The timeline of the lunar bombardment: Revisited

The Earth-Moon system has unique chemical and isotopic signatures compared with other planetary bodies; any successful model for the origin of this system therefore has to satisfy these chemical and isotopic constraints. The Moon is substantially depleted in volatile elements such as potassium compared with the Earth and the bulk solar composition, and it has long been thought to be the result of a catastrophic Moon-forming giant impact event. Volatile-element-depleted bodies such as the Moon were expected to be enriched in heavy potassium isotopes during the loss of volatiles; however such enrichment was never found. Here we report new high-precision potassium isotope data for the Earth, the Moon and chondritic meteorites. We found that the lunar rocks are significantly (>2σ) enriched in the heavy isotopes of potassium compared to the Earth and chondrites (by around 0.4 parts per thousand). The enrichment of the heavy isotope of potassium in lunar rocks compared with those of the Earth and chondrites can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour at an ambient pressure that is higher than 10 bar. We used these coupled constraints of the chemical loss and isotopic fractionation of K to compare two recent dynamic models that were used to explain the identical non-mass-dependent isotope composition of the Earth and the Moon. Our K isotope result is inconsistent with the low-energy disk equilibration model, but supports the high-energy, high-angular-momentum giant impact model for the origin of the Moon. High-precision potassium isotope data can also be used as a 'palaeo-barometer' to reveal the physical conditions during the Moon-forming event.

Potassium isotopic evidence for a high-energy giant impact origin of the Moon

The giant impact hypothesis remains the leading theory for lunar origin. However, current models struggle to explain the Moon's composition and isotopic similarity with Earth. Here we present a new lunar origin model. High-energy, high-angular momentum giant impacts can create a post-impact structure that exceeds the corotation limit (CoRoL), which defines the hottest thermal state and angular momentum possible for a corotating body. In a typical super-CoRoL body, traditional definitions of mantle, atmosphere and disk are not appropriate, and the body forms a new type of planetary structure, named a synestia. Using simulations of cooling synestias combined with dynamic, thermodynamic and geochemical calculations, we show that satellite formation from a synestia can produce the main features of our Moon. We find that cooling drives mixing of the structure, and condensation generates moonlets that orbit within the synestia, surrounded by tens of bars of bulk silicate Earth (BSE) vapor. The moonlets and growing moon are heated by the vapor until the first major element (Si) begins to vaporize and buffer the temperature. Moonlets equilibrate with BSE vapor at the temperature of silicate vaporization and the pressure of the structure, establishing the lunar isotopic composition and pattern of moderately volatile elements. Eventually, the cooling synestia recedes within the lunar orbit, terminating the main stage of lunar accretion. Our model shifts the paradigm for lunar origin from specifying a certain impact scenario to achieving a Moon-forming synestia. Giant impacts that produce potential Moon-forming synestias were common at the end of terrestrial planet formation.

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The origin of the Moon within a terrestrial synestia

We show that the assembly of the Solar System terrestrial planets can be successfully modelled with all of the mass initially confined to a narrow annulus between 0.7 and 1.0 AU. With this configuration, analogues of Mercury and Mars often form from the collisional evolution of material diffusing out of the annulus under the scattering of the forming Earth and Venus analogues. The final systems also possess eccentricities and inclinations that match the observations, without recourse to dynamical friction from remnant small body populations. Finally, the characteristic assembly timescale for Earth analogues is rapid in this model, and consistent with cosmochemical models based on the 182 Hf– 182 W isotopes. The agreement between this model and the observations suggests that terrestrial planet systems may also be formed in ‘planet traps’, as has been proposed recently for the cores of giant planets in our solar system and others.

