水星构造特征及演化

水星是离太阳最近的类地行星,它有着类似月球的外表和类似地球的内部,其重要的构造特征主要表现在以下方面:广泛分布的撞击坑;全球线性构造(格子构造)体系;叶片状悬崖;与Caloris盆地相关的构造;局部的拉张构造,其中叶片状悬崖是仅存在于水星的独特构造.类地行星(除地球以外)的构造形迹主要形成于星球历史的早、中期,同时小行星体的构造演化通常被认为是行星热演化以及外部作用(如强烈撞击或者潮汐)共同作用的结果.     

制约类地行星大气层和水体发育的主要因素

根据行星探测的资料,综合分析了水星、金星、地球(包括月球)、火星的大气层和水体的发育特征,对比了金星、火星的大气层与水体同地球的差异。类地行星质量小、体积小、密度大、旋转慢、卫星少甚至没有、挥发性元素较类木行星少、距离太阳较近,早期残留的原始大气层已经被早期太阳在金牛变星阶段的强烈太阳风所驱赶,加上巨大而频繁的撞击作用,使原始大气层被驱赶殆尽。现在的大气层是次生的,是由行星内部的去气作用形成的。类地行星的大气层、水体的发育和表生作用的特征与行星的质量大小(表征行星内部能量的大小和构造活动的强烈及持续时间)及行星与太阳的距离等因素有关。在类地行星中,地球和金星质量最大,逃逸速度最大,可将更多的气体“束缚”在它们表面,因此它们的大气有着复杂的组成和较大的密度。火星质量较小,逃逸速度不到地球的一半,在漫长的演化历史中,大气逐渐逸散进入太空,大气密度变得很稀薄。水星质量更小,而且最靠近太阳,不仅太阳风的驱赶作用强烈,而且表面温度高,气体分子的热运动更加剧烈,加剧了大气的逸散,所以水星的大气层极为稀薄,并且主要为太阳风成分。月球质量最小,几乎没有大气层,更没有水体的发育。行星的热演化历史对大气层和水体发育具有重要的制     

The split fate of the early Earth,Mars, Venus,and Moon

Plate tectonics shaped the Earth, whereas the Moon is a dry and inactive desert, Mars probably came to rest within the first billion years of its history, and Venus, although internally very active, has a dry inferno for its surface. Here we review the parameters that determined the fates of each of these planets and their geochemical expressions. The strong gravity field of a large planet allows for an enormous amount of gravitational energy to be released, causing the outer part of the planetary body to melt (magma ocean), helps retain water on the planet, and increases the pressure gradient. The weak gravity field and anhydrous conditions prevailing on the Moon stabilized, on top of its magma ocean, a thick buoyant plagioclase lithosphere, which insulated the molten interior. On Earth, the buoyant hydrous phases (serpentines) produced by reactions between the terrestrial magma ocean and the wet impactors received from the outer solar system isolated the magma and kept it molten for some few tens of million years. The planets from the inner solar system accreted dry: foundering of wet surface material softened the terrestrial mantle and set the scene for the onset of plate tectonics. This very same process also may have removed all the water from the surface of Venus and added enough water to its mantle to make its internal dynamics very strong and keep the surface very young. Because of a radius smaller than that of the Earth, not enough water could be drawn into the Martian mantle before it was lost to space and Martian plate tectonics never began. The radius of a planet is therefore the key parameter controlling most of its evolutional features.     

太阳星云凝聚过程的岩石学模型：（1）球粒陨石的凝聚成因

建立类地行星区太阳星云凝聚过程的岩石学模型，对于合理解释陨石、地球和类地行星的成因关系，探讨地球起源和估算地球的整体成分都有着重要意义。本文中根据天体化学和太阳系演化学说关于太阳星云物理化学条件的基本分析，以及实验凝聚岩石学的研究结果，推断在太阳星云盘的类地行星区中可能有星云的气－固和气－液－固两种凝聚作用发生。通过对球粒陨石中球粒和基质矿物成分及结构构造特征的对比，论证了绝大多数球粒的气－液－固凝聚成因和基质的气－固凝聚成因，并讨论了球粒陨石各化学群的凝聚成因模式。     

The early history of the earth—moon system

The study of the Earth—Moon system provides the connecting link between purely astronomical studies of the origin of the solar system and its planets, and geophysical and biological studies of the evolution of the Earth's geology, its surface features, atmosphere and hydrosphere, and of terrestrial life.A coherent account is presented here, based on the hypothesis that the Moon formed separately and was later captured by the Earth. The adoption of this hypothesis, together with the observed depletion of iron in the Moon, sets some important constraints on the development of condensation and agglomeration phenomena in the primeval solar nebula, which led to the formation of planetesimals, and ultimately to planets.Capture of the Moon also defines a severe heating event within the Earth, whereby its kinetic energy of rotation is largely dissipated internally by the mechanism of tidal friction. From this melting event dates the geologic, atmospheric, and oceanic history of the Earth. An attempt is made to account for the unique development of the Earth, especially in relation to Mars and Venus, its neighboring planets.     

