Inefficient compaction in small planetary cores -- application to the Moon

2021 ◽  
Author(s):  
Marine Lasbleis
Keyword(s):  
Magnetic Field ◽  
Latent Heat ◽  
Energy Budget ◽  
Inner Core ◽  
Light Elements ◽  
The Moon ◽  
Small Bodies ◽  
Typical Size ◽  
The Core ◽  
Planetary Cores

<div> <p>Growth of the solid inner core is generally considered to power the Earth's present geodynamo. Cristallisation of a solid central inner core has also been proposed to drive the lunar dynamo and to generate a magnetic field in smaller bodies. In a previous work, we estimated the compaction of planetary cores for different scenarios of growth (with or without supercooling) and different sizes of the inner core. Our main results indicated that small inner cores are unlikely to compact efficiently the liquid trapped during the first steps of the growth.</p> <p>This is especially true for small bodies for which the typical size of the core is similar to the compaction length. The light elements are thus trapped during the cristallisation, reducing the release of latent heat and of light elements. We present here a model to include the effect of an inefficient compaction in the energy budget of a planetary core and investigate the implications for the dynamo evolution in small bodies. We apply this model for the evolution of the core of the Moon. </p> </div>

Exoplanetary magnetic fields as a tool for future interior characterization

2020 ◽  
Author(s):  
Irene Bonati ◽  
Marine Lasbleis ◽  
Lena Noack
Keyword(s):  
Magnetic Field ◽  
Solar System ◽  
Magnetic Fields ◽  
Iron Content ◽  
Inner Core ◽  
Outer Core ◽  
Magnetic Activity ◽  
Light Elements ◽  
The Core ◽  
The Magnetic Field

<p>Most planets located within the solar system display evidence of past and/or current magnetic activity. Magnetic fields of rocky bodies are thought to be driven by thermo-chemical convection taking place in an electrically conducting fluid in their deep interior (the liquid outer core for Earth), and are thus evidence of strong internal dynamics. Furthermore, magnetism is thought to play a crucial role for the development and the long-term stability of habitable surface conditions, as it shields the upper atmosphere from mass loss induced by stellar winds and extreme space weather events.<span class="Apple-converted-space"> </span></p> <p>The discovery of a large number of rocky exoplanets motivates the search and the study of magnetic fields beyond the solar system. While current observations are limited to providing the planetary radius and minimum mass, future missions aimed at the exploration of exoplanetary atmospheres will open up new avenues for the inversion of interior properties starting from atmospheric parameters. Such a goal requires knowledge of the planetary cores and the development of exoplanetary magnetic fields, as well as their influence on atmospheric evolution and its interaction with the surrounding stellar wind.<span class="Apple-converted-space"> </span></p> <p>The aim of the current study is to identify trends and parameter(s) controlling the core evolution and magnetic field sustainment in super-Earths. To do this we investigate the evolution of the cores of planets having different masses (0.8-2 Earth masses) and iron inventories (bulk iron content and mantle iron number). Starting out from the internal temperature profile after the complete solidification of a global magma ocean (Noack and Lasbleis, 2020), we determine the size and the structure of the core, and model its thermal and magnetic evolution during the subsequent 5 billion years. By taking into account the energy release resulting from the growth of a solid inner core, we compute the thermal and compositional buoyancy fluxes, as well as the generated magnetic field strengths and lifetimes.<span class="Apple-converted-space"> </span></p> <p>Our findings show that while the planetary mass is not a controlling parameter, both the bulk iron content and the mantle iron number strongly influence inner core growth and the lifetime of the magnetic field. Iron-rich planets having a high mantle iron number tend to start out and end up with solid inner cores that are substantially larger than iron-poor bodies, sometimes even reaching up to the radius of the outer core and thus shutting down magnetic activity. We therefore find that there is a “sweet spot” for longer-lasting magnetic fields, located at intermediate bulk iron contents and low mantle iron numbers. <span class="Apple-converted-space"> </span></p> <p>We also varied the content of light elements in the core and found that the addition of a small fraction of light elements helps keeping the magnetic field active for longer, even at high bulk iron contents. Field strengths can reach up to several times the one of Earth, even though such a signal might still be too weak to be detected by current radio telescopes. Nevertheless, the development of new observation techniques and the multiple future missions devoted to atmospheric exploration will provide useful insights on the presence and frequency of planetary magnetic fields.</p>

scholarly journals Constraints on Earth’s inner core composition inferred from measurements of the sound velocity of hcp-iron in extreme conditions

