Stagnant lid tectonics:Perspectives from silicate planets,dwarf planets,large moons,and large asteroids

To better understand Earth's present tectonic style-plate tectonics—and how it may have evolved from single plate(stagnant lid) tectonics, it is instructive to consider how common it is among similar bodies in the Solar System. Plate tectonics is a style of convection for an active planetoid where lid fragment(plate) motions reflect sinking of dense lithosphere in subduction zones, causing upwelling of asthenosphere at divergent plate boundaries and accompanied by focused upwellings, or mantle plumes;any other tectonic style is usefully called "stagnant lid" or "fragmented lid". In 2015 humanity completed a 50+ year effort to survey the 30 largest planets, asteroids, satellites, and inner Kuiper Belt objects,which we informally call "planetoids" and use especially images of these bodies to infer their tectonic activity. The four largest planetoids are enveloped in gas and ice(Jupiter, Saturn, Uranus, and Neptune)and are not considered. The other 26 planetoids range in mass over 5 orders of magnitude and in diameter over 2 orders of magnitude, from massive Earth down to tiny Proteus; these bodies also range widely in density, from 1000 to 5500 kg/m~3. A gap separates 8 silicate planetoids with ρ = 3000 kg/m~3 or greater from 20 icy planetoids(including the gaseous and icy giant planets) with ρ = 2200 kg/m~3 or less. We define the "Tectonic Activity Index"(TAI), scoring each body from 0 to 3 based on evidence for recent volcanism, deformation, and resurfacing(inferred from impact crater density). Nine planetoids with TAI = 2 or greater are interpreted to be tectonically and convectively active whereas 17 with TAI 2 are inferred to be tectonically dead. We further infer that active planetoids have lithospheres or icy shells overlying asthenosphere or water/weak ice. TAI of silicate(rocky) planetoids positively correlates with their inferred Rayleigh number. We conclude that some type of stagnant lid tectonics is the dominant mode of heat loss and that plate tectonics is unusual. To make progress understanding Earth's tectonic history and the tectonic style of active exoplanets, we need to better understand the range and controls of active stagnant lid tectonics.     

Do terrestrial planets evolve according to the same scenario? Geological and petrological evidence

The evolution of terrestrial planets (the Earth, Venus, Mars, Mercury, and Moon) was proved to have proceeded according to similar scenarios. The primordial crusts of the Earth, Moon, and, perhaps, other terrestrial planets started to develop during the solidification of their global magmatic “oceans”, a process that propagated from below upward due to the difference in the adiabatic gradient and the melting point gradient. Consequently, the lowest melting components were “forced” toward the surfaces of the planets in the process of crystallization differentiation. These primordial crusts are preserved within ancient continents and have largely predetermined their inner structure and composition. Early tectono-magmatic activity at terrestrial planets was related to the ascent of mantle plumes of the first generation, which consisted of mantle material depleted during the development of the primordial crusts. Intermediate evolutionary stages of the Earth, Moon, and other terrestrial planets were marked by an irreversible change related to the origin of the liquid essentially iron cores of these planets. This process induced the ascent of mantle superplumes of the second generation (thermochemical), whose material was enriched in Fe, Ti, incompatible elements, and fluid components. The heads of these superplumes spread laterally at shallower depths and triggered significant transformations of the upper shells of the planets and the gradual replacement of their primordial crusts of continental type by secondary basaltic crusts. The change in the character of the tectono-magmatic activity was associated with modifications in the environment at the surface of the Earth, Mars, and Venus. The origin of thermochemical mantle plumes testifies that the tectono-magmatic process involved then material of principally different type, which had been previously “conserved” at deep portions of the planets. This was possible only if (1) the planetary bodies initially had a heterogeneous inner structure (with an iron core and silicate mantle made up of chondritic material); and (2) the planetary bodies were heated from their peripheral toward central portions due to the passage of a “thermal wave”, with the simultaneous cooling of the outer shells. The examples of the Earth and Moon demonstrate that the passage of such a “wave” through the silicate mantles of the planets was associated with the generation of mantle plumes of the first generation. When the “wave” reached the cores, whose composition was close to the low-temperature Fe + FeS eutectic, these cores started to melt and gave rise to superplumes of the second generation. The “waves” are thought to have been induced by the acceleration of the rotation of these newly formed planets due to the decrease of their radii because of the compaction of their material. When this process was completed, the rotation of the planets stabilized, and the planets entered their second evolutionary stage. It is demonstrated that terrestrial planets are spontaneously evolving systems, whose evolution was accompanied by the irreversible changes in their tectono-magmatic processes. The evolution of most of these planets (except the Earth) is now completed, so that they “dead” planetary bodies.     

