Formation,history and energetics of cores in the terrestrial planets

The cores of the terrestrial planets Earth, Moon, Mercury, Venus and Mars differ substantially in size and in history. Though no planet other than the Earth has a conclusively demonstrated core, the probable cores in Mercury and Mars and Earth's core show a decrease in relative core size with solar distance. The Moon does not fit this sequence and Venus may not. Core formation must have been early (prior to ～4 · 10⁹ yr. ago) in the Earth, by virtue of the existence of ancient rock units and ancient paleomagnetism and from UPb partitioning arguments, and in Mercury, because the consequences of core infall would have included extensional tectonic features which are not observed even on Mercury's oldest terrain. If a small core exists in the Moon, still an open question, completion of core formation may be placed several hundred million years after the end of heavy bombardment on tectonic and thermal grounds. Core formation time on Mars is loosely constrained, but may have been substantially later than for the other terrestrial planets. The magnitude and extent of early heating to drive global differentiation appear to have decreased with distance from the sun for at least the smaller bodies Mercury, Moon and Mars.Energy sources to maintain a molten state and to fuel convection and magnetic dynamos in the cores of the terrestrial planets include principally gravitational energy, heat of fusion, and long-lived radioactivity. The gravitational energy of core infall is quantifiable and substantial for all bodies but the Moon, but was likely spent too early in the history of most planets to prove a significant residual heat source to drive a present dynamo. The energy from inner core freezing in the Earth and in Mercury is at best marginally able to match even the conductive heat loss along an outer core adiabat. Radioactive decay in the core offers an attractive but unproven energy source to maintain core convection.     

Gravitational energy of core evolution: implications for thermal history and geodynamo power

Using density–pressure relationships for mantle silicate and core alloy closely matching PREM we have constructed six models of the Earth in different evolutionary states. Gravitational energies and elastic strain energies are calculated for models with homogeneous composition, separated mantle and liquid core, separated inner and outer cores with the inner core either liquid or solid and models with increased densities, representing cooling of either the mantle or core. In this way we have isolated the gravitational energy released by each of several evolutionary processes and subtracted the consequent increase in strain energy to obtain the net energy released as heat or geodynamo power. Radiogenic heat (∼7.8×10³⁰ J) is found to contribute only about 25% of the total heat budget, the balance originating as residual gravitational energy from the original accretion and from core separation (14×10³⁰ J). The total energy of compositional convection, driven by inner core formation, is 3.68×10²⁸ J and this is the most important (or even the only) energy source for the dynamo for the most recent 2 billion years. It appears unlikely that the inner core existed much before that time. The total net (gravitational minus strain) energy released in the core by the process of inner core formation, 11.92×10²⁸ J, is not much less than the thermal energy released in this process, 15.1×10²⁸ J. In the mantle the net (gravitational minus strain) energy released by thermal contraction is about 20% of the heat release. All of the numerical results are presented in a manner that allows simple rescaling to any revised density estimates.     

Core cooling by subsolidus mantle convection

Although vigorous mantle convection early in the thermal history of the Earth is shown to be capable of removing several times the latent heat content of the core, we are able to construct a thermal evolution model of the Earth in which the core does not solidify. The large amount of energy removed from the model Earth's core by mantle convection is supplied by the internal energy of the core which is assumed to cool from an initial high temperature given by the silicate melting temperature at the core-mantle boundary. For the smaller terrestrial planets, the iron and silicate melting temperatures at the core-mantle boundaries are more comparable than for the Earth, and the cores of these planets may not possess enough internal energy to prevent core solidification by mantle convection. Our models incorporate temperature-dependent mantle viscosity and radiogenic heat sources in the mantle. The Earth models are constrained by the present surface heat flux and mantle viscosity. Internal heat sources produce only about 55% of the Earth model's present surface heat flow.     