2016 ◽  
Vol 2 (2) ◽  
pp. e1500802 ◽  
Author(s):  
Tatsuya Sakamaki ◽  
Eiji Ohtani ◽  
Hiroshi Fukui ◽  
Seiji Kamada ◽  
Suguru Takahashi ◽  
...  
Keyword(s):  
Sound Velocity ◽  
Inner Core ◽  
Earth Model ◽  
Light Elements ◽  
Earth’S Core ◽  
Heated Sample ◽  
Earth's Core ◽  
The Core ◽  
Core Composition ◽  
Earth’S Inner Core

Hexagonal close-packed iron (hcp-Fe) is a main component of Earth’s inner core. The difference in density between hcp-Fe and the inner core in the Preliminary Reference Earth Model (PREM) shows a density deficit, which implies an existence of light elements in the core. Sound velocities then provide an important constraint on the amount and kind of light elements in the core. Although seismological observations provide density–sound velocity data of Earth’s core, there are few measurements in controlled laboratory conditions for comparison. We report the compressional sound velocity (VP) of hcp-Fe up to 163 GPa and 3000 K using inelastic x-ray scattering from a laser-heated sample in a diamond anvil cell. We propose a new high-temperature Birch’s law for hcp-Fe, which gives us the VP of pure hcp-Fe up to core conditions. We find that Earth’s inner core has a 4 to 5% smaller density and a 4 to 10% smaller VP than hcp-Fe. Our results demonstrate that components other than Fe in Earth’s core are required to explain Earth’s core density and velocity deficits compared to hcp-Fe. Assuming that the temperature effects on iron alloys are the same as those on hcp-Fe, we narrow down light elements in the inner core in terms of the velocity deficit. Hydrogen is a good candidate; thus, Earth’s core may be a hidden hydrogen reservoir. Silicon and sulfur are also possible candidates and could show good agreement with PREM if we consider the presence of some melt in the inner core, anelasticity, and/or a premelting effect.

The early magnetic field and primeval satellite system of the Moon: clues to planetary formation

1994 ◽  
Vol 349 (1690) ◽  
pp. 181-196 ◽  
Keyword(s):  
Magnetic Field ◽  
Magnetic Anomalies ◽  
Satellite System ◽  
Iron Core ◽  
Planetary Formation ◽  
The Moon ◽  
The Core ◽  
Basin Formation ◽  
The Mean ◽  
The Moment

From the stable remanent magnetization of the Apollo igneous rocks and high-grade breccias the existence of a primeval lunar magnetic field was inferred. The palaeointensities of the samples rise rapidly to a maximum at 3.9 Ga, then decrease exponentially to 3.2 Ga, strongly suggesting that the Moon had a field generated in a core, the existence of which was inferred from its non-hydrostatic figure. Modelling of the Apollo 15 and 16 subsatellite magnetic anomalies, by P. J. Coleman, L. L. Hood and C. T. Russell, gave palaeomagnetic directions of crustal strata. This enabled N pole positions to be calculated, which were empirically found to form three bipolar groups, the mean poles of which define (on the core dynamo hypothesis) three axes of rotation different from the present. These were dated as Pre-Nectarian, Lower Nectarian, and Upper Nectarian-Imbrian. Multi-ring basins of these ages were found to lie close to the corresponding palaeo-equators. The impacting bodies were therefore satellites, not asteroids or comets. Their velocities, before collision, can be shown (from basin asymmetries) to be nearly equatorial. The consequent changes in the moment of inertia tensor by basin formation caused these successive reorientations of the Moon relative to its axis of rotation in space. The three mean poles form a 90° spherical triangle. The explanation is that the Moon had three satellites: the orbits of each decayed, they broke up at the Roche limit into smaller bodies, which produced impact basins near the equator. The Moon then reorientated according to Euler’s principle before the next group of impacts. Lunar palaeomagnetism, and especially the inferences that the Moon has an iron core that segregated late and had a primeval satellite system, may provide important constraints on theories of lunar and planetary formation.

