A heat shield called the thermal protection system (TPS) is an important structure in hypersonic vehicles as it prevents hot air from entering vehicles and potential impacts from space debris. With ...the increase in demand for low-cost reusable launch vehicles as well as for searching and exploration of new planets in both unmanned and manned missions, the need for developing an effective TPS has increased across many countries. The structural design of TPSs has become more prominent in the early stage of hypersonic vehicle development. Sandwich structures that have the advantages of low density and high performance are integrated into the structural design of an effective TPS. This paper provides a comprehensive review of recent research efforts on sandwich structures for TPSs. The topics discussed in this paper include aspects of structural and material design, mechanical and thermomechanical performances, and manufacturing methods. In particular, we review and discuss the structural design as well as the material design of sandwich structures for different TPS types with various configurations, including corrugated cores, lattice cores, multilayer cores, foams, honeycomb cores, bio-inspired cores. The materials used for the sandwich structures, such as various types of laminated composite, ceramic matrix composite, and metals, are included. We also discuss the performance of the TPS sandwich structures in terms of temperature gradients, deformation limits, and mechanical strengths and provide a discussion on the manufacturing methods of TPS sandwich structures for hypersonic vehicles. Finally, further research directions and challenges of sandwich structures for TPSs are presented.
As the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported ...iron‐melting curves show large discrepancies (up to 1000 K at the Earth's core–mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid‐liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core‐mantle boundary (4250 ± 250 K), here determined by X‐ray absorption experiments at the Fe K‐edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.
Plain Language Summary
Iron is the main constituent of planetary cores; however, there are still large controversies regarding its melting temperature and phase diagram under planetary interior conditions. The present study reconciles different experimental approaches using laser‐heated diamond anvil cell with different in situ X‐ray diagnostics (absorption, diffraction, and Mossbauer spectroscopy). The main reason of discrepancies (over 1000 K at core‐mantle boundary conditions) is attributed to carbon contamination from the diamond anvils and metrology issues related to thermal pressure overestimation. A high‐melting temperature for iron at core‐mantle boundary pressure would imply the presence of volatile elements in the liquid outer core, such as sulfur, carbon, or hydrogen, in order to lower its crystallization temperature and avoid extended melting of the surrounding silicate mantle.
Key Points
Melting curve and phase diagram of pure Fe has been measured by in situ X‐ray absorption up to 130 GPa
Overall agreement between different in situ studies leads to a melting temperature of 4250 ± 250 K at core‐mantle boundary pressure
Discrepancies with previous measurements is unambiguously attributed to carbon contamination from the diamonds or thermal pressure overestimation
Earth's core is structured in a solid inner core, mainly composed of iron, and a liquid outer core. The temperature at the inner core boundary is expected to be close to the melting point of iron at ...330 gigapascal (GPa). Despite intensive experimental and theoretical efforts, there is little consensus on the melting behavior of iron at these extreme pressures and temperatures. We present static laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast x-ray diffraction as a primary melting diagnostic. When extrapolating to higher pressures, we conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 kelvin. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the mantle.
This article aims to study the core losses behavior at high frequency (MHz range) with superimposed dc bias in ferrite. Inductive compensation method is employed in the measurement circuit to reduce ...sensitivity to phase errors. An addition to the circuit is proposed in this work to make easier dc bias generation. Furthermore, a detailed measurement accuracy consideration is also discussed. The tested MnZn ferrite has a nominal relative permeability of 1500 and 800, which was tested at frequency from 500 kHz to 3 MHz. The measurement results are explained thoroughly with three controlling parameters: excitation frequency, peak ac flux density, and dc bias. There are several important findings. First, a higher dc bias creates higher losses at the same frequency and ac flux density. Second, at the same dc bias and ac flux density, higher frequency generates a lower relative losses increase. This second point has not been seen in the previous literature and is elaborated more in Section <xref ref-type="sec" rid="sec3b">III-B . Third, the measurement result shows how dc bias increases the hysteresis loop area and coercivity. The first and third findings confirm the existing literature findings. As a final step, an improved Steinmetz Premagnetization Graph and artificial neural network are used to create core loss prediction model. The measurement data and the built model can be accessed online for use by other magnetics designer.