Metal/silicate fractionation in the solar system

Fractionation between the metal and silicate components of objects in the inner solar system has long been recognized as a necessity in order to explain the observed density variations of the terrestrial planets and the H-group, L-group dichotomy of the ordinary chondrites. This paper discusses the densities of the terrestrial planets in light of current physical and chemical models of processes in the solar nebula. It is shown that the observed density trends in the inner solar system need not be the result of special fractionation processes, and that the densities of the planets may be direct results of simultaneous application of both physical and chemical restraints on the structure of the nebula, most notably the variation of temperature with heliocentric distance. The density of Mercury is easily attributed to accretion at temperatures so high that MgSiO₃ is only partially retained but Fe metal is condensed. The densities of the other terrestrial planets are shown to be due to different degrees of retention of S, O and H as FeS, FeO and hydrous silicates produced in chemical equilibrium between condensates and solar-composition gases. It is proposed that Mercury and Venus Have cores of Fe⁰, Earth has a core of Fe⁰ containing substantial amounts of FeS, and Mars has a quite small core of FeS with more FeO in its mantle than in Earth's. Geophysical and geochemical consequences of these conclusions are discussed.     

Gravitational heating of planets

Analytical estimates for three important and general planetary heating processes, excluding radioactive heating, are presented: accretional heating, adiabatic compression and core formation. The relative importance of these processes appears to be as follows. Accretional heating is important for almost all planets and satellites including asteroid-size bodies. Heating due to core formation becomes important for objects which are similar to, or larger than the terrestrial planets. Compressional heating is important only for the outer cores and the envelopes of the giant planets, provided that they are heated, before compression, up to about 1000 K.     

Concerning the occurrence of water in the planetary bodies

The study of the cosmic chemical abundance of the elements suggests that water (which is a combination of the first and second most abundant chemically active elements) is likely to be the most abundant chemical compound in the solar system.It is found that water indeed appears to be a common constituent of planetary bodies even though its presence is not always directly detectable. The amount involved, and the form it takes, varies from one object to another. The Earth has surface liquid water and crustal hydrate materials and only Mars of the terrestrial planets is also likely to have non-atmospheric water and that in frozen form near the surface. The mantles of the icy satellites, and particularly those of Jupiter and Saturn, are the most extended locations of water in the solar system although Uranus and Neptune are likely to have substantial mid-mantle internal water components. Only Mercury and Moon appear to be devoid of water. The smaller bodies such as comets are excluded from the discussion even though they are now known to be composed largely of water-ice.     

Thermal histories of the core and mantle

Recognition that the cooling of the core is accomplished by conduction of heat into a thermal boundary layer (D″) at the base of the mantle, partly decouples calculations of the thermal histories of the core and mantle. Both are controlled by the temperature-dependent rheology of the mantle, but in different ways. Thermal parameters of the Earth are more tightly constrained than hitherto by demanding that they satisfy both core and mantle histories. We require evolution from an early state, in which the temperatures of the top of the core and the base of the mantle were both very close to the mantle solidus, to the present state in which a temperature increment, estimated to be ～ 800 K, has developed across D″. The thermal history is not very dependent upon the assumption of Newtonian or non-Newtonian mantle rheology. The thermal boundary layer at the base of the mantle (i.e., D″) developed within the first few hundred million years and the temperature increment across it is still increasing slowly. In our preferred model the present temperature at the top of the core is 3800 K and the mantle temperature, extrapolated to the core boundary without the thermal boundary layer, is 3000 K. The mantle solidus is 3860 K. These temperatures could be varied within quite wide limits without seriously affecting our conclusions. Core gravitational energy release is found to have been remarkably constant at ～ 3 × 10¹¹ W. nearly 20% of the core heat flux, for the past 3 × 10⁹ y, although the total terrestrial heat flux has decreased by a factor of 2 or 3 in that time. This gravitational energy can power the “chemical” dynamo in spite of a core heat flux that is less than that required by conduction down an adiabatic gradient in the outer core; part of the gravitational energy is used to redistribute the excess heat back into the core, leaving 1.8 × 10¹¹ W to drive the dynamo. At no time was the dynamo thermally driven and the present radioactive heating in the core is negligibly small. The dynamo can persist indefinitely into the future; available power 10¹⁰ y from now is estimated to be 0.3 × 10¹¹ W if linear mantle rheology is assumed or more if mantle rheology is non-linear. The assumption that the gravitational constant decreases with time imposes an implausible rate of decrease in dynamo energy. With conventional thermodynamics it also requires radiogenic heating of the mantle considerably in excess of the likely content of radioactive elements.     