The primeval axis rotation of the Moon

1984 ◽  
Vol 313 (1524) ◽  
pp. 77-83 ◽  
Keyword(s):  
Magnetic Field ◽  
Dominant Role ◽  
The Moon ◽  
Tidal Friction ◽  
The Core ◽  
Axis Of Rotation ◽  
Euler’S Theorem ◽  
Dipole Magnetic Field ◽  
Axis Rotation

The palaeopoles of the Moon have been calculated from the palaeomagnetic directions of lunar crustal strata, determined by Coleman, Russell and Hood from Apollo 15 and 16 sub-satellite data. Because of the dominant role of the Coriolis force in the core dynamo hydromagnetics, the lunar dipole magnetic field would have been aligned along the axis of rotation. As the palaeopoles lie in bipolar groupings along 3 different axes, far from the present axis (the magnetizations are dated at 4.2 Ga, 4.0 Ga and 3.85 Ga ago), it is concluded that the Moon was reoriented at least three times. These polar displacements are attributed to the creation of multi-ring basins at these times by the collision of fragments of at least three lunar satellites, the orbits of which decayed by tidal friction. The actual paths of the pole are explained by Euler’s theorem applied to a moon in which the interior can flow by solid state creep.

Interaction of Interplanetary Shocks with the Moon

2020 ◽  
Author(s):  
Xiaoyan Zhou ◽  
Nojan Omidi
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Field Strength ◽  
Shock Front ◽  
Magnetic Field Strength ◽  
Interplanetary Shocks ◽  
The Moon ◽  
Energetic Ions ◽  
The Core ◽  
The Magnetic Field

<p>In this presentation, we use data from THEMIS-ARTEMIS spacecraft and electromagnetic hybrid (kinetic ions, fluid electrons) simulations to describe the nature of the interaction between interplanetary shocks and the Moon. In the absence of a global magnetic field and an ionosphere at the Moon, solar wind interaction is controlled by (1) absorption of the core solar wind protons on the dayside; (2) access of supra-thermal and energetic ions in the solar wind to the lunar tail; (3) penetration and passage of the IMF through the lunar body. This results in a lunar tail populated by energetic ions and enhanced magnetic field in the central tail region. In general, ARTEMIS observations show a clear jump in the magnetic field strength associated with the passage of the interplanetary shock regardless of the position in the tail. Compared to the shock front observed in the solar wind, the magnetic field strength in the tail is stronger both upstream and downstream of the shock which is consistent with the expectations of larger field strengths in the tail. In addition, the transition from upstream to downstream magnetic field strength takes longer time as compared to the solar wind, indicating the broadening in space of the shock transition region. In contrast, plasma observations show that depending on the position of the spacecraft in the tail, a density enhancement in association with the shock front may or may not be observed. Using the observed solar wind conditions, we have used hybrid simulations to examine the interaction of interplanetary shocks with the Moon. The results indicate that by virtue of IMF passage through the lunar body, the magnetic field shock front also passes through the Moon and as such a jump in the magnetic field strength is observed throughout the lunar tail in association with the passage of the shock. As expected, the field strength in the upstream and downstream regions in the tail are larger than the corresponding values in the solar wind. In addition, the passage of the shock through the lunar tail is associated with the broadening of the shock front. The absorption of the core solar wind protons on the dayside introduces a density hole in the shock front as it passes through the Moon and the lunar tail and, as such, the shock front as a whole is disrupted. This hole is gradually filled with the ambient plasma while it travels further down the tail until eventually the shock front is fully restored a few lunar radii away from the Moon. The simulation results are found to be consistent with ARTEMIS observations. Here we also discuss the impacts of shock Mach number on the interaction. These results depict the lunar environment under transient solar wind conditions, which provide helpful information for the NASA’s plan to return humans to the Moon.</p>