The terrestrial planets accreted from a diverse suite of solar system materials ranging from strongly O-deficient materials similar to enstatite chondrites via ordinary chondrite materials to fully ...oxidised carbonaceous chondrite and cometary materials. Heliocentric zoning with increasingly oxidised planetesimals outwards through the protoplanetary disc is broadly reflected in core fraction and FeOmantle concentration, ranging from 68 wt% core and 0.5 wt% FeOmantle for Mercury to 18 wt% core and 24 wt% FeOmantle for Vesta. Mercury, Venus and Earth grew mostly from materials which were isotopically similar to enstatite chondrites, although Earth and Venus also received more oxidised material. The elevated (Mg + Fe)/Si ratio, compared to chondrites, in the bulk silicate fraction of the terrestrial planets, except for Mercury, may be related to a combination of nebular fractionation associated with forsterite condensation, concentration of olivine-rich chondrules near the mid-plane of the accretion disc and multi-cycle impact erosion of protocrusts. For the extremely reduced Mercury the silicate magma ocean (MO) and a core with 15 wt% Si might have equilibrated with a melt layer of FeS at the core-mantle boundary (CMB). Recent data from the MESSENGER mission combined with experimentally derived phase relations, support estimates of about 0.5 wt% FeO and 10 wt% S in the MO and the current mantle. Core segregation at very high temperatures for the largest planets, Venus and Earth, led to cores with high Si content, even at relatively high oxygen fugacities and FeOmantle contents, because increasing temperature shifts the equilibrium:SiO2MO+2Fecore=2FeOMO+Sicoretowards the products (right side). The hot protocores of Venus and Earth might have started with about 5–7 wt% Si and 2–3 wt% O. Mars and Vesta segregated S-rich cores at high oxygen fugacity and low pressure.
Strong partitioning of Fe and Mg to melt and solids, respectively, caused neutrally buoyant bridgmanite (bm) to crystallise from the MO at 1700–1860 km depth (72–80 GPa), resulting in a separate basal magma ocean (BMO) within Earth, and probably also in Venus. Slow cooling of a thermally insulated BMO and core, accompanied by protracted crystallisation of bm and ferropericlase (fp), would facilitate core-BMO chemical exchange by reversing the equilibrium SiO2MO + 2Fecore = 2FeOMO + Sicore towards the reactants. Transfer of silica crystals and a liquid SiO2 component from the core to the BMO, and liquid FeO and Fe2O3 components from the BMO to the core, would increase the Si/Mg, Mg/Fe and bm/fp ratio of the resulting cumulates. Because the solidus temperature of peridotite is <200–300 K above the present temperature of the outermost core, and the melting interval of late-stage BMO melt enriched in Al, Fe, Ca and Na would be lower than that of peridotite, the BMO might have persisted through the Hadean and possibly also the Archean. Low solid state diffusion rates, especially in bm, would have restricted the core-mantle interaction upon BMO solidification, but limited core-mantle interaction could possibly occur via partially molten ultra-low velocity zones. An outermost stagnant low-density and low-velocity core layer (E′-layer), with reduced Si and elevated O contents relative to the convecting core, appears to trace the core-BMO exchange. The E′-layer is compositionally gradational towards the convecting core at 445 km below the CMB. High thermal conductivity and minimal convective entrainment in the low-viscosity core fluid might have developed and stabilised such a gradational layer since the Hadean or early Archean.