Origin and thermal evolution of icy satellites

The paper reviews the problem of formation and evolution of the so-called regular satellites of the giant planets, and it consists of two parts: the first describes the possible origin of the satellites, the second studies their evolution, attempting to stress the relations of the present status of the satellites with their evolutionary history.The formation of regular satellite systems around giant planets is probably related to the formation of the central planet. Some characteristics of regular satellite systems are quite similar, and suggest a common origin in a disk present around the central body. This disk can originate through different mechanisms which we will describe, paying attention to the so-called accretion disk model, in which the satellite-forming material is captured. The disk phase links the formation of the primary body with the formation of satellites. The subsequent stages of the disk's evolution can lead first to the formation of intermediate size bodies, and through the collisional evolution of these bodies, to the birth of satellite embryos able to gravitationally capture smaller bodies.Given the scenario in which icy satellites may be formed by homogeneous accretion of planetesimals made of a mixtures of ice and silicates, if no melting occurs during accretion, the satellites have a homogeneous ice-rock composition. For the smaller satellites this homogeneous structure should not be substantially modified; only sporadic local events, such as large impacts, can modify the surface structure of the smaller satellites. For the larger satellites, if some degree of melting appears during accretion, a differentiation of the silicate part occurs, the amount of differentiation and hence the core size depending on the fraction of gravitational potential energy retained during the accumulation process. Melting and differentiation soon after the accretion, for the larger satellites, could also depend on the convective evolution in presence of phase transitions and generate an intermediate rock layer, considerably denser than the underlying, still homogeneous core, and unstable to overturning on a geologic time scale. Moreover the liquid water mantle could be a transient feature because the mantle would freeze over several hundred million years. For these large bodies the stable configuration is expected to be one consisting of a silicate core and a mantle of mixed rock and ice.     

火星和月球热历史的参量化模型研究

通过类地行星热历史的比较研究,可以更全面地了解它们的热演化过程．火星和月球不具有板块构造,研究它们的热演化过程时,考虑了岩石层逐渐加厚对行星内部对流的影响,同时也考虑了由对流传热转变为传导传热对它们热历史的影响．参量化模型计算结果表明：火星和月球岩石层随温度的逐渐降低目前大约分别增厚到320km和250km左右;并且,火星幔和月幔分别于1．6Ga前和3Ga前停止热对流,这与天文和空间探测资料一致．     

Stable calcium isotopic composition of meteorites and rocky planets

New measurements of mass-dependent calcium isotope effects in meteorites, lunar and terrestrial samples show that Earth, Moon, Mars, and differentiated asteroids (e.g., 4-Vesta and the angrite and aubrite parent bodies) are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution (± 0.07‰ SD for the ⁴⁴Ca/⁴⁰Ca ratio). In contrast, enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes (ca. + 0.5‰) and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes (ca. ? 0.5‰). The calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes. The isotopic variations probably survive from the earliest stages of nebular condensation, and indicate that condensation occurred under non-equilibrium (undercooled nebular gas) conditions. Some of this early high-temperature calcium isotope heterogeneity is recorded by refractory inclusions (Niederer and Papanastassiou, 1984) and survived in planetesimals, but virtually none of it survived through terrestrial planet accretion. The new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets; enstatite and carbonaceous chondrites are not. The enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction.     

Cosmoradiogenic ghosts and the origin of Ca-Al-rich inclusions

The Ca-Al-rich inclusions within Allende are described as quickly frozen non-equilibrated partial melts arising from energetic collisions between centimeter-sized mechanical accumulations of cold presolar grains. The resulting minerals are refractory-rich because refractory supernova condensates are the most persistent components of the preheated accumulates. The shock heating drives off most of the more volatile matrix that had accumulated cold around the refractory cores, which quickly recrystallize while picking up isotopically homogenized trace elements. This picture is advanced to account for the isotopic anomalies in those elements for which fractionation of stardust from gas also fractionates a special isotope whose stellar condensation history can be expected to have been special.I call the anomaly that would have existed before the special component was added anisotopic ghost. These ghosts can be larger than the special anomaly surviving today in meteorites and planets. I argue that ghosts in²⁶Mg/²⁴Mg,⁸⁷Sr/⁸⁶Sr, and^206,207Pb/²⁰⁴Pb have caused erroneous cosmoradiogenic estimates of large age differences between meteorites, their special phases, and even the Moon.     

