Toward understanding early Earth evolution: Prescription for approach from terrestrial noble gas and light element records in lunar soils

Because of the almost total lack of geological record on the Earth''s surface before 4 billion years ago, the history of the Earth during this period is still enigmatic. Here we describe a practical approach to tackle the formidable problems caused by this lack. We propose that examinations of lunar soils for light elements such as He, N, O, Ne, and Ar would shed a new light on this dark age in the Earth''s history and resolve three of the most fundamental questions in earth science: the onset time of the geomagnetic field, the appearance of an oxygen atmosphere, and the secular variation of an Earth–Moon dynamical system.     

Carbon and other light element contents in the Earth’s core based on first-principles molecular dynamics

Carbon (C) is one of the candidate light elements proposed to account for the density deficit of the Earth’s core. In addition, C significantly affects siderophile and chalcophile element partitioning between metal and silicate and thus the distribution of these elements in the Earth’s core and mantle. Derivation of the accretion and core–mantle segregation history of the Earth requires, therefore, an accurate knowledge of the C abundance in the Earth’s core. Previous estimates of the C content of the core differ by a factor of ∼20 due to differences in assumptions and methods, and because the metal–silicate partition coefficient of C was previously unknown. Here we use two-phase first-principles molecular dynamics to derive this partition coefficient of C between liquid iron and silicate melt. We calculate a value of 9 ± 3 at 3,200 K and 40 GPa. Using this partition coefficient and the most recent estimates of bulk Earth or mantle C contents, we infer that the Earth’s core contains 0.1–0.7 wt% of C. Carbon thus plays a moderate role in the density deficit of the core and in the distribution of siderophile and chalcophile elements during core–mantle segregation processes. The partition coefficients of nitrogen (N), hydrogen, helium, phosphorus, magnesium, oxygen, and silicon are also inferred and found to be in close agreement with experiments and other geochemical constraints. Contents of these elements in the core derived from applying these partition coefficients match those derived by using the cosmochemical volatility curve and geochemical mass balance arguments. N is an exception, indicating its retention in a mantle phase instead of in the core.The high solubility of C in liquid iron (1, 2) and the existence of graphite and cohenite (Fe,Ni)₃C in iron meteorites (3) suggest that C could account for the density deficit of the Earth’s core (4–6). Carbon content also greatly influences siderophile and chalcophile element partitioning between metal and silicate, and hence their distribution in the Earth’s core and mantle (7, 8). In particular, its influence on Pb partitioning would affect the inference of the age of the Earth (9). Therefore, it is critically important to constrain the C content of the Earth’s core.There are several ways to infer the C content of the core. The first method is a solubility and phase diagram study in the Fe–C system, which generally gives a high C content in the Earth’s core, on the order of 2–4 wt% (1). The implicit assumption is that C is supersaturated during the Earth’s accretion process, and so this study will generally give the maximum C content of the core. The second method involves density and bulk modulus measurements of solid Fe₃C and Fe₇C₃ under the inner core pressures and comparison with the seismic data of the inner core. This kind of study results in a C content of 1–1.5 wt% (10, 11) and assumes that C is the only light element in the inner core, thus giving only an upper limit for the C content in the inner core. Application of the density and bulk modulus measurements of solid and liquid Fe₃C to the outer core exclude C as a major alloying element in the Earth’s outer core (12, 13). The third method is based on carbonaceous chondrite composition, the element volatility curve of the bulk Earth, and the mass balance between primitive mantle and core. This method gives the smallest C content of the core: 0.2 wt% (14).Thus, the C content of the core has proven to be difficult to constrain, and current estimates differ by a factor of ∼20. Some limits on the geochemical role of C in the core-forming process can be provided by P, which becomes lithophile during C saturation (7). Because P is depleted rather than enriched in the mantle, core formation probably did not occur at C saturation (7). Thus, we contend that the starting bulk composition of the Earth may have contained a lower C content than some of the experiments outlined above indicated.Carbon is thought to have entered the core through core–mantle segregation processes in the early history of the Earth during the magma ocean stage. Inferring the amount of C involved thus requires determining the partition coefficient of C between liquid iron and silicate melt experimentally. However, as yet no such data exist, as pointed out by Dasgupta and Walker (15). Many partitioning experiments for siderophile elements use C as a sample encapsulating material, thus greatly affecting the behavior of these elements.In the present study, we use the two-phase first-principles molecular dynamics (FPMD) method (16) to determine the partition coefficient of C and a few other light elements between liquid iron and silicate melt. This partition coefficient can then be combined with the composition of the bulk Earth or the primitive mantle (i.e., bulk silicate Earth) to infer the C content of the Earth’s core.     

Tipping elements in the Earth's climate system

The term “tipping point” commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term “tipping element” to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.     