Formation Of The Terrestrial Planets From A Narrow Annulus

A number of distinct methodologies are available for determining the oxygen isotope composition of minerals and rocks, these include laser-assisted fluorination, secondary ion mass spectrometry (SIMS) and UV laser ablation. In this review we focus on laser-assisted fluorination, which currently achieves the highest levels of precision available for oxygen isotope analysis. In particular, we examine how results using this method have furthered our understanding of early-formed differentiated meteorites. Due to its rapid reaction times and low blank levels, laser-assisted fluorination has now largely superseded the conventional externally-heated Ni “bomb” technique for bulk analysis. Unlike UV laser ablation and SIMS analysis, laser-assisted fluorination is not capable of focused spot analysis. While laser fluorination is now a mature technology, further analytical improvements are possible via refinements to the construction of sample chambers, clean-up lines and the use of ultra-high resolution mass spectrometers. High-precision oxygen isotope analysis has proved to be a particularly powerful technique for investigating the formation and evolution of early-formed differentiated asteroids and has provided unique insights into the interrelationships between various groups of achondrites. A clear example of this is seen in samples that lie close to the terrestrial fractionation line (TFL). Based on the data from conventional oxygen isotope analysis, it was suggested that the main-group pallasites, the howardite eucrite diogenite suite (HEDs) and mesosiderites could all be derived from a single common parent body. However, high precision analysis demonstrates that main-group pallasites have a Δ 17 O composition that is fully resolvable from that of the HEDs and mesosiderites, indicating the involvement of at least two parent bodies. The range of Δ 17 O values exhibited by an achondrite group provides a useful means of assessing the extent to which their parent body underwent melting and isotopic homogenization. Oxygen isotope analysis can also highlight relationships between ungrouped achondrites and the more well-populated groups. A clear example of this is the proposed link between the evolved GRA 06128/9 meteorites and the brachinites. The evidence from oxygen isotopes, in conjunction with that from other techniques, indicates that we have samples from approximately 110 asteroidal parent bodies (∼60 irons, ∼35 achondrites and stony-iron, and  ∼15 chondrites) in our global meteorite collection. However, compared to the likely size of the original protoplanetary asteroid population, this is an extremely low value. In addition, almost all of the differentiated samples (achondrites, stony-iron and irons) are derived from parent bodies that were highly disrupted early in their evolution. High-precision oxygen isotope analysis of achondrites provides some important insights into the origin of mass-independent variation in the early Solar System. In particular, the evidence from various primitive achondrite groups indicates that both the slope 1 (Y&R) and CCAM lines are of primordial significance. Δ 17 O differences between water ice and silicate-rich solids were probably the initial source of the slope 1 anomaly. These phases most likely acquired their isotopic composition as a result of UV photo-dissociation of CO that took place either in the early solar nebula or precursor giant molecular cloud. Such small-scale isotopic heterogeneities were propagated into larger-sized bodies, such as asteroids and planets, as a result of early Solar System processes, including dehydration, aqueous alteration, melting and collisional interactions. There is increasing evidence that chondritic parent bodies accreted relatively late compared to achondritic asteroids. This may account for the fact that apart from a few notable exceptions’ such as the aubrite-enstatite chondrite association, known chondrite groups could not have been the parents to the main achondrite groups.

Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies

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The Origin of the Moon Within a Terrestrial Synestia