On the possible existence of superheavy elements in the primeval Moon

The remarkably complete and well-dated record of the primeval history of the Moon poses two problems concerned with early heat sources. The early melting and differentiation, especially the formation of a lunar core, requires a source with a half-life of a few times 10⁸ years. Convection and magnetic field generation in the iron core requires a heat source soluble in iron with a half-life of the order of 10⁸ years. It is shown that both requirements are quantitatively met by supposing that superheavy elements existed in the early Moon and the relationship of this result with the search for their existence today is discussed.     

Back to the Moon?

Ian A Crawford makes the case for a return to the Moon, where an archive of information from the early history of the terrestrial planets demands the attention of human observers and explorers on the spot.     

A geochemical record of environmental changes in sediments from Sishili Bay,northern Yellow Sea,China: Anthropogenic influence on organic matter sources and composition over the last 100 years

Total organic carbon (TOC), total nitrogen (TN), δ¹³C and δ¹⁵N were measured in sediment cores at three sites in Sishili Bay, China, to track the impacts of anthropogenic activities on the coastal environment over the last 100 years. The increased TOC and TN in the upper section of sediment cores indicated a eutrophic process since 1975. In comparison, the TOC and TN in the sediment core near to a scallop aquaculture area displayed a much slower increase, indicating the contribution of scallop aquaculture in mitigating eutrophication. Combined information from δ¹³C, δ¹⁵N and TOC:TN indicated an increased terrestrial signal, although organic matter sources in Sishili Bay featured a mixture of terrestrial and marine sources, with phytoplankton being dominant. Increased fertilizer use since 1970s contributed to the eutrophic process in Sishili Bay since 1975, and increased sewage discharge from 1990s has added to this process.     

Planetary seismology

Since 1969, seismology has been extended beyond the Earth, and seismic sensors have been placed on the surface of other bodies of the solar system. A Lunar seismic network thus operated for the 8 years after 1969, with up to 4 stations, and detected some 1000 Moonquakes per year. A single seismic station was also operated on the Martian surface for 19 months since 1977. Unfortunately, it did not detect any Marsquakes, but produced useful information for future experiments. Remotesensing seismic experiments using Doppler shift observation have also been applied to Jupiter in the last two years and are beginning to return information on the normal modes. Planetary seismology is thus now well developed, and will provide increasing information on the structure and dynamics of the planets and bodies of the solar system. In this paper we review the state of the art in planetary seismology. For the terrestrial planets, we compare the seismic sources, structure and experiments on Earth, Moon and Mars. Such a comparison is useful in evaluating the design of past or future experiments. Results in the seismology of giant planets are also reviewed, stressing the connection between methods and theory.     

The age of ferroan anorthosite 60025: oldest crust on a young Moon?

SmNd isotopic data for mineral separates from the ferroan anorthosite 60025 define a precise isochron of 4.44 ± 0.02Ga age. This age is roughly 110 m.y. younger than the formation of the first large solid objects in the solar nebula, as recorded by the radiometric ages of the differentiated meteorites. In the magma ocean model for early lunar differentiation, ferroan anorthosites are the first crustal rocks to form on the Moon. If the Moon is as old as the oldest meteorites, the relatively young age determined for 60025 implies either that the magma ocean did not form synchronously with lunar formation, or that the magma ocean required over 100 m.y. before reaching the stage of ferroan anorthosite crystallization. Alternatively, we propose that the accumulated body of radiogenic isotope data for lunar rocks permit the Moon to be as young as 4.44–4.51 Ga. If so, isotopic evidence for chemical differentiation on the Earth at about this same time suggests that the formation of the Moon is reflected in the chemical evolution of the Earth. This, in turn, is consistent with the idea that the materials that now make up the Moon were derived from the Earth, perhaps ejected by collision between the Earth and another very large planetesimal during the final stages of accumulation of the terrestrial planets. Terrestrial origin models for the Moon lessen the requirement that the Earth and Moon each have near chondritic relative abundances of the refractory elements and could require that certain chemical and isotopic characteristics of both bodies be considered in the framework of the chemical mass-balance of the combined Earth-Moon system.     

The historical acceleration of the earth

In this paper, we review the current state of knowledge about the acceleration of the Earth's spin, and about the closely related acceleration of the Moon. It is now established at a high confidence level that the acceleration of the Moon, when taken respect to Universal time, has changed by a large amount, and that it has even changed sign, within historic times. This almost certainly means that the acceleration of the Earth's spin has also changed by a large amount. At present we do not have enough information to say whether the changes have been in the contributions from tidal friction, in the contributions that do not arise from tidal friction, or both. Further, we do not know yet whether or not the variations in the Earth's rotation can account for the observed fluctuations in the longitudes of the Sun, the Moon, and the planets.     

