Toward a mineral physics reference model for the Moon's core

International audienceIron is the main constituent of terrestrial planetary cores, taking on a hexagonal closed packed structure under the conditions of Earth’s inner core, and a face-centered cubic (fcc) structure at the more moderate pressures of smaller bodies, such as the Moon, Mercury, or Mars. Here we present sound velocity and density measurements of fcc iron at pressures and temperatures characteristic of small planetary interiors. The results indicate that the seismic velocities currently proposed for the Moon’s inner core are well below those of fcc iron or plausible iron alloys. Our dataset provides strong constraints to seismic models of the lunar core and cores of small telluric planets, and allows us to build a direct compositional and velocity model of the Moon’s core

Lattice dynamics of MgSiO3_3 perovskite (bridgmanite) studied by inelastic x-ray scattering and ab initio calculations

We have determined the lattice dynamics of MgSiO$_3$ perovskite (bridgmanite) by a combination of single-crystal inelastic x-ray scattering and ab initio calculations. We observe a remarkable agreement between experiment and theory, and provide accurate results for phonon dispersion relations, phonon density of states and the full elasticity tensor. The present work constitutes an important milestone to extend this kind of combined studies to extreme conditions of pressure and temperature, directly relevant for the physics and the chemistry of Earth's lower mantle

Signature of a polyamorphic transition in the THz spectrum of vitreous GeO2

The THz spectrum of density fluctuations, $S(Q, \omega)$, of vitreous GeO$_2$ at ambient temperature was measured by inelastic x-ray scattering from ambient pressure up to pressures well beyond that of the known $\alpha$-quartz to rutile polyamorphic (PA) transition. We observe significant differences in the spectral shape measured below and above the PA transition, in particular, in the 30-80 meV range. Guided by first-principle lattice dynamics calculations, we interpret the changes in the phonon dispersion as the evolution from a quartz-like to a rutile-like coordination. Notably, such a crossover is accompanied by a cusp-like behavior in the pressure dependence of the elastic response of the system. Overall, the presented results highlight the complex fingerprint of PA phenomena on the high-frequency phonon dispersion.Comment: 18 pages, 5 figure

Composition of the Earth's inner core from high-pressure sound velocity measurements in Fe-Ni-Si alloys

Editor: R.D. van der Hilst Keywords: Fe-Ni-Si alloy aggregate compressional and shear sound velocities high pressure inner core light elements We performed room-temperature sound velocity and density measurements on a polycrystalline alloy, Fe 0.89 Ni 0.04 Si 0.07 , in the hexagonal close-packed (hcp) phase up to 108 GPa. Over the investigated pressure range the aggregate compressional sound velocity is ∼ 9% higher than in pure iron at the same density. The measured aggregate compressional (V P ) and shear (V S ) sound velocities, extrapolated to core densities and corrected for anharmonic temperature effects, are compared with seismic profiles. Our results provide constraints on the silicon abundance in the core, suggesting a model that simultaneously matches the primary seismic observables, density, P-wave and S-wave velocities, for an inner core containing 4 to 5 wt.% of Ni and 1 to 2 wt.% of Si

Steroid treatment in the management of destructive thyrotoxicosis induced by PD1 blockade

Objective: Destructive thyroiditis is the most common endocrine immune related adverse event (iRAEs) in patients treated with anti-PD1/PD-L1 agents. Given its self-limited course, current guidelines recommend no treatment for this iRAE. Nevertheless in patients with enlarged thyroid volume and a poor performance status, thyrotoxicosis may be particularly severe and harmful. Aim of the study is to evaluate if steroid treatment might be useful in improving thyrotoxicosis in subjects with a poor performance status. Methods: We conducted a retrospective study, comparing the course of thyrotoxicosis of 4 patients treated with oral prednisone at the dosage of 25 mg/d (tapered to discontinuation in three weeks) and an enlarged thyroid volume to that of 8 patients with similar thyroid volume who were left untreated. Results: The levels of thyroid hormones were lower in subjects treated compared to those untreated at time 7, 14, 21, 28, 35, 42, 60 and 90 days (P<0.05 at each time). The time to remission of thyrotoxicosis was 24 days in patients treated with steroids and 120 days in untreated patients (P<0.001). At 6 months, the rate of evolution to hypothyroidism was similar in the 2 groups (4/4 in steroid group vs 7/8 in untreated group, P=0.74) and no difference was found in tumor progression (P=0.89). Conclusions: Our preliminary data suggest that in patients with a poor performance status experiencing a severe destructive thyrotoxicosis induced by PD-1 blockade, a short period of administration of oral prednisone is effective in obtaining a quick reduction of the levels of thyroid hormones

Phase transition boundary between fcc and hcp structures in Fe-Si alloy and its implications for terrestrial planetary cores

International audienc

Initial results from the InSight mission on Mars

NASA’s InSight (Interior exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018. It aims to determine the interior structure, composition and thermal state of Mars, as well as constrain present-day seismicity and impact cratering rates. Such information is key to understanding the differentiation and subsequent thermal evolution of Mars, and thus the forces that shape the planet’s surface geology and volatile processes. Here we report an overview of the first ten months of geophysical observations by InSight. As of 30 September 2019, 174 seismic events have been recorded by the lander’s seismometer, including over 20 events of moment magnitude Mw = 3–4. The detections thus far are consistent with tectonic origins, with no impact-induced seismicity yet observed, and indi- cate a seismically active planet. An assessment of these detections suggests that the frequency of global seismic events below approximately Mw = 3 is similar to that of terrestrial intraplate seismic activity, but there are fewer larger quakes; no quakes exceeding Mw = 4 have been observed. The lander’s other instruments—two cameras, atmospheric pressure, temperature and wind sensors, a magnetometer and a radiometer—have yielded much more than the intended supporting data for seismometer noise characterization: magnetic field measurements indicate a local magnetic field that is ten-times stronger than orbital estimates and meteorological measurements reveal a more dynamic atmosphere than expected, hosting baroclinic and gravity waves and convective vortices. With the mission due to last for an entire Martian year or longer, these results will be built on by further measurements by the InSight lander

Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

International audienceDetermining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth's core but is experimentally challenging. Here, we review techniques and methodologies used to measure sound velocities in metals at megabar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth's inner core composition

Properties of iron alloys under the Earth's core conditions

International audienceThe Earth's core is constituted of iron and nickel alloyed with lighter elements. In view of their affinity with the metallic phase, their relative high abundance in the solar system and their moderate volatility, a list of potential light elements have been established, including sulfur, silicon and oxygen. We will review the effects of these elements on different aspects of Fe-X high pressure phase diagrams under Earth's core conditions, such as melting temperature depression, solid-liquid partitioning during crystallization, and crystalline structure of the solid phases. Once extrapolated to the inner-outer core boundary, these petrological properties can be used to constrain the Earth's core propertie

Sound velocities and thermodynamical properties of hcp iron at high pressure and temperature

International audienceSound velocities and thermodynamical properties of hcp iron have been computed using ab initio calculations over an extended density and temperature range, encompassing the conditions directly relevant for the Earth's inner core. At room temperature, and up to 350 GPa, an excellent agreement is obtained between present results and experimental data for many thermodynamical quantities: phonon density of states, vibrational entropy, heat capacity, Grüneisen parameter and thermal expansion. With increasing temperature, along an isochore, we observe a strong decrease of the phonon frequencies, demonstrating that intrinsic anharmonic effects cannot be neglected. We also carefully compare previous theoretical data for the sound velocities and try to explain the discrepancies observed with experiments. Finally, we propose a temperature dependant Birch's law that we compare with previous experimental wor