George Helffrich's research while affiliated with Earth-Life Science Institute (ELSI) and other places

In this study we inverted a large set of normal-mode centre frequencies and quality (attenuation) factors, including geodetic data (mass, moment of inertia and tidal response), using self-consistently-built models of the radial elastic and anelastic seismic structure of the Earth. The mantle models are constructed using petrologic phase equilibria in combination with a laboratory-based visco-elastic model that connects dissipation from seismic to tidal periods, whereas seismic properties for a well-mixed and homogeneous core are computed using equations-of-state. Relative to the preliminary seismic reference model (PREM), we find that for the models to fit the observations, mantle P- and S-wave velocities have to be slightly faster and slower, respectively, while outer-core P-wave velocity is slower on account of a different velocity gradient, whereas inner-core velocity structure is similar, within the uncertainties of the inferred model parameters. In terms of density, we find that the mantle is less dense and the outer core more dense than PREM, while the inner core is similar to PREM. These changes are driven in part by the astronomic-geodetic data. The laboratory-based visco-elastic model considered here resolves the anelastic response of Earth’s mantle from long-period seismic (∼100 s) to tidal (18.6 yrs) periods, accounting for both normal-mode and tidal dissipation measurements. To study the impact of the inferred mantle seismic velocity structure, we computed P- and S-wave travel times and compared these to the observations of globally-averaged P- and S-wave travel times from the reprocessed ISC catalog that resulted in an excellent match. In an attempt to further refine the seismic P-wave velocity structure of the outer core, we also considered multiple core-mantle-boundary underside-reflected body wave travel time data. While the match to the underside reflections clearly improves as a result of a steeper velocity gradient in the outer core relative to the normal-modes- and astronomic-geodetic-data-only case, subtle differences nevertheless persist that appear to support a change in velocity gradient in the outermost core, evocative of a stably stratified layer. Finally, as a potential means of refining core composition, we considered the density contrast across the inner-core boundary (ICB) based on our inverted models. The most probable ICB density difference found here is 0.3–0.45 g/cm3, which is in the lower range of earlier body-wave- and normal-mode-based predictions. This suggests that the compositional heterogeneity associated with light-element partitioning, which is considered the principal driving mechanism for the compositional convection that powers the geodynamo, may be less effective than previously thought, calling for exsolution of solids from the liquid outer core as a possible additional source of energy. This would also help address the problem of a young inner core.

While hydrogen could be an important light alloying element in planetary iron cores, phase relations in the Fe‐FeH system remain largely unknown at high pressures and temperatures (P‐T). A speculative Fe‐H2 phase diagram has been proposed assuming continuous solid solution between Fe and FeH and eutectic melting between FeH and H2. Recent studies revealed that stoichiometric FeH becomes non‐magnetic above ∼40 GPa, which might affect its melting behavior. Here we examined the melting curve of non‐magnetic FeH between 43 and 152 GPa by a combination of laser‐heated diamond‐anvil cell techniques and synchrotron X‐ray diffraction (XRD) analyses. The melting temperature was determined by employing the appearance of additional hazy XRD signals upon quenching temperature as a melting criterion. We also performed thermodynamic modeling, which well reproduces the change in the curvature of FeH melting curve upon the loss of magnetism and extrapolates the experimental constraints to inner core pressures. The XRD data showed that non‐magnetic FeH melts congruently at temperatures higher than the known eutectic melting curve for FeHx (x > 1). Combined with the fact that the endmembers exhibit different crystal structures, these results indicate that Fe and non‐magnetic FeH form a eutectic system. The dT/dP slope of the FeH melting curve is comparable to that for Fe, suggesting that the eutectic liquid composition of FeH0.42 (Fe + 0.75 wt% H) previously estimated at ∼40 GPa changes little with increasing pressure.