The primordial bm-dominated cumulates with high Mg/Fe ratios and viscosities may have become convectively aggregated into large refractory domains, remaining neutrally buoyant in the middle to upper parts of the lower mantle and resistant to convective destruction. Late-stage dense BMO cumulates with elevated Fe/Mg ratios relative to the bulk mantle composition might represent a suitable material for 100–200 km thick thermochemical piles at the bottom of the large low S-wave velocity provinces (LLSVPs) under Africa and the Pacific. A degree-2 convection pattern, possibly initiated and stabilised during Earth's early rapid rotation, involving antipodally ascending columns in equatorial positions and an intermediary descending longitudinal belt, might have swept the late-stage, dense bridgmanitic cumulates with high Fe/Mg-ratios, viscosity and bulk modulus towards the root zones of the upwelling columns.
Display omitted
•Core and mantle compositional estimates for the terrestrial planets are provided•Melting and silicate-metal partitioning led to magma oceans (MO) and cores•Core-mantle separation in Earth and Venus occurred at high temperature and pressure•Mineral-melt density relations led to a separate basal MO (BMO) in Earth and Venus•Cooling of Earth and Venus cores led to core-BMO chemical exchange
Hunt begins for ancient Antarctic ice Voosen, Paul
Science (American Association for the Advancement of Science),
2021-Oct-22, 2021-10-22, 20211022, Letnik:
374, Številka:
6566
Journal Article
Recenzirano
Six teams plan to drill cores that could shed light on warm climates 1.5 million years ago
Six teams plan to drill cores that could shed light on warm climates 1.5 million years ago
The European Project for Ice Coring in Antarctica Dome ice core from Dome C (EDC) has allowed for the reconstruction of atmospheric CO2 concentrations for the last 800,000years. Here we revisit the ...oldest part of the EDC CO2 record using different air extraction methods and sections of the core. For our established cracker system, we found an analytical artifact, which increases over the deepest 200m and reaches 10.1±2.4ppm in the oldest/deepest part. The governing mechanism is not yet fully understood, but it is related to insufficient gas extraction in combination with ice relaxation during storage and ice structure. The corrected record presented here resolves partly - but not completely - the issue with a different correlation between CO2 and Antarctic temperatures found in this oldest part of the records. In addition, we provide here an update of 800,000years atmospheric CO2 history including recent studies covering the last glacial cycle.
Convection is a fundamental physical process in the fluid cores of planets. It is the primary transport mechanism for heat and chemical species and the primary energy source for planetary magnetic ...fields. Key properties of convection-such as the characteristic flow velocity and length scale-are poorly quantified in planetary cores owing to the strong dependence of these properties on planetary rotation, buoyancy driving and magnetic fields, all of which are difficult to model using realistic conditions. In the absence of strong magnetic fields, the convective flows of the core are expected to be in a regime of rapidly rotating turbulence
, which remains largely unexplored. Here we use a combination of non-magnetic numerical models designed to explore this regime to show that the convective length scale becomes independent of the viscosity when realistic parameter values are approached and is entirely determined by the flow velocity and the planetary rotation. The velocity decreases very rapidly at smaller scales, so this turbulent convective length scale is a lower limit for the energy-carrying length scales in the flow. Using this approach, we can model realistically the dynamics of small non-magnetic cores such as the Moon. Although modelling the conditions of larger planetary cores remains out of reach, the fact that the turbulent convective length scale is independent of the viscosity allows a reliable extrapolation to these objects. For the Earth's core conditions, we find that the turbulent convective length scale in the absence of magnetic fields would be about 30 kilometres, which is orders of magnitude larger than the ten-metre viscous length scale. The need to resolve the numerically inaccessible viscous scale could therefore be relaxed in future more realistic geodynamo simulations, at least in weakly magnetized regions.