The bulk composition of Mars

An accurate assessment of the bulk chemical composition of Mars is fundamental to understanding planetary accretion, differentiation, mantle evolution, the nature of the igneous parent rocks that were altered to produce sediments on Mars, and the initial concentrations of volatiles such as H, Cl and S, important constituents of the Martian surface. This paper reviews the three main approaches that have been used to estimate the bulk chemical composition of Mars: geochemical/cosmochemical, isotopic, and geophysical. The standard model is one developed by Wänke and Dreibus in a series of papers, which is based on compositions of Martian meteorites. Since their groundbreaking work, substantial amounts of data have become available to allow a reassessment of the composition of Mars from elemental data, including tests of the basic assumptions in the geochemical models. The results adjust some of the concentrations in the Wänke–Dreibus model, but in general confirm its accuracy. Bulk silicate Mars has roughly uniform depletion of moderately volatile elements such as K (0.6 × CI), and strong depletion of highly volatile elements (e.g., Tl). The highly volatile elements are within uncertainties uniformly depleted at about 0.06 CI abundances. The highly volatile chalcophile elements are likewise roughly uniformly depleted, but with more scatter, with normalized abundances of 0.03 CI. Bulk planetary H₂O is much higher than estimated previously: it appears to be slightly less than in Earth, but D/H is similar in Earth and Mars, indicating a common source of water-bearing material in the inner solar system. K/Th ranges from ∼3000 to ∼5000 among the terrestrial planets, a small range compared to CI chondrites (19,000). FeO varies throughout the inner solar system: ∼3 wt% in Mercury, 8 wt% in Earth and Venus, and 18 wt% in Mars. These differences can be produced by varying oxidation conditions, hence do not suggest the terrestrial planets were formed from fundamentally different materials. The broad chemical similarities among the terrestrial planets indicate substantial mixing throughout the inner solar system during planet formation, as suggested by dynamical models.     

Similar-sized collisions and the diversity of planets

It is assumed in models of terrestrial planet formation that colliding bodies simply merge. From this the dynamical and chemical properties (and habitability) of finished planets have been computed, and our own and other planetary systems compared to the results of these calculations. But efficient mergers may be exceptions to the rule, for the similar-sized collisions (SSCs) that dominate terrestrial planet formation, simply because moderately off-axis SSCs are grazing; their centers of mass overshoot. In a “hit and run” collision the smaller body narrowly avoids accretion and is profoundly deformed and altered by gravitational and mechanical torques, shears, tides, and impact shocks. Consequences to the larger body are minor in inverse proportion to its relative mass. Over the possible impact angles, hit-and-run is the most common outcome for impact velocities v_imp between $～1.2$ and 2.7 times the mutual escape velocity v_esc between similar-sized planets. Slower collisions are usually accretionary, and faster SSCs are erosive or disruptive, and thus the prevalence of hit-and-run is sensitive to the velocity regime during epochs of accretion. Consequences of hit-and-run are diverse. If barely grazing, the target strips much of the exterior from the impactor—any atmosphere and ocean, much of the crust—and unloads its deep interior from hydrostatic pressure for about an hour. If closer to head-on $(～30\text{–}45°)$ a hit-and-run can cause the impactor core to plow through the target mantle, graze the target core, and emerge as a chain of diverse new planetoids on escaping trajectories. A hypothesis is developed for the diversity of next-largest bodies (NLBs) in an accreting planetary system—the bodies from which asteroids and meteorites derive. Because nearly all the NLBs eventually get accreted by the largest (Venus and Earth in our terrestrial system) or by the Sun, or otherwise lost, those we see today have survived the attrition of merger, evolving with each close call towards denser and volatile-poor bulk composition. This hypothesis would explain the observed density diversity of differentiated asteroids, and of dwarf planets beyond Neptune, in terms of episodic global-scale losses of rock or ice mantles, respectively. In an event similar to the Moon-forming giant impact, Mercury might have lost its original crust and upper mantle when it emerged from a modest velocity hit and run collision with a larger embryo or planet. In systems with super-Earths, profound diversity and diminished habitability is predicted among the unaccreted Earth-mass planets, as many of these will have be stripped of their atmospheres, oceans and crusts.     