Investigation of the Structure and Magnetic Properties of Bulk Amorphous FeCoYB Alloy

2017 ◽  
Vol 68 (9) ◽  
pp. 2162-2165 ◽  
Author(s):  
Katarzyna Bloch ◽  
Mihail Aurel Titu ◽  
Andrei Victor Sandu
Keyword(s):  
Magnetic Field ◽  
Magnetic Properties ◽  
Hyperfine Field ◽  
Irreversible Processes ◽  
Magnetization Process ◽  
Casting Method ◽  
The Core ◽  
Injection Casting ◽  
Core Losses ◽  
Microstructural Studies

The paper presents the results of structural and microstructural studies for the bulk Fe65Co10Y5B20 and Fe63Co10Y7B20 alloys. All the rods obtained by the injection casting method were fully amorphous. It was found on the basis of analysis of distribution of hyperfine field induction that the samples of Fe65Co10Y5B20 alloy are characterised with greater atomic packing density. Addition of Y to the bulk amorphous Fe65Co10Y5B20 alloy leads to the decrease of the average induction of hyperfine field value. In a strong magnetic field (i.e. greater than 0.4HC), during the magnetization process of the alloys, where irreversible processes take place, the core losses associated with magnetization and de-magnetization were investigated.

Ups and Downs

2018 ◽  
Author(s):  
Roy Livermore
Keyword(s):  
Mantle Convection ◽  
Laboratory Experiments ◽  
Inner Core ◽  
Computer Modelling ◽  
Great Depth ◽  
New Horizons ◽  
The Core ◽  
The Earth ◽  
The World ◽  
The 1960S

Despite the dumbing-down of education in recent years, it would be unusual to find a ten-year-old who could not name the major continents on a map of the world. Yet how many adults have the faintest idea of the structures that exist within the Earth? Understandably, knowledge is limited by the fact that the Earth’s interior is less accessible than the surface of Pluto, mapped in 2016 by the NASA New Horizons spacecraft. Indeed, Pluto, 7.5 billion kilometres from Earth, was discovered six years earlier than the similar-sized inner core of our planet. Fortunately, modern seismic techniques enable us to image the mantle right down to the core, while laboratory experiments simulating the pressures and temperatures at great depth, combined with computer modelling of mantle convection, help identify its mineral and chemical composition. The results are providing the most rapid advances in our understanding of how this planet works since the great revolution of the 1960s.

scholarly journals Hydromagnetic dynamos in rotating spherical fluid shells in dependence on the Prandtl number, density stratification and electromagnetic boundary conditions

2014 ◽  
Vol 44 (4) ◽  
pp. 293-312 ◽  
Author(s):  
Tomáš Šoltis ◽  
Ján Šimkanin
Keyword(s):  
Magnetic Field ◽  
Electrical Conductivity ◽  
Magnetic Fields ◽  
Prandtl Number ◽  
Inner Core ◽  
Polar Regions ◽  
Number Density ◽  
Magnetic Diffusion ◽  
Spherical Fluid ◽  
The Magnetic Field

Abstract We present an investigation of dynamo in a simultaneous dependence on the non-uniform stratification, electrical conductivity of the inner core and the Prandtl number. Computations are performed using the MAG dynamo code. In all the investigated cases, the generated magnetic fields are dipolar. Our results show that the dynamos, especially magnetic field structures, are independent in our investigated cases on the electrical conductivity of the inner core. This is in agreement with results obtained in previous analyses. The influence of non-uniform stratification is for our parameters weak, which is understandable because most of the shell is unstably stratified, and the stably stratified region is only a thin layer near the CMB. The teleconvection is not observed in our study. However, the influence of the Prandtl number is strong. The generated magnetic fields do not become weak in the polar regions because the magnetic field inside the tangent cylinder is always regenerated due to the weak magnetic diffusion.

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