Potassium isotope composition of Mars reveals a mechanism of planetary volatile retention

The abundances of water and highly to moderately volatile elements in planets are considered critical to mantle convection, surface evolution processes, and habitability. From the first flyby space probes to the more recent “Perseverance” and “Tianwen-1” missions, “follow the water,” and, more broadly, “volatiles,” has been one of the key themes of martian exploration. Ratios of volatiles relative to refractory elements (e.g., K/Th, Rb/Sr) are consistent with a higher volatile content for Mars than for Earth, despite the contrasting present-day surface conditions of those bodies. This study presents K isotope data from a spectrum of martian lithologies as an isotopic tracer for comparing the inventories of highly and moderately volatile elements and compounds of planetary bodies. Here, we show that meteorites from Mars have systematically heavier K isotopic compositions than the bulk silicate Earth, implying a greater loss of K from Mars than from Earth. The average “bulk silicate” δ⁴¹K values of Earth, Moon, Mars, and the asteroid 4-Vesta correlate with surface gravity, the Mn/Na “volatility” ratio, and most notably, bulk planet H₂O abundance. These relationships indicate that planetary volatile abundances result from variable volatile loss during accretionary growth in which larger mass bodies preferentially retain volatile elements over lower mass objects. There is likely a threshold on the size requirements of rocky (exo)planets to retain enough H₂O to enable habitability and plate tectonics, with mass exceeding that of Mars.

Examining the presence, distribution, and abundance of volatile elements and compounds, including water, on Mars has been a central theme of space exploration for the past 50 y. The majority of all past, ongoing, and future Mars missions involve the direct or indirect study of volatile element inventories, including the recent “Perseverance” and “Tianwen-1” missions (1, 2). The direct study of volatiles in martian meteorites, along with remote sensing efforts, have significantly broadened understanding of the volatile inventory of Mars and spurred the development of competing bulk chemistry models for Mars. These models can broadly be divided into three groups: 1) those based on cosmochemical implications of elemental ratios (3–5), 2) those attempting to reproduce the martian O isotope composition by mixing different proportions of chondritic materials (6, 7) and equating these to volatile abundances, and 3) those combining spacecraft data and meteorite chemistry to estimate the composition of bulk silicate Mars (8). These models all adopted the ratios of volatile element K to the refractory elements U and Th as a proxy of volatile depletion because these elements are all highly incompatible and lithophile during igneous processes, that is, such ratios are not strongly affected by partial melting followed by melt fractionation that leads to the formation of basaltic rocks and their derivatives that constitute the present-day martian crust. Furthermore, the concentrations of K, Th, and U of this martian crust can be measured remotely from orbit using gamma-ray spectrometry (GRS). All previous models for the composition of bulk silicate Mars, as well as GRS data of exposed martian surface materials, have shown that Mars has elevated K/Th as well as higher contents of a greater suite of moderately volatile elements relative to Earth (Fig. 1), together implying a volatile-rich early Mars (8–10). A caveat with these models is the inherent difficulty in determining the K/Th of bulk silicate Mars from surface data as well as the marked inconsistency between meteorite analyses and GRS data of martian surface regions (9).Open in a separate windowFig. 1.Potassium to thorium ratios versus the corresponding K concentrations of martian meteorites (basaltic, olivine-phyric, and lherzolitic shergottites and other categories), the martian surface detected by the Mars Odyssey Gamma Ray Spectrometer, and terrestrial mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). To avoid potential terrestrial contamination effects, only meteorite falls and Antarctic finds are plotted. Data are from compilations for martian meteorites (47, 57–63), Mars Odyssey GRS data from ref. 64, MORB (65), and for OIB from GEOROC. The bulk silicate Earth has a K abundance of 240 ppm and a K/Th of 2,900 (66). Note the systematically high K/Th for martian surface materials measured by GRS compared with martian meteorites. Terrestrial rocks, especially MORB, exhibit a wide K/Th range overlapping with martian meteorite samples.An alternative means of examining the volatile history of Mars is by measuring the isotopic ratios of moderately volatile elements (MVE) in martian meteorites. Of the MVEs, which are defined as having 50% equilibrium condensation temperatures (50%T_c) of less than 1,335 K at a total pressure of 10⁻⁴ bar for a solar system gas composition (11), K is one of the most abundant (50% T_c = 1,006 K(11)). The isotopic ratios of K in igneous rocks from planetary bodies are insensitive to igneous processes [e.g., melting and fractional crystallization (12)] and secondary effects such as impact-induced vaporization (13) and eruptive degassing (14) and thus are a strong proxy for volatile depletion in planetary interiors. Here, the K isotope compositions of 20 martian meteorites are reported. These meteorites have previously been established to originate from Mars on the basis of triple-oxygen isotope systematics, trapped noble gas inventories, and the generally young crystallization ages (<1.34 Ga as with the majority of martian meteorites) (15–18). The 20 examined meteorites cover a range of rock types (basaltic, olivine-phyric, lherzolitic, and picritic shergottites, nakhlites [clinopyroxene-rich cumulates], a chassignite [cumulate dunite], and a basaltic crustal breccia [NWA 7034], SI Appendix, Table S1) and geochemical signatures (incompatible element-enriched, -intermediate, and -depleted shergottites). With the exception of the basaltic breccia NWA 7034, only observed meteorite falls and Antarctic meteorite finds were considered in order to avoid uncertainties related to terrestrial contamination and alteration, which commonly affect hot desert finds (19).     