太阳系探测的进展与比较行星学的主要科学问题

回顾了太阳系的探测历程,综合分析了太阳系探测的发展趋势。未来的太阳系探测将以月球与火星探测为主线,适度开展太阳系其他行星及其卫星、小行星和彗星的考察性探测。21世纪将是全面探测太阳系并为人类社会长期可持续发展服务的新时代。随着太阳系探测的进展,通过系统比较地球与类地行星的大气层与水体的形成演化过程、地形地貌与地质构造特征、岩石类型、热历史与内部结构等方面的共性与特性研究,表明行星的质量大小和行星与太阳的距离的相互耦合,制约了行星的形成和演化的复杂过程。比较行星学已成为指导太阳系探测的科学理论体系。     

碰撞作用与类地行星起源探讨

类地行星挥发性元素普遍亏损很可能是由于太阳星云早期剧烈的太阳活动引起的。当气体、尘粒、挥发性元素和水被驱赶出内太阳系时，只有米级到公里级的物质保存下来并堆积成星子，最终吸积星子形成类地行星。我们认为类地行星的初始物质主要是已分异的星子和一些未分异的球粒陨石质星子或不同类型的陨石母体，最靠近太阳形成的星子具有最低的ＦｅＯ／（ＦｅＯ＋ＭｇＯ）值，水星是在靠近太阳的高度还原条件下吸积成分类似ＥＨ球粒陨石的星子形成的。地球的初始物质为分异的铁陨石及Ｈ群球粒陨石。随着距太阳距离增大及温度降低，陨石形成的部位大致为：ＥＨ、ＥＬ－ＩＡＢ－ＳＮＣ（辉玻无球粒陨石、辉橄无球粒陨石、纯橄无球粒陨石）－Ｅｕｃ（钙长辉长无球粒陨石）－Ｈ、Ｌ、ＬＬ－ＣＶ、ＣＭ、ＣＯ－Ｃｌ－彗星。物体之间、星子之间及行星与星子之间的碰撞对太阳系的形成和演化起着重要的作用。     

Planet formation processes revealed by meteorites

The history of the solar system is locked within the planets, asteroids and other objects that orbit the Sun. While remote observations of these celestial bodies are essential for understanding planetary processes, much of the geological and geochemical information regarding solar system heritage comes directly from the study of rocks and other materials originating from them. The diversity of materials available for study from planetary bodies largely comes from meteorites; fragments of rock that fall through Earth's atmosphere after impact‐extraction from their parent planet or asteroid. These extra‐terrestrial objects are fundamental scientific materials, providing information on past conditions within planets, and on their surfaces, and revealing the timing of key events that affected a planet's evolution. Meteorites can be sub‐divided into four main groups: (1) chondrites, which are unmelted and variably metamorphosed ‘cosmic sediments’ composed of particles that made up the early solar nebula; (2) achondrites, which represent predominantly silicate materials from asteroids and planets that have partially to fully melted, from a broadly chondritic initial composition; (3) iron meteorites, which represent Fe‐Ni samples from the cores of asteroids and planetesimals; and (4) stony‐iron meteorites such as pallasites and mesosiderites, which are mixtures of metal and dominantly basaltic materials. Meteorite studies are rapidly expanding our understanding of how the solar system formed and when and how key events such as planetary accretion and differentiation occurred. Together with a burgeoning collection of classified meteorites, these scientific advances herald an unprecedented period of further scientific challenges and discoveries, an exciting prospect for understanding our origins.     

太阳系演化中的地球和月球起源(英文)