Introduction: The canonical giant impact model for lunar origin fails to explain key observables for the Moon [1], including the isotopic similarity between the Earth and Moon, the large mass of the Moon and the moderately volatile element depletion of the Moon. Various attempts have been made to resolve these issues with the giant impact model, most recently by using coupled physical-chemical models of the canonical disk [2], but none has yet provided a satisfactory explanation of the observations. Here we present a new model for the formation of the Moon from the aftermath of a giant impact. High energy, high angular momentum giant impacts leave the Earth in a state that exceeds the hot spin stability limit (HSSL) in which the mantle atmosphere and disk (MAD) form a well-mixed continuous extended structure that reaches beyond the Roche limit [3]. The growing Moon equilibrates with the bulk silicate Earth (BSE) vapor and naturally generates the observed bulk lunar chemistry and isotopic similarity with Earth. Cooling the post-impact MAD structure: The continuous majority-vapor structure cools rapidly by radiation from the outer regions of the structure. Silicate droplets form and fall inwards into higher pressure and temperature vapor. Droplets that fall within the Roche limit are vaporized; droplets that fall outside the limit can persist by accreting onto the growing Moon. We have modeled the cooling of a post-impact MAD structure using a modified smooth particle hydrodynamics (SPH) code. The particle entropy and radial mass distribution are adjusted by algorithms that approximate radiative cooling and convective mixing while conserving angular momentum. At each time step, condensing liquid beyond Roche is removed from the calculation and tracked as the growing moon. The Moon is formed rapidly. In an example case (Figs. 1 and 2), the moon forms in 10 to 20 yrs within 10’s bars of BSE vapor. The structure contracts within the Roche limit in ~50 yrs. Lunar chemistry: The MAD structure is being driven to mix rapidly by the torrential rain of droplets and thermal convection. The mixing timescale is on the order of weeks, much shorter than the lunar formation timescale. The Moon condenses from, and equilibrates with, a well mixed BSE vapor, naturally explaining the isotopic similarity between the Earth and the Moon. We have used a Gibbs free energy minimization code to calculate the phase diagram of BSE vapor at the pressures and temperatures relevant to lunar formation [4]. We calculate the composition of the Moon formed from the MAD structure. Droplets that condense at the surface of the structure acquire the refractory element composition of the bulk vapor, but the equilibrium composition contains a lower abundance of moderately volatile elements than observed in the moon. As the droplets fall, they equilibrate both chemically and thermally with the surrounding gas. If the droplets remain small, they evaporate quickly, but larger moonlets can survive in the gas much longer than the timescale of lunar formation due to the large latent heat of vaporization of silicates. As silica is the first major component to begin to vaporize, the moonlets equilibrate at the temperature dictated by silica vaporization (Fig. 3). The moonlets acquire the equilibrium composition of the condensate at the pressure of gas in the structure and a temperature dictated by silica vaporization. The predicted composition of the condensate at these P-T conditions is in excellent agreement with the bulk lunar chemistry including the magnitude of the depletion in moderately volatile elements and other trace elements (Fig. 4). Little isotopic fractionation between the Earth and Moon would be expected [5]. As cooling continues, the radius of the structure contracts, leaving the Moon surrounded by its own gravitationally bound silicate atmosphere (Fig. 1). Volatiles remain in the vapor structure bound to the Earth. The Earth continues to contract and cool, eventually falling below the HSSL and reaching a corotating state with a silicate atmosphere and mostly liquid mantle. During the Moon’s tidal evolution, angular momentum is removed from the Earth-Moon system. A new tidal evolution model can explain both the angular momentum removal and lunar inclination [6]. Conclusions: Our model for the formation of the Moon by equilibration with the BSE after a high energy, high angular momentum giant impact predicts the observed isotopic and elemental composition of the Moon. The criteria for a successful Moon forming impact shifts from a specific set of impact parameters to a range of impacts that produce the required post-impact state beyond the hot spin stability limit.

A New Model for Lunar Origin: Equilibration with Earth Beyond the Hot Spin Stability Limit

An understanding of the fundamental processes by which continental rifts form and develop is necessary to fully understand plate tectonics and the associated earthquake and volcano hazards. Fault slip, magma chamber inflation and magma intrusion produce characteristic patterns of surface deformation which can be the key to characterising the active tectonic and magmatic processes. The ISMER project uses archived and scheduled satellite radar images to analyse surface displacements along the length of the East African Rift over the past 15 years. Where available we use seismological observations from global catalogues and local networks to target our observations, and perform systematic surveys over key areas to identify aseismic processes. In this article, we summarise published findings on the Kenyan (1997–2008) and Main Ethiopian Rifts and the 2009 Malawi earthquakes and describe new results from the Western Branch, the Kenyan Rift (2008–2010) and a series of seismic swarms. Due to InSAR studies such as these, the number of volcanoes known to be active in East Africa has gone from 3 (Nyiragongo, Nyamuragira and Oldonyo Lengai) to >14 with implications for volcanic hazard and geothermal potential.

ISMER—Active Magmatic Processes in the East African Rift: A Satellite Radar Perspective

Remote Sensing Advances for Earth System Science

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