Nominally hydrous magmatism on the Moon

For the past 40 years, the Moon has been described as nearly devoid of indigenous water; however, evidence for water both on the lunar surface and within the lunar interior have recently emerged, calling into question this long-standing lunar dogma. In the present study, hydroxyl (as well as fluoride and chloride) was analyzed by secondary ion mass spectrometry in apatite [Ca₅(PO₄)₃(F,Cl,OH)] from three different lunar samples in order to obtain quantitative constraints on the abundance of water in the lunar interior. This work confirms that hundreds to thousands of ppm water (of the structural form hydroxyl) is present in apatite from the Moon. Moreover, two of the studied samples likely had water preserved from magmatic processes, which would qualify the water as being indigenous to the Moon. The presence of hydroxyl in apatite from a number of different types of lunar rocks indicates that water may be ubiquitous within the lunar interior, potentially as early as the time of lunar formation. The water contents analyzed for the lunar apatite indicate minimum water contents of their lunar source region to range from 64 ppb to 5 ppm H₂O. This lower limit range of water contents is at least two orders of magnitude greater than the previously reported value for the bulk Moon, and the actual source region water contents could be significantly higher.     

Fast accretion of the earth with a late moon-forming giant impact

Constraints on the formation history of the Earth are critical for understanding of planet formation processes. (182)Hf-(182)W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and (182)Hf-(182)W and (87)Rb-(87)Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to > 100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the (182)W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (> 52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in < 10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.     

Conditions and extent of volatile loss from the Moon during formation of the Procellarum basin

Rocks from the lunar interior are depleted in moderately volatile elements (MVEs) compared to terrestrial rocks. Most MVEs are also enriched in their heavier isotopes compared to those in terrestrial rocks. Such elemental depletion and heavy isotope enrichments have been attributed to liquid–vapor exchange and vapor loss from the protolunar disk, incomplete accretion of MVEs during condensation of the Moon, and degassing of MVEs during lunar magma ocean crystallization. New Monte Carlo simulation results suggest that the lunar MVE depletion is consistent with evaporative loss at 1,670 ± 129 K and an oxygen fugacity +2.3 ± 2.1 log units above the fayalite-magnetite-quartz buffer. Here, we propose that these chemical and isotopic features could have resulted from the formation of the putative Procellarum basin early in the Moon’s history, during which nearside magma ocean melts would have been exposed at the surface, allowing equilibration with any primitive atmosphere together with MVE loss and isotopic fractionation.

Returned samples of basaltic rocks from the Moon provided evidence decades ago that the Moon is depleted in volatile elements compared to the Earth (1), with lunar basalt abundances of moderately volatile elements (MVEs) being ∼1/5 that of terrestrial basalt abundances for alkali elements and ∼1/40 for other MVE, such as Zn, Ag, In, and Cd (2). The theme of lunar volatiles thus seemed settled. Yet, the unambiguous detection in 2008 of lunar indigenous hydrogen and other volatile elements, such as F, Cl, and S in pyroclastic glasses (3), heralded a new era of research into lunar volatiles, overturning the decades-old paradigm of a bone-dry Moon (e.g., refs. 4 and 5). Here, we define volatile elements as those with 50% condensation temperatures below these of the major rock-forming elements Fe, Mg, and Si (6). This paradigm shift was accompanied by new measurements of volatile stable isotope compositions (e.g., H, C, N, Cl, K, Cr, Cu, Zn, Ga, Rb, and Sn) in a wealth of bulk lunar samples (7–18) and in the mineral phases and melt inclusions they host (19–28). These studies have shown that the stable isotope compositions of most MVEs (e.g., K, Zn, Ga, and Rb) are enriched in their heavier isotopes with respect to the bulk silicate Earth (BSE) (9, 11, 13–15, 17). Such heavy isotope enrichment is associated with elemental depletion, which has been variously attributed to liquid–vapor exchange and vapor loss from the protolunar disk (17, 18), incomplete accretion of MVEs during condensation of the Moon (13, 29, 30), and degassing of these elements during lunar magma ocean crystallization (9, 11, 14, 15, 25, 31). Almost all these hypotheses have typically assumed that the MVE depletions and associated MVE isotope fractionations are relevant to the whole Moon. However, our lunar sample collections are biased, as all Apollo and Luna returned samples come from the lunar nearside from within or around the anomalous Procellarum KREEP Terrane (PKT) region (e.g., ref. 32), where KREEP stands for enriched in K, REEs, and P. Barnes et al. (26) proposed that the heavy Cl isotope signature measured in KREEP-rich Apollo samples resulted from metal-chloride degassing from late-stage lunar magma ocean melts in response to a large crust-breaching impact event, spatially associated with the PKT region, which facilitated exposure of these late-stage melts to the lunar surface. Here, we further investigate whether a localized impact event could have been responsible for the general MVE depletion and heavy MVE isotope enrichment measured in lunar samples.     

Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth

After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post–Snowball Earth hothouse.The Snowball Earth hypothesis predicts that Neoproterozoic glaciations were global and synchronous at low latitudes and that deglaciation occurred as a result of the buildup of pCO₂ to extreme levels resulting in a “greenhouse” aftermath (1, 2). The temporal framework of Cryogenian glaciations is built on chemostratigraphy and correlation of lithologically distinct units, such as glaciogenic deposits, iron formation, and cap carbonates (3), tied to the few successions that contain volcanic rocks dated using U-Pb zircon geochronology (4). In strata lacking horizons suitable for U-Pb geochronology, Re-Os geochronology can provide depositional ages on organic-rich sedimentary rocks bracketing glaciogenic strata (5, 6). Moreover, Os isotope stratigraphy can be used as a proxy to test for supergreenhouse weathering during deglaciation (7). In a Snowball Earth scenario, we can make specific predictions for Cryogenian weathering: CO₂ consumption via silicate weathering should increase before glaciation, stagnate during the glaciation, and increase again during deglaciation. However, the use of a single weathering proxy to provide evidence for such a scenario, such as Sr isotopes from marine carbonates, is limited both by lithological constraints and an inability to distinguish between the amount of weathering and the composition of what is being weathered (8). The short residence time of Os in the present-day oceans (<10 ky) (9) provides a complementary higher resolution archive to Sr isotopes, and thus, insights into the nature of extreme fluctuations in the Earth’s climate as documented herein.     

A model to explain the various paradoxes associated with mantle noble gas geochemistry

As a result of an energetic accretion, the Earth is a volatile-poor and strongly differentiated planet. The volatile elements can be accounted for by a late veneer (≈1% of total mass of the Earth). The incompatible elements are strongly concentrated into the exosphere (atmosphere, oceans, sediments, and crust) and upper mantle. Recent geochemical models invoke a large primordial undegassed reservoir with chondritic abundances of uranium and helium, which is clearly at odds with mass and energy balance calculations. The basic assumption behind these models is that excess “primordial” ³He is responsible for ³He/⁴He ratios higher than the average for midocean ridge basalts. The evidence however favors depletion of ³He and excessive depletion of ⁴He and, therefore, favors a refractory, residual (low U, Th) source Petrological processes such as melt-crystal and melt-gas separation fractionate helium from U and Th and, with time, generate inhomogeneities in the ³He/⁴He ratio. A self-consistent model for noble gases involves a gas-poor planet with trapping of CO₂ and noble gases in the shallow mantle. Such trapped gases are released by later tectonic and magmatic processes. Most of the mantle was depleted and degassed during the accretion process. High ³He/⁴He gases are viewed as products of ancient gas exsolution stored in low U environments, rather than products of primordial reservoirs.     

Biochemical evolution II: Origin of life in tubular microstructures on weathered feldspar surfaces

Mineral surfaces were important during the emergence of life on Earth because the assembly of the necessary complex biomolecules by random collisions in dilute aqueous solutions is implausible. Most silicate mineral surfaces are hydrophilic and organophobic and unsuitable for catalytic reactions, but some silica-rich surfaces of partly dealuminated feldspars and zeolites are organophilic and potentially catalytic. Weathered alkali feldspar crystals from granitic rocks at Shap, north west England, contain abundant tubular etch pits, typically 0.4–0.6 μm wide, forming an orthogonal honeycomb network in a surface zone 50 μm thick, with 2–3 × 10⁶ intersections per mm² of crystal surface. Surviving metamorphic rocks demonstrate that granites and acidic surface water were present on the Earth’s surface by ~3.8 Ga. By analogy with Shap granite, honeycombed feldspar has considerable potential as a natural catalytic surface for the start of biochemical evolution. Biomolecules should have become available by catalysis of amino acids, etc. The honeycomb would have provided access to various mineral inclusions in the feldspar, particularly apatite and oxides, which contain phosphorus and transition metals necessary for energetic life. The organized environment would have protected complex molecules from dispersion into dilute solutions, from hydrolysis, and from UV radiation. Sub-micrometer tubes in the honeycomb might have acted as rudimentary cell walls for proto-organisms, which ultimately evolved a lipid lid giving further shelter from the hostile outside environment. A lid would finally have become a complete cell wall permitting detachment and flotation in primordial “soup.” Etch features on weathered alkali feldspar from Shap match the shape of overlying soil bacteria.     