堆积的地球及其初始不均一性

从天体化学和地球科学的研究成果出发，认为地球是在一较窄的类地行星区域内，主要由硅酸盐质星子随机吸积而成。在星子形成之前，初始太阳星云已经历了挥发性元素的强烈亏损事件，同时也已发生了硫化物、金属和硅酸盐成分之间的分馏作用，随着行星的形成，行星内部的分馏作用将会持续进行。在形成地球的独立吸积区内，混合作用不彻底，星子群之间的化学成分不均一，因此，构成地球的将是一套具有各自独立化学成分组成的星子群，而不同于地球上现已发现的任何陨石或者它们的组合。     

The nature,origin and modification of insoluble organic matter in chondrites,the major source of Earth’s C and N

All chondrites accreted ∼3.5 wt.% C in their matrices, the bulk of which was in a macromolecular solvent and acid insoluble organic material (IOM). Similar material to IOM is found in interplanetary dust particles (IDPs) and comets. The IOM accounts for almost all of the C and N in chondrites, and a significant fraction of the H. Chondrites and, to a lesser extent, comets were probably the major sources of volatiles for the Earth and the other terrestrial planets. Hence, IOM was both the major source of Earth’s volatiles and a potential source of complex prebiotic molecules.Large enrichments in D and ¹⁵N, relative to the bulk solar isotopic compositions, suggest that IOM or its precursors formed in very cold, radiation-rich environments. Whether these environments were in the interstellar medium (ISM) or the outer Solar System is unresolved. Nevertheless, the elemental and isotopic compositions and functional group chemistry of IOM provide important clues to the origin(s) of organic matter in protoplanetary disks. IOM is modified relatively easily by thermal and aqueous processes, so that it can also be used to constrain the conditions in the solar nebula prior to chondrite accretion and the conditions in the chondrite parent bodies after accretion.Here we review what is known about the abundances, compositions and physical nature of IOM in the most primitive chondrites. We also discuss how the IOM has been modified by thermal metamorphism and aqueous alteration in the chondrite parent bodies, and how these changes may be used both as petrologic indicators of the intensity of parent body processing and as tools for classification. Finally, we critically assess the various proposed mechanisms for the formation of IOM in the ISM or Solar System.     

放射性元素集中于上地壳的原因及其地球动力学意义

地热学和陨石学的研究都指出, 地球上放射性热源浓度随深度指数衰减.但为何会存在这样的分布特征仍是地球科学的疑谜.通过地球排气现象的研究指出, 正是类地行星及小行星演化中的排气作用, 将放射性元素带到了类地行星及小行星的表面; 而且演化时间越长的类地行星, 深部的放射性元素也越少.目前流行的地球动力学理论是以地球内部的热为根本动力的学说, 地球深部放射性热源数量的稀少对“热机动力机制”提出了质疑.由于地球动力学依靠的能源只有地球内部的热和地球自转动能2种, “地球自转动力机制”应更多地被重视和研究      

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.     

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。     

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.     

再论“关于板块运动启动时间的争论”

笔者在"关于板块运动启动时间的争论"一文(陆松年等,2016)基础上,参阅公开发表的部分文章,介绍了前板块岩浆洋和停滞盖研究的新进展,并再次讨论有关板块运动启动时间的不同认识和观点。根据太阳系30个类地行星天体图像资料,硅酸盐质行星体在它的生命期由于冷却和岩石圈增厚似乎经历过多种构造样式,包括岩浆洋、多类型停滞盖以及板块构造。硅酸盐类行星由于增生、分异、撞击和放射性活动,使星体变热,发生熔融反应而形成"岩浆洋"。硅酸盐星球体早期很可能是大面积但寿命很短的岩浆洋,之后的星球可能受两种构造模型控制:停滞盖构造和板块构造。已有资料表明,停滞盖构造是类地行星的主要构造类型,而板块构造在类地行星中是十分罕见的。停滞盖构造是一个单一的、围绕天体的板块,基本由硅酸盐质岩石圈组成。停滞盖构造有三种主要变化形态——热管、滴状及柱状体、拆沉及上涌流。板块构造启动时间从>4.4Ga到0.85Ga至少有11种不同观点:认为板块构造始于冥古宙的有1种、始于始-古太古代的有3种、始于中-新太古代的有4种、始于古元古代有1种、始于新元古代的有2种。本文重点介绍了地球冥古宙-太古宙板块运动启动的地质学证据和特点。     