We report the first Ni and Cr stable isotope data for ureilite meteorites that are the mantle residue of a carbon‐rich differentiated planet. Ureilites have similar Ni stable isotope compositions as ...chondrites, suggesting that the core‐mantle differentiation of ureilite parent body (UPB) did not fractionate Ni isotopes. Since the size of Earth is potentially larger than that of UPB; with diameter >690 km), resulting in higher temperatures at the core‐mantle boundary of Earth, it can be predicted that the terrestrial core formation may not directly cause Ni stable isotope fractionation. On the other hand, we also report high‐precision Cr stable isotope composition of ureilites, including one ureilitic trachyandesite (ALM‐A) that is enriched in lighter Cr stable isotopes relative to the main‐group ureilites, which suggests that the partial melting occurred on UPB. The globally heavy Cr in the UPB compared to chondrites can be caused by sulfur‐rich core formation processes.
Plain Language Summary
The stable isotope fractionation of siderophile elements is robust to trace the planetary core formation processes. However, whether nickel (Ni) isotopes fractionate during the core formation is highly debated, since the origin of Ni stable isotope difference between bulk silicate Earth and chondrites is not clear. Here, we report high‐precision Ni stable isotope data (expressed as δ60/58Ni, the per mil deviation of Ni60/Ni58 ratios relative to NIST SRM 986) for ureilite meteorites that come from the mantle of a carbon‐rich differentiated body. Ureilites have an average δ60/58Ni value of 0.26 ± 0.13‰ (2SD, N = 22) that is highly consistent with that of chondrites with δ60/58Ni = 0.23 ± 0.14‰ (2SD, N = 37), which suggests that planetary core formation does not effectively fractionate Ni stable isotopes. There is a ureilite trachyandesite that enriches lighter Cr stable isotopes (δ53Cr = −0.11 ± 0.02‰; δ53Cr as the per‐mil deviation of Cr53/52Cr ratios relative to NIST SRM 979) relative to the main‐group ureilites (δ53Cr = −0.05 ± 0.04‰; 2SD, N = 10), which suggests that the partial melting occurred on ureilite parent body (UPB). The globally heavy Cr in the UPB compared to chondrites can be caused by sulfur‐rich core formation processes.
Key Points
Planetary magmatic processes do not fractionate Ni stable isotopes
Ureilite parent body (UPB) has similar δ60/58Ni values as chondrites, and Earth's core formation does not fractionate Ni stable isotopes
Cr stable isotopes recorded partial melting and sulfur‐rich core formation on UPB
The phase transition between a face-centered cubic (fcc) and hexagonal close-packed (hcp) structures in Fe-4wt% Si alloy was examined in an internally resistive heated diamond-anvil cell (DAC) under ...high-pressure (P) and high-temperature (T) conditions to 71 GPa and 2000 K by in situ synchrotron X-ray diffraction. Complementary laser-heated DAC experiments were performed in Fe-6.5wt% Si. The fcc-hcp phase transition boundaries in the Fe-Si alloys are located at higher temperatures than that in pure Fe, indicating that the addition of Si expands the hcp stability field. The dP/dT slope of the boundary of the entrant fcc phase in Fe-4wt% Si is similar to that of pure Fe, but the two-phases region is observed over a temperature range increasing with pressure, going from 50 K at 15 GPa to 150 K at 40 GPa. The triple point, where the fcc, hcp, and liquid phases coexist in Fe-4wt% Si, is placed at 90-105 GPa and 3300-3600 K with the melting curve same as in Fe is assumed. This supports the idea that the hcp phase is stable at Earth's inner core conditions. The stable structures of the inner cores of the other terrestrial planets are also discussed based on their P-T conditions relative to the triple point. In view of the reduced P-T conditions of the core of Mercury (well below the triple point), an Fe-Si alloy with a Si content up to 6.5 wt% would likely crystallize an inner core with an fcc structure. Both cores from Venus and Mars are currently believed to be totally molten. Upon secular cooling, Venus is expected to crystallize an inner core with an hcp structure, as the pressures are similar to those of the Earth's core (far beyond the triple point). Martian inner core will take an hcp or fcc structure depending on the actual Si content and temperature.
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