The negligible chondritic contribution in the lunar soils water

Recent data from Apollo samples demonstrate the presence of water in the lunar interior and at the surface, challenging previous assumption that the Moon was free of water. However, the source(s) of this water remains enigmatic. The external flux of particles and solid materials that reach the surface of the airless Moon constitute a hydrogen (H) surface reservoir that can be converted to water (or OH) during proton implantation in rocks or remobilization during magmatic events. Our original goal was thus to quantify the relative contributions to this H surface reservoir. To this end, we report NanoSIMS measurements of D/H and ⁷Li/⁶Li ratios on agglutinates, volcanic glasses, and plagioclase grains from the Apollo sample collection. Clear correlations emerge between cosmogenic D and ⁶Li revealing that almost all D is produced by spallation reactions both on the surface and in the interior of the grains. In grain interiors, no evidence of chondritic water has been found. This observation allows us to constrain the H isotopic ratio of hypothetical juvenile lunar water to δD ≤ −550‰. On the grain surface, the hydroxyl concentrations are significant and the D/H ratios indicate that they originate from solar wind implantation. The scattering distribution of the data around the theoretical D vs. ⁶Li spallation correlation is compatible with a chondritic contribution <15%. In conclusion, (i) solar wind implantation is the major mechanism responsible for hydroxyls on the lunar surface, and (ii) the postulated chondritic lunar water is not retained in the regolith.Three types of sources could contribute to lunar superficial and mantle water, namely: (i) a primordial indigenous source identified in apatites (1–4), volcanic glasses (5, 6), and plagioclase phases (7) supporting a common origin of water for the Earth−Moon system (8–10); (ii) an addition of H₂O-rich material via impacts of carbonaceous chondrites (CCs) and cometary materials (11, 12); and (iii) a proton implantation by the solar wind (SW) (13–18). Because magmatic water was incorporated in apatites, i.e., in the last minerals crystallized from lunar melts, the D/H ratio of these minerals was used to identify the source of this water. Indeed, all inner solar system objects (Earth, Moon, CCs) show an average water D/H ratio around 150 × 10⁻⁶ with variations lying between 125 × 10⁻⁶ and 220 × 10⁻⁶. However, in lunar materials, a variety of processes may have altered this D/H ratio, namely: isotopic fractionation during the outgassing of the melt under vacuum, the reduction of water into H₂ by the highly reduced lunar melts, or the contribution of D from spallation reactions. The possible oxidation of SW H into water during silicate melting could also be considered as a possible source for this mantellic water. Indeed, production of water by SW implantation is now considered as a ubiquitous process in the solar system (13, 19) and one of the possible mechanisms for bringing water to the Moon’s surface. However, its contribution relative to chondritic or cometary sources is still debated (20).The D/H ratio (reported here in δD units) is commonly used to identify water sources. However, the Moon being an airless body unprotected by a planetary magnetic field, space weathering (21) modifies the δD of implanted H or of water adsorbed on grains, complicating the identification of the sources. Several types of space contributions can be distinguished: (i) water vapor deposition (22) resulting from carbonaceous chondrite or comet impacts; (ii) low-energy SW particles (∼1 keV/u) that are implanted in silicate grains (23), yielding a 200-nm-thick rim; and (iii) high-energy solar (SCR; 0.5–1.0 MeV/u) and galactic (GCR, 0.1–10 GeV/u) cosmic rays that penetrate the rocks down to a few centimeters to a few meters, respectively. These high-energy particles are responsible for the production of cosmogenic D and ⁶Li via the so-called spallation reactions (24, 25). As a consequence, the D/H ratios of the rim and of the interior of grains do not record the same information: (i) The rim contains SW H, cosmogenic elements, and water redeposited after the impacts of water-rich bodies, whereas (ii) the interior of grains contains the cosmogenic elements and lunar volatiles trapped in the melt.To estimate the relative proportions of SW and cosmogenic D in the hydrogen budget of grains in soils, we use the ⁷Li/⁶Li ratio as a record of the average concentration of spallation products (26). This approach offers two advantages: (i) The amount of cosmogenic D is considered as a free parameter and does not rely on the usual assumptions of theoretical calculations of spallation yields (5, 8, 9, 13), and (ii) in addition to a small isotope fractionation restricted to 6‰ (27), departure of the ⁷Li/⁶Li ratio toward low values can be unambiguously attributed to the contribution of the cosmogenic ⁶Li (28) [the cosmogenic ⁷Li/⁶Li ratio lies between 1.4 and 2.0 (29) while the lunar ratio is 12.15].     

Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets

Achondrite meteorites have anomalous enrichments in ³³S, relative to chondrites, which have been attributed to photochemistry in the solar nebula. However, the putative photochemical reactions remain elusive, and predicted accompanying ³³S depletions have not previously been found, which could indicate an erroneous assumption regarding the origins of the ³³S anomalies, or of the bulk solar system S-isotope composition. Here, we report well-resolved anomalous ³³S depletions in IIIF iron meteorites (<−0.02 per mil), and ³³S enrichments in other magmatic iron meteorite groups. The ³³S depletions support the idea that differentiated planetesimals inherited sulfur that was photochemically derived from gases in the early inner solar system (<∼2 AU), and that bulk inner solar system S-isotope composition was chondritic (consistent with IAB iron meteorites, Earth, Moon, and Mars). The range of mass-independent sulfur isotope compositions may reflect spatial or temporal changes influenced by photochemical processes. A tentative correlation between S isotopes and Hf-W core segregation ages suggests that the two systems may be influenced by common factors, such as nebular location and volatile content.Of all of the extraterrestrial materials found on Earth, iron meteorites have always been the most conspicuous. So-called “magmatic” iron meteorites are likely to be samples from the cores of magmatically differentiated protoplanetary parent bodies (1), whereas “nonmagmatic” iron meteorites are commonly suggested to sample solidified melt pockets that formed via impacts onto nondifferentiated (chondritic) parent bodies (2). Individual members from an iron meteorite group are assumed to derive from a common parent body, based on shared chemical and isotopic characteristics (1–4). Chemical and isotopic differences among the different iron groups provide convincing evidence that different individual parent body planetesimals incorporated genetically distinct precursor materials (1–4).Recent observations that several achondrite meteorite groups possess small, mass-independent ³³S enrichments relative to chondrites (5) have led to the conclusion that sulfur isotopes were heterogeneously distributed among the materials that accreted to form early solar system planetesimals. This observation, coupled with the ancient ¹⁸²Hf-¹⁸²W ages (within 1–3 My of solar system formation) of magmatic irons (6–8), provides impetus to search for systematic variations in mass-independent sulfur isotope compositions among the iron groups.     

Identification of specific Rp-phosphate oxygens in the tRNA anticodon loop required for ribosomal P-site binding

tRNA binding to the ribosomal P site is dependent not only on correct codon–anticodon interaction but also involves identification of structural elements of tRNA by the ribosome. By using a phosphorothioate substitution–interference approach, we identified specific nonbridging Rp-phosphate oxygens in the anticodon loop of tRNA^Phe from Escherichia coli which are required for P-site binding. Stereo-specific involvement of phosphate oxygens at these positions was confirmed by using synthetic anticodon arm analogues at which single Rp- or Sp-phosphorothioates were incorporated. Identical interference results with yeast tRNA^Phe and E. coli tRNA_f^Met indicate a common backbone conformation or common recognition elements in the anticodon loop of tRNAs. N-ethyl-N-nitrosourea modification–interference experiments with natural tRNAs point to the importance of the same phosphates in the loop. Guided by the crystal structure of tRNA^Phe, we propose that specific Rp-phosphate oxygens are required for anticodon loop (“U-turn”) stabilization or are involved in interactions with the ribosome on correct tRNA–mRNA complex formation.     

From the Cover: Volcanic history of the Imbrium basin: A close-up view from the lunar rover Yutu

We report the surface exploration by the lunar rover Yutu that landed on the young lava flow in the northeastern part of the Mare Imbrium, which is the largest basin on the nearside of the Moon and is filled with several basalt units estimated to date from 3.5 to 2.0 Ga. The onboard lunar penetrating radar conducted a 114-m-long profile, which measured a thickness of ∼5 m of the lunar regolith layer and detected three underlying basalt units at depths of 195, 215, and 345 m. The radar measurements suggest underestimation of the global lunar regolith thickness by other methods and reveal a vast volume of the last volcano eruption. The in situ spectral reflectance and elemental analysis of the lunar soil at the landing site suggest that the young basalt could be derived from an ilmenite-rich mantle reservoir and then assimilated by 10–20% of the last residual melt of the lunar magma ocean.The surface of the Moon is covered by regolith, a mixed layer of fine-grained lunar soil and ejecta deposits, which is crucial to understanding the global composition of the Moon. The lunar regolith has also recorded the complex history of the surface processes, and it is the main reservoir of ³He and other solar wind gases. The thickness of the lunar regolith was estimated to be from 2 to 8 m in the maria and up to 8–16 m in the highland areas using various methods (1), including crater morphology (2, 3), seismology with low spatial resolution (4), radar wave scattering (5), and microwave brightness temperature (6). However, no in situ measurement of spectral reflectance, elemental compositions, lunar regolith thickness, or subsurface structures has been carried out.The surface of the Moon is dominated with numerous large basins. They were formed about 3.9 Ga (7, 8), probably by the late heavy bombardment, and then filled with dark lava flows derived from partial melting of the lunar mantle, within a period mainly during 3.8–3.1 Ga (7). The Imbrium basin is the largest and was formed on Procellarum KREEP [potassium (K), rare earth elements (REE), and phosphorus (P)] Terrane (9), a unique terrain highly enriched in U, Th, and K radionuclides and other incompatible trace elements referred to as KREEP (10) and considered as the last residual melt of the Lunar Magma Ocean (11). The presence of the KREEPy materials, indicated by high concentrations of radionuclides U, Th, and K (9), around the rims of the Imbrium basin suggests that they are likely the basin-forming ejecta deposits. At least three main lava flows, dated from 3.5 Ga to 2.0–2.3 Ga (7, 12), have been recognized in Mare Imbrium with distinct FeO and TiO₂ concentrations (13, 14), which brought up interior information of this KREEP-rich terrain. The old and low-Ti basalt unit has been sampled by the Apollo 15 mission that landed at the eastern rim of the Imbrium basin. Information of other lava flows in Mare Imbrium was obtained only by remote sensing from orbit. On December 14, 2013, Chang’e-3 successfully landed on the young and high-Ti lava flow in the northeastern Mare Imbrium, about 10 km south from the old low-Ti basalt unit (Fig. 1).Open in a separate windowFig. 1.The landing site of Chang’e-3 (red cross), on the high-Ti basalt (dark gray) near the boundary in contact with the low-Ti basalt (light gray). The background image was taken by Chang’e-1.The lunar rover Yutu (named for the jade rabbit on the Moon in a Chinese fairy tale) was equipped with an active particle-induced X-ray spectrometer (APXS), a visible to near-infrared (450–945 nm) imaging spectrometer and short-wave infrared (900–2,395 nm) spectrometer (VNIS), and a lunar penetrating radar (LPR), accompanied by a stereo camera and a navigating camera. Originally, the mission planned to have the lunar rover measure chemical and mineral compositions of the lunar soil and various types of ejecta rocks and to carry out a LPR profile of the lunar regolith and subsurface structures in the first 3 mo. The mission was scheduled to extend up to 1 y and to explore the old low-Ti lava flow ∼10 km north. Unfortunately, some of Yutu’s mechanical parts failed to move just before the rover prepared for sleeping at the end of the second month due to unknown faults probably in the control system. During the first 2 mo, Yutu successfully carried out two APXS and four VNIS analyses of the lunar soil and performed a 114-m-long LPR profile along the rover track in the landing area (Fig. 2). These in situ measurements provide insights into the volcanic history of Mare Imbrium and the ground-truth data for calibration of the orbital data.Open in a separate windowFig. 2.Chang’e-3 landing site and the rover Yutu’s track. Crater A is blocky, indicating penetration through the regolith. Crater B is the largest one without blocks in the landing area. The APXS (LS1–LS2) and VNIS (CD5–CD8) analysis positions and the rover navigation points are marked. The image was composed from the series images taken by the Chang’e-3 landing camera.     