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     

类地天体与地球地质作用的比较研究

<正> 一、概述 地球是作为一员星球在银河系中运动和发展的。因而,它的形成和演化是应服从于一般星球形成和演化规律的。特别是与地球相邻近,而且特征也相近的星球,在形成和演化方面应有更多共同特点。 比较法是认识、鉴别事物的重要方法。事物是从与其他事物比较中反映出它的特性和共性的,在现在科学水平上,能与地球相比较的天体在亿万星球中还是极少数,我们将这些与地球相邻近,而特征又近似的天体称类地天体。     

Irreversible evolution of tectono-magmatic processes at the Earth and Moon: Petrological data

The tectono-magmatic evolution of the Earth and Moon started after the solidification of their magmatic “oceans”, whose in-situ crystallization produced the primordial crusts of the planets, with the composition of these crusts depending on the depths of the “oceans”. A principally important feature of the irreversible evolution of the planetary bodies, regardless of their sizes and proportions of their metallic cores and silicate shells, was a fundamental change in the course of their tectono-magmatic processes during intermediate evolutionary stages. Early in the geological evolution of the Earth and Moon, their magmatic melts were highly magnesian and were derived from mantle sources depleted during the solidification of the magmatic “oceans”; this situation can be described in terms of plume tectonics. Later, geochemically enriched basalts with high concentrations of Fe, Ti, and incompatible elements became widespread. These rocks were typical of Phanerozoic within-plate magmatism. The style of tectonic activity has also changed: plate tectonics became widespread at the Earth, and large depressions (maria) started to develop at the Moon. The latter were characterized by a significantly thinned crust and basaltic magmatism. These events are thought to have been related to mantle superplumes of the second generation (thermochemical), which are produced (Dobretsov et al., 2001) at the boundary between the liquid core and silicate mantle owing to the accumulation of fluid at this interface. Because of their lower density, these superplumes ascended higher than their precursors did, and the spreading of their head parts resulted in active interaction with the superjacent thinned lithosphere and a change in the tectonic regime, with the replacement of the primordial crust by the secondary basaltic one. This change took place at 2.3–2.0 Ga on the Earth and at 4.2–3.9 Ga on the Moon. Analogous scenarios (with small differences) were also likely typical of Mars and Venus, whose vast basaltic plains developed during their second evolutionary stages. The change in the style of tectonic-magmatic activity was associated with important environmental changes on the surfaces of the planets, which gave rise to their secondary atmospheres. The occurrence of a fundamental change in the tectono-magmatic evolution of the planetary bodies with the transition from depleted to geochemically enriched melts implies that these planets were originally heterogeneous and had metal cores and silicate shells enriched in the material of carbonaceous chondrites. The involvement of principally different material (that had never before participated in these processes) in tectono-magmatic processes was possible only if these bodies were heated from their outer to inner levels via the passage of a heating wave (zone) with the associated cooling of the outermost shells. The early evolutionary stages of the planets, when the waves passed through their silicate mantles, were characterized by the of development of super-plumes of the first generation. The metallic cores were the last to melt, and this processes brought about the development of thermochemical super-plumes.     

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.     

Formation of planets during the evolution of single and binary stars

Current views of the origin and evolution of single and binary stars suggest that the planets can form aroundmain-sequence single and binary stars, degenerate dwarfs, neutron stars, and stellarmass black holes according to several scenarios. Planets can arise during the formation of a star mainly due to excess angular momentum leading to the formation of an accretion-decretion disk of gas and dust around a single star or the components of a binary. It is the evolution of such disks that gives rise to planetary systems. A disk can arise around a star during its evolution due to the accretion of matter from dense interstellar clouds of gas and dust onto the star, the accretion of mass froma companion in a binary system, and the loss of matter during the contraction of a rapidly rotating star, in particular, if the star rotates as a rigid body and the rotation accelerates with its evolution along the main sequence. The fraction of stars with planetary systems is theoretically estimated as 30–40%, which is close to the current observational estimate of ∼34%.     