宇宙中恒星的演化始于巨星的形成 ,后者的质量是太阳系的数百倍 ,寿命估计为数百万年。重元素合成于巨星的内部。它们控制了巨星爆炸过程中 (超新星 )形成的气态云和盘状物的冷凝加速度。冷凝和旋转的加速导致后代恒星质量越来越小 ,寿命越来越长 ,直到形成像太阳这样的小星体 ,其质量为 1.989× 10 30 kg ,寿命已有几十亿年。这些小恒星的形成是冷凝过程中产生的水成冰氢星子不断聚集的结果。上一代巨星的原始星盘中的物质只有一小部分参与了冰氢星子的形成。这些星体形成于致密、高速旋转的原始恒星星盘中 ,周围环绕着巨行星和褐矮星。由于星体达到恒星状态 ,它们开始影响原恒星盘 ,结果导致星体相互分散 ,同时 ,最近的巨星发生表面去气作用。后者可以从巨星到恒星的质量衰减得到证实。UpsilonAndromedae、5 5Cancri和HD16 84 4 3等天体的巨行星记载了这样的事实。太阳系中的表面去气作用主要反映在近太阳巨星的流体外壳完全消失。由于流体外壳消失 ,铁硅酸盐熔融核暴露地表 ,形成小的类地行星。木星也经历过表面去气作用 ,依据是木星具有很高的平均密度 (1.3g cm3) ,几乎是土星密度 (0 .7g cm3)的两倍。因此 ,类地行星的形成经历了两个阶段 :原行星 (其父巨星具有重的熔融核 )和正常行星 (在其父行星     

类地行星热物理条件与热演化影响因素分析——以火星地幔热柱演化为例

太阳系内类地行星具有相似的岩石层包围金属核的圈层结构,在行星幔的热演化历史起源方面具有同时性和同源性,并且都在早期变形重力位能加热的基础上随放射性热能衰减而冷却。但是,由于半径、密度、粘度以及表层构造属性等物理条件的差异,其热演化历史各具特色。依据基本的热对流和热传导方程,我们计算分析了类地行星热物理条件差异对行星幔热演化历史的影响。计算表明,类地行星热演化的早期,行星幔热对流是主要的散热方式。半径较大的行星表面热流密度大,平均散热量也大。半径较小的行星内部温差小,粘滞系数高,对流能力低,提早进入传导散热状态,且传导散热的岩石层也比大行星厚。不同边界层热物理条件下,类地行星幔热演化历史会分别出现逐渐冷却的平稳式、包含热柱上涌的波动式、行星幔幕次翻转的周期式等特点不同的热演化过程。火星内部曾经存在的地幔热柱构造与火星地幔热动力学演化过程密切相关。我们从火星地幔热动力学演化模型出发,定量计算与地幔热柱构造演化相关的地幔热动力学演化特征,通过三维球壳数值模拟,研究了火星地幔热演化历史上可能存在的热柱活动造成的火星热演化历史的非单调变化,火星地幔对流环结构随时间的演变方式,以及与边界相关的地幔热柱对火星地形的影响。     

On the evolution of terrestrial planets:Bi-stability,stochastic effects,and the non-uniqueness of tectonic states

The Earth is the only body in the solar system for which significant observational constraints are accessible to such a degree that they can be used to discriminate between competing models of Earth's tectonic evolution.It is a natural tendency to use observations of the Earth to inform more general models of planetary evolution.However,our understating of Earth's evolution is far from complete.In recent years,there has been growing geodynamic and geochemical evidence that suggests that plate tectonics may not have operated on the early Earth,with both the timing of its onset and the length of its activity far from certain.Recently,the potential of tectonic bi-stability(multiple stable,energetically allowed solutions)has been shown to be dynamically viable,both from analytical analysis and through numeric experiments in two and three dimensions.This indicates that multiple tectonic modes may operate on a single planetary body at different times within its temporal evolution.It also allows for the potential that feedback mechanisms between the internal dynamics and surface processes(e.g.,surface temperature changes driven by long term climate evolution),acting at different thermal evolution times,can cause terrestrial worlds to alternate between multiple tectonic states over giga-year timescales.The implication within this framework is that terrestrial planets have the potential to migrate through tectonic regimes at similar‘thermal evolution times'(e.g.,points were they have a similar bulk mantle temperature and energies),but at very different'temporal times'(time since planetary formation).It can be further shown that identical planets at similar stages of their evolution may exhibit different tectonic regimes due to random variations.Here,we will discuss constraints on the tectonic evolution of the Earth and present a novel framework of planetary evolution that moves toward probabilistic arguments based on general physical principals,as opposed to particular rheologies,and incorporates the potential of tectonic regime transitions and multiple tectonics states being viable at equivalent physical and chemical conditions.     