From the Cover: Dynamic model constraints on oxygen-17 depletion in atmospheric O2 after a snowball Earth

A large perturbation in atmospheric CO₂ and O₂ or bioproductivity will result in a drastic pulse of ¹⁷O change in atmospheric O₂, as seen in the Marinoan Oxygen-17 Depletion (MOSD) event in the immediate aftermath of a global deglaciation 635 Mya. The exact nature of the perturbation, however, is debated. Here we constructed a coupled, four-box, and quick-response biosphere–atmosphere model to examine both the steady state and dynamics of the MOSD event. Our model shows that the ultra-high CO₂ concentrations proposed by the “snowball’ Earth hypothesis produce a typical MOSD duration of less than 10⁶ y and a magnitude of ¹⁷O depletion reaching approximately −35‰. Both numbers are in remarkable agreement with geological constraints from South China and Svalbard. Moderate CO₂ and low O₂ concentration (e.g., 3,200 parts per million by volume and 0.01 bar, respectively) could produce distinct sulfate ¹⁷O depletion only if postglacial marine bioproductivity was impossibly low. Our dynamic model also suggests that a snowball in which the ocean is isolated from the atmosphere by a continuous ice cover may be distinguished from one in which cracks in the ice permit ocean–atmosphere exchange only if partial pressure of atmospheric O₂ is larger than 0.02 bar during the snowball period and records of weathering-derived sulfate are available for the very first few tens of thousands of years after the onset of the meltdown. In any case, a snowball Earth is a precondition for the observed MOSD event.     

Proterozoic ocean redox and biogeochemical stasis

The partial pressure of oxygen in Earth’s atmosphere has increased dramatically through time, and this increase is thought to have occurred in two rapid steps at both ends of the Proterozoic Eon (∼2.5–0.543 Ga). However, the trajectory and mechanisms of Earth’s oxygenation are still poorly constrained, and little is known regarding attendant changes in ocean ventilation and seafloor redox. We have a particularly poor understanding of ocean chemistry during the mid-Proterozoic (∼1.8–0.8 Ga). Given the coupling between redox-sensitive trace element cycles and planktonic productivity, various models for mid-Proterozoic ocean chemistry imply different effects on the biogeochemical cycling of major and trace nutrients, with potential ecological constraints on emerging eukaryotic life. Here, we exploit the differing redox behavior of molybdenum and chromium to provide constraints on seafloor redox evolution by coupling a large database of sedimentary metal enrichments to a mass balance model that includes spatially variant metal burial rates. We find that the metal enrichment record implies a Proterozoic deep ocean characterized by pervasive anoxia relative to the Phanerozoic (at least ∼30–40% of modern seafloor area) but a relatively small extent of euxinic (anoxic and sulfidic) seafloor (less than ∼1–10% of modern seafloor area). Our model suggests that the oceanic Mo reservoir is extremely sensitive to perturbations in the extent of sulfidic seafloor and that the record of Mo and chromium enrichments through time is consistent with the possibility of a Mo–N colimited marine biosphere during many periods of Earth’s history.     