We developed a thermodynamic model to explore the joint solubility of Mg, Si, and O in liquid Fe on the basis of high‐pressure metal‐silicate partitioning data in the literature, with more Mg kept in the metal when Si and O are present. With <1.7 ± 0.5 wt% Mg, the metal in the young Earth's core retains Mg as the core evolves and crystallizes SiO2. Higher Mg concentrations require either the late addition of metal that equilibrated with silicate in a super‐liquidus magma ocean or the assimilation of silicate into the core at the time of a giant impact. Above 1.7 wt% Mg, (Mg,Fe)‐silicate melts also exsolve from the core and transfer core‐hosted elements to the mantle. Fractional crystallization of the core‐derived silicate melts in the core or at the core‐mantle boundary could, additionally, yield a persistent molten silicate layer that could also contribute to ultralow velocity zone formation in the lowermost mantle.

Abstract Mercury, the Solar System’s innermost planet, has an unusually massive core prompting speculation that the planet lost silicate after it formed. Using the unusually high sulfur and low iron composition of its surface and space geodetic constraints on its core composition, we show Mercury’s chemistry to be compatible with formation in a larger planet at minimum 1.4–2.5 times Mercury’s present mass and possibly 2–4 times its mass by similarity with other rocky Solar System bodies. To do this, we apply an experimentally determined metal-silicate partitioning model for sulfur to Mercury’s silicate. The model is validated by applying it to Vesta, which, when evaluated at the conditions of Vestan self-differentiation, yields sulfur contents in its silicate in the range of HED meteorites. Mercury could have lost a substantial fraction of its rocky material through impacts or by being itself a remnant impactor. Independent of any stripping, because a significant amount of silicon resides in Mercury’s core, silicate meteoritic debris from Mercury would likely be characterized by 30Si isotopic enrichment >+ 0.10‰ relative to parent sources that could aid identification of a new meteorite class.

Plain Language Summary Seismic waves travel through Earth's solid inner core at different speeds in different directions. This happens because the metal crystals in the core line up to give the inner core a grain, like wood. The outermost part of the inner core's surface seems to lack a grain, however, because the speeds in it do not depend on direction. This study explores the possibility that there is a grain to the surface of the inner core too—it is just oriented differently. If the crystals align themselves to all point outward, you will not see changes in speed with direction with the usual seismic waves that travel across the inner core. Waves that reflect from the inner core's surface will seem much brighter, however, if the grains all point outward. It turns out that reflections from the inner core are much brighter than you would expect, lending credence to the idea of texture in it everywhere. As the crystals accumulate on the inner core's surface, they eventually get pressed into the grain orientation that the rest of the inner core has.

The chemical composition of carbonaceous chondrites permits up to 0.5 ± 0.4 wt% N in the Earth's core, which potentially affects the melting temperature, density profile, and phase relation of the core iron alloy. Here we have determined the melting curve and the high‐temperature equation of state (EoS) of β‐Fe7N3, which is stable above 40 GPa as the most Fe‐rich iron‐nitride. Experiments were performed in a laser‐heated diamond‐anvil cell up to 136 GPa, together with synchrotron X‐ray diffraction measurements. We found that multiple melting criteria gave similar melting temperatures: (1) the appearance of diffuse X‐ray scattering, (2) discontinuity in the laser power versus temperature relation, and (3) the reduction in diffraction peak intensity from solid. The validity of these melting criteria was confirmed by textural observation of recovered samples. We also observed rapid recrystallization at temperatures lower than the melting temperatures. The results demonstrate that β‐Fe7N3 melts congruently at about 3,100 K at 135 GPa, lower than the melting temperatures of FeSi and FeO and similar to that of FeS. The thermal EoS indicates that the density of Fe7(C,N)3 matches the observed inner core density. Combining the melting curve and the EoS of β‐Fe7N3, we also obtain the EoS of liquid Fe7N3. It shows that the density and compressibility of Fe + 10 wt% N is compatible with the outer core density profile. It supports the presence of some nitrogen in the liquid and solid parts of the core, although its concentration is difficult to constrain from the core density.

The methods advocated by the authors in their recent publication seem quite powerful and useful innovations in the methods used to process ambient noise data. However, insufficient details are given in the descriptions of the methods used to reproduce the results shown in the paper. In the hope that others may not pursue processing dead ends, I summarize my own attempts to reproduce the details of their study and suggest what additional methodological details could be provided to make the results more robust and reproducible. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.