Ga zircon population. One inferred magma type has a high alumina activity. This magma likely assimilated Feldspathic Highlands Terrane (FHT) anorthosites, though impact-generated melts of an alumina-rich target rock is a viable alternative. The other magma signature bears more similarities to KREEP basalts from the Procellarum KREEP Terrane (PKT), reflecting lower apparent alumina activities. Melt diversity seems to disappear after 4.0 Ga, with zircon recording magma compositions that largely fall in-between the two main groups found for pre-4.0 Ga samples. We interpret <4 Ga zircons to have formed from a mixture of PKT- and FHT-like rocks, consistent with the upper ∼15 km of the crust being thoroughly mixed and re-melted by basin-forming impacts during the pre-Nectarian period.
As much as half of lunar surface rocks may have originated between 4.4 and 3.9 billion years and thus observations of, and samples from, Moon could attest to conditions then extant in the inner solar system. The concept of a lunar cataclysm at ~3.9 Ga grew from seemingly contradictory observations of elemental fractionation in lunar highland rocks. U–Pb—and some Rb–Sr—data suggested recrystallization occurred between about 4.0 and 3.8 Ga. The Late Heavy Bombardment (LHB) concept that emerged appeared supported by ~3.9 Ga ⁴⁰Ar/³⁹Ar “plateau ages” of lunar impact melt rocks, although no similar spike in ages was seen in the likely more globally distributed lunar meteorites. While the ⁴⁰Ar/³⁹Ar step-heating method can reveal intragrain isotope variations, this capability has several method-specific requirements that, if not met, preclude thermochronologic interpretations. Three such issues effectively rule out the use of virtually all lunar ⁴⁰Ar/³⁹Ar data as support for the LHB hypothesis: (1) the “plateau age” approach used is an aphysical concept for the thermally disturbed samples typical of most lunar impact melt rocks, (2) laboratory artifacts destroy preserved diffusion information, or create false apparent age gradients; and (3) obtaining meaningful thermal history information from extraterrestrial samples that have differing activation energies for Ar diffusion in their K-bearing phases requires a different laboratory protocol than was used on lunar rocks. Possibly due to these issues, no case in which multiple chronometric techniques have yielded intrasample concordancy of a lunar melt rock has yet been documented. Advancements in mass spectrometry now permit ⁴⁰Ar/³⁹Ar and U–Pb dating to be undertaken on small (10 s-of-μm diameter) in situ spots on glasses and accessory minerals in lunar rocks. This approach has the potential to transcend the analytical challenge posed by the continuous impact reworking of the lunar regolith that produces fine-scale polygenetic breccias of multiple age and origins. The longstanding assumption that lunar melt rocks originated from discrete, basin-forming events is obviated by lunar imaging that show impact melts formed in small highland craters and clusters of ‘light plains’ deposits radiating outward >2000 km from large impact basins. The latter underscores how poorly the spatial relationships between large basins and their surrounding deposits were understood when impact chronologies were developed in the 1970s. The assumption that a specific lunar melt rock from a given landing site is representative of one of the basin-forming impacts is deeply flawed. Establishing a reliable, quantitative planetary impact chronology requires that all analyzed rocks be equally suitable for the application of specific chronometers. This may not be possible given the large contrasts in incompatible trace element distributions across the lunar surface (e.g., Procellarum KREEP terrane, South Pole Aiken basin). A conservative view of the lunar chronological record is that the large nearside basins are older than 3.82 Ga but these data are consistent with most of them being older than 3.92 Ga and possibly older than 4.35 Ga.
The dimensions of large craters formed by impact are controlled to a large extent by gravity, whereas the volume of impact melt created during the same event is essentially independent of gravity. This "differential scaling" fosters size-dependent changes in the dynamics of impact-crater and basin formation as well as in the final morphologies of the resulting structures. A variety of such effects can be observed in the lunar cratering record, and some predictions can be made on the basis of calculations of impact melting and crater dimensions. Among them are the following: (1) as event magnitude increases, the volume of melt created relative to that of the crater will grow, and more will be retained inside the rim of the crater or basin. (2) The depth of melting will exceed the depth of excavation at diameters that essentially coincide with both the inflection in the depth-diameter trend and the simple-to-complex transition. (3) The volume of melt will exceed that of the transient cavity at a cavity diameter on the order of the diameter of the Moon; this would arguably correspond to a Moon-melting event. (4) Small lunar craters only rarely display exterior flows of impact melt because the relatively small volumes of melt created can become choked with clasts, increasing the melt's viscosity and chilling it rapidly. Larger craters and basins should suffer little from such a process. (5) Deep melting near the projectile's axis of penetration during larger events will yield a progression in central-structure morphology; with growing event magnitude, this sequence should range from single peaks through multiple peaks to peak rings. (6) The minimum depth of origin of central-peak material should coincide with the maximum depth of melting; the main central peak in a crater the size of Tycho should have had a preimpact depth of close to 15 km.
Three of the principal variables in scaling impact-crater dimensions are the impact velocity, the projectile size, and the gravitational acceleration of the target body. The amount of impact melt generated by an impact, however, is independent of gravity, but will grow in direct proportion to the projectile dimensions and as an increasing function of the impact velocity. Thus, if the impact velocity and gravitational acceleration were held constant and projectiles of increasing size were considered, the amount of melt generated relative to the dimensions of the final crater would grow at a steady rate. Using the Earth and the Moon for comparison, this paper examines the effects of differential scaling on the depth of origin of central-peak material, on the amount of stratigraphic uplift associated with the formation of those peaks, and on the clast contents of impact melts. When craters of similar size are compared, central peaks should be derived from greater depths on Earth because of relatively deeper melting. The amount of stratigraphic uplift, however, should be greater on the Moon. A lunar crater will be larger than its terrestrial counterpart formed by an identical projectile, but the terrestrial crater will be accompanied by substantially more impact melt. As a large fraction of the melt would have lined the transient cavity during the excavation stage of the impact event, a greater fraction of the lunar melt will have been in contact with clastic materials on the cavity wall. Thus, the clast contents of lunar impact melts should be higher than in those in terrestrial craters of similar size.
Recent explorations of the oldest known rocks of marine sedimentary origin from the southwestern coast of Greenland suggest that they preserve a biogeochemical record of early life. On the basis of the age of these rocks, the emergence of the biosphere appears to overlap with a period of intense global bombardment. This finding could also be consistent with evidence from molecular biology that places the ancestry of primitive bacteria living in extreme thermal environments near the last common ancestor of all known life. To make new advances in understanding the physical, chemical, and biological states of early environments for life through this unique Greenland record, we must fully exploit the spectrum of biosignatures available; these efforts must also be coupled with an understanding of the complex geologic history of the rocks hosting these signatures. The new methods employed here will eventually become applicable to other worlds when samples become available for study early in the 21st century.
One possible definition for the origin of life on Earth is the time at which the interval between devastating environmental insults by impact exceeded the timescale for establishing self-replicating proto-organisms. A quantitative relationship for the Hadean (pre-3,800 Myr ago) and Early Archean (3,800 to 3,400 Myr) impact flux can be derived from the lunar and terrestrial impact records. Also, the effects of impact-related processes on the various environments proposed for abiogenesis (the development of life through chemical evolution from inorganic materials) can be estimated. Using a range of plausible values for the timescale for abiogenesis, the interval in time when life might first have bootstrapped itself into existence can be found for each environment. We find that if the deep marine hydrothermal setting provided a suitable site, abiogenesis could have happened as early as 4,000 to 4,200 Myr ago, whereas at the surface of the Earth abiogenesis could have occurred between 3,700 and 4,000 Myr.