The Surface of Nanoparticle Silicon as Studied by Solid-State NMR

The surface structure and adjacent interior of commercially available silicon nanopowder (np-Si) was studied using multinuclear, solid-state NMR spectroscopy. The results are consistent with an overall picture in which the bulk of the np-Si interior consists of highly ordered (“crystalline”) silicon atoms, each bound tetrahedrally to four other silicon atoms. From a combination of ¹H, ²⁹Si and ²H magic-angle-spinning (MAS) NMR results and quantum mechanical ²⁹Si chemical shift calculations, silicon atoms on the surface of “as-received” np-Si were found to exist in a variety of chemical structures, with apparent populations in the order (a) (Si–O–)₃Si–H > (b) (Si–O–)₃SiOH > (c) (HO–)_nSi(Si)_m(–OSi)_4−m−n ≈ (d) (Si–O–)₂Si(H)OH > (e) (Si–O–)₂Si(–OH)₂ > (f) (Si–O–)₄Si, where Si stands for a surface silicon atom and Si represents another silicon atom that is attached to Si by either a Si–Si bond or a Si–O–Si linkage. The relative populations of each of these structures can be modified by chemical treatment, including with O₂ gas at elevated temperature. A deliberately oxidized sample displays an increased population of (Si–O–)₃Si–H, as well as (Si–O–)₃SiOH sites. Considerable heterogeneity of some surface structures was observed. A combination of ¹H and ²H MAS experiments provide evidence for a substantial population of silanol (Si–OH) moieties, some of which are not readily H-exchangeable, along with the dominant Si–H sites, on the surface of “as-received” np-Si; the silanol moieties are enhanced by deliberate oxidation. An extension of the DEPTH background suppression method is also demonstrated that permits measurement of the T₂ relaxation parameter simultaneously with background suppression.     

147Sm-143Nd systematics of Earth are inconsistent with a superchondritic Sm/Nd ratio

The relationship between the compositions of the Earth and chondritic meteorites is at the center of many important debates. A basic assumption in most models for the Earth’s composition is that the refractory elements are present in chondritic proportions relative to each other. This assumption is now challenged by recent ¹⁴²Nd/¹⁴⁴Nd ratio studies suggesting that the bulk silicate Earth (BSE) might have an Sm/Nd ratio 6% higher than chondrites (i.e., the BSE is superchondritic). This has led to the proposal that the present-day ¹⁴³Nd/¹⁴⁴Nd ratio of BSE is similar to that of some deep mantle plumes rather than chondrites. Our reexamination of the long-lived ¹⁴⁷Sm-¹⁴³Nd isotope systematics of the depleted mantle and the continental crust shows that the BSE, reconstructed using the depleted mantle and continental crust, has ¹⁴³Nd/¹⁴⁴Nd and Sm/Nd ratios close to chondritic values. The small difference in the ratio of ¹⁴²Nd/¹⁴⁴Nd between ordinary chondrites and the Earth must be due to a process different from mantle-crust differentiation, such as incomplete mixing of distinct nucleosynthetic components in the solar nebula.     

Tricubic polynomial interpolation

A new triangular “finite element“ is described; it involves the 12-parameter family of all quartic polynomial functions that are “tricubic“ in that their variation is cubic along any parallel to any side of the triangle. An interpolation scheme is described that approximates quite accurately any smooth function on any triangulated domain by a continuously differentiable function, tricubic on each triangular element.     

Quantum Monte Carlo computations of phase stability,equations of state,and elasticity of high-pressure silica

Silica (SiO₂) is an abundant component of the Earth whose crystalline polymorphs play key roles in its structure and dynamics. First principle density functional theory (DFT) methods have often been used to accurately predict properties of silicates, but fundamental failures occur. Such failures occur even in silica, the simplest silicate, and understanding pure silica is a prerequisite to understanding the rocky part of the Earth. Here, we study silica with quantum Monte Carlo (QMC), which until now was not computationally possible for such complex materials, and find that QMC overcomes the failures of DFT. QMC is a benchmark method that does not rely on density functionals but rather explicitly treats the electrons and their interactions via a stochastic solution of Schrödinger’s equation. Using ground-state QMC plus phonons within the quasiharmonic approximation of density functional perturbation theory, we obtain the thermal pressure and equations of state of silica phases up to Earth’s core–mantle boundary. Our results provide the best constrained equations of state and phase boundaries available for silica. QMC indicates a transition to the dense α-PbO₂ structure above the core-insulating D” layer, but the absence of a seismic signature suggests the transition does not contribute significantly to global seismic discontinuities in the lower mantle. However, the transition could still provide seismic signals from deeply subducted oceanic crust. We also find an accurate shear elastic constant for stishovite and its geophysically important softening with pressure.