Genetic relations between meteorites and terrestrial and lunar rocks

According to their genesis, meteorites are classified into heliocentric (which originate from the asteroid belt) and planetocentric (which are fragments of the satellites of giant planets, including the Proto-Earth). Heliocentric meteorites (chondrites and primitive meteorites genetically related to them) used in this study as a characteristic of initial phases of the origin of the terrestrial planets. Synthesis of information on planetocentric meteorites (achondrites and iron meteorites) provides the basis for a model for the genesis of the satellites of giant planets and the Moon. The origin and primary layering of the Earth was initially analogously to that of planets of the HH chondritic type, as follows from similarities between the Earth’s primary crust and mantle and the chondrules of Fe-richest chondrites. The development of the Earth’s mantle and crust precluded its explosive breakup during the transition from its protoplanetary to planetary evolutionary stage, whereas chondritic planets underwent explosive breakup into asteroids. Lunar silicate rocks are poorer in Fe than achondrites, and this is explained in the model for the genesis of the Moon by the separation of a small metallic core, which sometime (at 3–4 Ga) induced the planet’s magnetic field. Iron from this core was involved into the generation of lunar depressions (lunar maria) filled with Fe- and Ti-rich rocks. In contrast to the parent planets of achondrites, the Moon has a olivine mantle, and this fact predetermined the isotopically heavier oxygen isotopic composition of lunar rocks. This effect also predetermined the specifics of the Earth’s rocks, whose oxygen became systematically isotopically heavier from the Precambrian to Paleozoic and Mesozoic in the course of olivinization of the peridotite mantle, a processes that formed the so-called roots of continents.     

Primitive Earth environments: organic syntheses and the origin and early evolution of life

The Precambrian is the longest and least understood period in the history of our planet. There is no direct evidence which bears on the primitive Earth environment, the synthesis of organic compounds and the origin of life on our planet, since the geological record only goes back to 3.8 Ga ago. To overcome this deficiency it is necessary to extrapolate backwards from 3.8 Ga ago to the time when the Earth was formed, 4.6 Ga ago, and also make use of additional scientific approaches. These include models of the solar nebula and planet formation, comparative observations with other cosmic environments, and experimental simulation studies under plausible primitive Earth conditions, particularly in regard to the prebiological synthesis of organic compounds. A large number of interstellar organic molecules have been detected by radioastronomy in the interstellar medium, particularly in places where other solar systems may be in the process of formation. It is remarkable that 10 of these molecules are known to be the precursors of the most important biochemical monomers. Furthermore, some of these monomers, e.g., amino acids, purines and pyrimidines, have also been detected in carbonaceous chondrites, which were presumably synthesized on the parent bodies of the meteorites or possibly in the solar nebula. Comparative studies on the terrestrial planets indicate that their primitive atmospheres resulted primarily from outgassing of CO₂ and other partially oxidized (CO, N₂) and reduced (H₂, CH₄, NH₃, H₂S) volatiles. Furthermore, comets and meteorites are thought to have made a significant contribution to the reduced carbon precursors for the prebiological synthesis of biochemical compounds. A brief review of the experiments carried out in the laboratory under possible primitive Earth conditions shows that amino acids, purines, pyrimidines, monosaccharides, fatty acids and other compounds are readily obtained using different energy sources. However, the formation of oligopeptides, oligonucleotides and phospholipids requires more restrictive conditions. They have been synthesized by means of condensing agents, such as cyanamide, as well as by heating under relatively dry conditions of less than 100°C. Experiments have been carried out on the formation of liposomes from prebiotically synthesized phospholipids. Also, using 2-methylimidazolide derivatives of nucleotides, the template directed synthesis of polynucleotides has been demonstrated. In the latter experiments a high fidelity of complementary base pairing has been observed, which if further improved would approach that of enzyme catalyzed replication. However, it is not known how the prebiologically synthesized polymers became organized into a self-replicating system. At any rate the first living systems were probably very rudimentary heterotrophic micro-organisms which used organic compounds from the environment for both cell components and energy. They were probably followed by fermenters and eventually by autotrophic CO₂ reducing micro-organisms similar to the methanogenic and photosynthetic bacteria.     

Differentiation of crusts and cores of the terrestrial planets: Lessons for the early Earth?

It now appears probable that all of the terrestrial planets underwent some form of global chemical differentiation to produce crusts, mantles, and cores of variable relative mass fractions. There is direct seismic evidence for a crust on the Moon, and indirect evidence for distinct crusts on Mars and Venus. Substantial portions of these crusts have been in place since the time that heavy bombardment of the inner solar system ceased 4 Ga ago. There is direct evidence for a sizeable core on Mars, indirect evidence for one on Mercury, and bounds on a possible small core for the Moon. Core formation is an important heat source confined to times prior to 4 Ga ago for Mercury and the Earth, but was not closely linked to crustal formation on the Moon nor, apparently, on Mars. The tectonic and volcanic histories of the surfaces of the terrestrial planets Moon, Mars, and Mercury can be used, with simple thermal history models, to restrict the earliest chemical differentiation to be shallow (outer 200–400 km) for the first two bodies and much more extensive for Mercury. Extension of these models to an Earth-size planet leads to the prediction of a hot and vigorously convecting mantle with an easily deformable crust immediately following core formation, and of the gradual development of a lithosphere and of plates with some lateral rigidity in Late Archean—Proterozoic times.     