Moments of the lunar disturbing force

Summary The components of the moments of the external force due to the gravitational effect of the Moon are derived, which causes disturbances in the motion of the Earth round its mass centre, taking into account the gravitational fields of both bodies in the form of a development in terms of harmonics upto degree n=4. This paper ties up with [3–5] and the notations are identical.     

Inversion of seismic and gravity data for the composition and core sizes of the Moon

We model the internal structure of the Moon, initially homogeneous and later differentiated due to partial melting. The chemical composition and the internal structure of the Moon are retrieved by the Monte-Carlo inversion of the gravity (the mass and the moment of inertia), seismic (compressional and shear velocities), and petrological (balance equations) data. For the computation of phase equilibrium relations and physical properties, we have used a method of minimization of the Gibbs free energy combined with a Mie-Gr@uneisen equation of state within the CaO-FeO-MgO-Al₂O₃-SiO₂ system. The lunar models with a different degree of constraints on the solution are considered. For all models, the geophysically and geochemically permissible ranges of seismic velocities and concentrations in three mantle zones and the sizes of Fe-10%S core are estimated. The lunar mantle is chemically stratified; different mantle zones, where orthopyroxene is the dominant phase, have different concentrations of FeO, Al₂O₃, and CaO. The silicate portion of the Moon (crust + mantle) may contain 3.5–5.5% Al₂O₃ and 10.5–12.5% FeO. The chemical boundary between the middle and the lower mantle lies at a depth of 620–750 km. The lunar models with and without a chemical boundary at a depth of 250–300 km are both possible. The main parameters of the crust, the mantle, and the core of the Moon are estimated. At the depths of the lower mantle, the P and S velocities range from 7.88 to 8.10 km/s and from 4.40 to 4.55 km/s, respectively. The radius of a Fe-10%S core is 340 ± 30 km.     

Core formation and chemical equilibrium in the Earth—I. Physical considerations

Current models of planetary formation suggest a hierarchy in the size of planetesimals from which planets were formed, causing formation of a hot magma ocean through which metal-silicate separation (core formation) may have occurred. We analyze chemical equilibrium during metal-silicate separation and show that the size of iron as well as the thermodynamic conditions of equilibrium plays a key role in determining the chemistry of the mantle (silicates) and core (iron) after core formation. A fluid dynamical analysis shows that the hydrodynamically stable size of iron droplets is less than 10⁻² m for which both chemical and thermal equilibrium should have been established during the separation from the surrounding silicate magma. However, iron may have been separated from silicates as larger bodies when accumulation of iron on rheological boundaries and resultant large scale gravitational instability occurred or when the core of colliding planetesimals directly plunged into the pre-existing core. In these cases, iron to form the core will be chemically in dis-equilibrium with surrounding silicates during separation. The relative role of equilibrium and dis-equilibrium separation has been examined taking into account of the effects of rheological structure of a growing earth that contains a completely molten near surface layer followed by a partially molten deep magma ocean and finally a solid innermost proto-nucleus. We show that the separation of iron through a completely molten magma ocean likely occurred with iron droplets assuming a hydrodynamically stable size ( 10⁻² m) at chemical equilibrium, but the sinking iron droplets are likely to have been accumulated on top of the partially molten layer to form a layer (or a lake) of molten iron which sank to deeper portions as a larger droplet. The degree of chemical equilibrium during this process is determined by the size of droplets which is in turn controlled by the size and frequency of accreting planetesimals and the rheological properties of silicate matrix. For a plausible range of parameters, most of the iron that formed the core is likely to have been separated as large droplets or bodies and chemical equilibrium with silicate occurred only at relatively low temperatures and pressures in a shallow magma ocean or in their parental bodies. However, a small portion of iron that separated as small droplets was in chemical equilibrium with silicate at high temperatures and pressures in a deep magma ocean during the later stage of core formation. Therefore the chemistry of the core is mostly controlled by the chemical equilibrium with silicates at relatively low temperatures and pressures, whereas the chemistry of the mantle controlled by the interaction with iron during core formation is likely to have been determined mostly by the chemical equilibrium with a small amount of iron at high temperatures and pressures.