Styles of mantle convection and their influence on planetary evolution

Mantle convection is the method of heat elimination for silicate mantles in terrestrial bodies, provided they are not too small or too hot. Bodies that are small (～Moon or smaller, possibly even Mercury) may rely largely on conduction or melt migration, and bodies that are very hot (Io, very early Earth) may use massive melt migration (magma oceans) and heat pipes. In the standard, simple picture, we can use scaling laws to determine the secular cooling of a planet, likelihood and form of volcanism, and the possibility of a core dynamo. Contrary to popular belief, small planets do not cool faster than larger planets (provided they convect) but they do tend to have a slightly lower internal temperature at all times and thus may cease to be volcanically active at an earlier epoch. On the other hand, a larger volume fraction of a small planet may be involved in melt generation. However, our understanding of heat transfer by mantle convection is limited by three very important, largely unsolved problems: The complexities of rheology, the effects of compositional gradients, and the effects of phase transitions, especially melting. The most striking manifestation of the role of rheology lies in the difference between a mobile lid mode (plate tectonics for Earth) and a stagnant lid mode (other large terrestrial bodies). This difference may arise because of the role of water, but perhaps also because of melting, or size (gravity), or the vagaries of history. It has profound effects for the differences in history of Earth, Venus and Mars, including their surface geology, volatile reservoirs and magnetic fields. Since thermal convection is driven by small density differences, it can also be greatly altered or limited by compositional or phase effects. Melt migration introduces additional complications to the heat transport as well as being a source for the irreversible differentiation that might promote layering. Our limited understanding and ability to model these processes continues to limit the development of a predictive framework for the differences among the terrestrial planets.     

http://dx.doi.org/10.1016/j.gsf.2016.06.011

We present a new united theory of planet formation,which includes magneto-rotational instability(MRl) and porous aggregation of solid particles in a consistent way.We show that the "tandem planet formation" regime is likely to result in solar system-like planetary systems.In the tandem planet formation regime,planetesimals form at two distinct sites:the outer and inner edges of the MRl suppressed region.The former is likely to be the source of the outer gas giants,and the latter is the source for the inner volatile-free rocky planets.Our study spans disks with a various range of accretion rates,and we find that tandem planet formation can occur for M = 10~(7.3)- 10~(-6.9)M_⊙yr~(-1).The rocky planets form between 0.4-2 AU,while the icy planets form between 6-30 All;no planets form in 2—6 AU region for any accretion rate.This is consistent with the gap in the solid component distribution in the solar system,which has only a relatively small Mars and a very small amount of material in the main asteroid belt from 2-6 AU.The tandem regime is consistent with the idea that the Earth was initially formed as a completely volatile-free planet.Water and other volatile elements came later through the accretion of icy material by occasional inward scattering from the outer regions.Reactions between reductive minerals,such as schreibersite(Fe-jP),and water are essential to supply energy and nutrients for primitive life on Earth.     

New W-isotope evidence for rapid terrestrial accretion and very early core formation

The short-lived ¹⁸²Hf-¹⁸²W-isotope system is an ideal clock to trace core formation and accretion processes of planets. Planetary accretion and metal/silicate fractionation chronologies are calculated relative to the chondritic ¹⁸²Hf-¹⁸²W-isotope evolution. Here, we report new high-precision W-isotope data for the carbonaceous chondrite Allende that are much less radiogenic than previously reported and are in good agreement with published internal Hf-W chronometry of enstatite chondrites. If the W-isotope composition of terrestrial rocks, representing the bulk silicate Earth, is homogeneous and 2.24 ε_182W units more radiogenic than that of the bulk Earth, metal/silicate differentiation of the Earth occurred very early. The new W-isotope data constrain the mean time of terrestrial core formation to 34 million years after the start of solar system accretion. Early terrestrial core formation implies rapid terrestrial accretion, thus permitting formation of the Moon by giant impact while ¹⁸²Hf was still alive. This could explain why lunar W-isotopes are more radiogenic than the terrestrial value.