Impacts in the primordial history of terrestrial planets

Collisions played a very important role in the formation of terrestrial planets. These planets are believed to have formed from a system of planetary embryos, with masses comparable to that of the Moon or of Mars. Giant collisions between proto-planets and embryos were, therefore, the rule. The collision which gave origin to the Earth’s moon was just one of these collisions. We review the state of the art concerning numerical modeling of the terrestrial planets accretion process and we compare the results with the available observational or geochemical constraints. After the completion of the formation process, the history of the bombardment of the terrestrial planets was peculiar. After a period most likely characterized by a weak bombardment rate, about 3.9 Gyr ago, the planets experienced the ‘Late Heavy Bombardment’, a cataclysmic episode characterized by a bombardment rate of about 20,000 times the current one, during a time-span of 50–150 Myr. We review a recent model that has been proposed to explain the origin of this cataclysm.     

Earth and Mars – Distinct inner solar system products

Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk's lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth's mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (∼0.5% of the mass of the Earth's mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.     

Water and astrobiology

Water is formed from two of the three most abundant elements in the universe and so is abundant in interstellar space, in our Solar System, and on Earth, where it is an essential compound for the existence of life as we know it. Water ice acts as a substrate and reactant in interstellar clouds of gas and dust, enabling the formation of organic compounds that are important precursors to life and that eventually became incorporated into comets and asteroids in the early Solar System. Laboratory experiments have allowed us to infer the reaction pathways and mechanisms by which some of these compounds are formed. In these reactions, water can act as an energy transfer medium, increasing product yields, or it can lower yields by diluting reaction centers. Water can also destroy organic compounds when water ice decomposes under ionizing radiation and the decomposition products attack the compounds; whether this happens depends critically on temperature and structure of the ice, whether crystalline or amorphous. Ice structure and temperature also largely determine its gas content. As the solar nebula collapsed, icy mantles on interstellar grains probably sublimated and then recondensed onto other grains, thus influencing the transport of energy, mass, and angular momentum in the disk. Icy grains also influenced the temperature structure of the disk because they influence mean disk opacity. Outside the “snow line” at 3–5 AU icy grains accreted to become part of comets and planetesimals that occupy the region of the outer planets, the Kuiper belt, and the Oort cloud. Water was acquired by the growing Earth by several mechanisms. Evidence from noble gas isotopes indicates that Earth achieved sufficient mass fast enough to capture an early H-rich atmosphere from the Solar nebula itself. Although the remnant of this primary atmosphere is now found only in the mantle, it may also reside in the core, which could contain most of the H on Earth (or none at all). The bulk silicate Earth contains only 500–1100 ppm H₂O, an amount small enough to explain by “wet” accretion, although most of it probably accumulated with the latter half of Earth's mass from wetter planetary embryos originating beyond 1.5 AU. Degassing on impact delivered water to Earth's surface, where it dissolved into a magma ocean, a process that likely saved it from loss during subsequent catastrophic impacts such as the Moon-forming giant impact, which resulted in >99% loss of the noble gas inventory. Although most of Earth's water probably came from meteoritic material, the depletion on Earth of Xe relative to Kr strongly suggests a role for comets. The role of water in supporting life is an essential one on Earth and probably elsewhere, given the unusual properties of water compared with other potentially abundant compounds. Its dipolarity, high boiling point and heat of vaporization and, for ice, melting temperature; its expansion on freezing; and its solvent properties make it an ideal medium for life. Life originated early on Earth, indicating an abundance of water, nutrients, precursor molecules, substrates, and appropriate physical and chemical conditions. Life adapted quickly to (and may have originated in) extreme environments, of heat, cold, dryness, saltiness, and acidity. This adaptation to extreme conditions bodes well for the prospect of finding life elsewhere in our Solar System and in planetary systems around other stars.     

History of the solar system

A brief introduction is given to the various approaches which have been followed in attempting to construct models of the origin of the solar system. The author then outlines in more detail one of the more recent approaches, involving models of a massive primitive solar nebula. In this approach a massive gaseous disk is first formed during the course of star formation, and the Sun must subsequently form from the disk as a result of hydrodynamical dissipation processes. Both the dissipation and the accompanying formation of the planets are estimated to require only a few thousand years. As a consequence, there was little time for the Earth to radiate its energy of gravitational accretion, and the primitive Earth must have been extremely hot.     

The evolution of planets. Venus as the Earth’s probable future

The general evolution of planets in the Solar System is discussed with a focus on the structure and history of Venus compared with the Earth. The history of the planets of the terrestrial group has been similar and included at least six correlated stages. Many common features the terrestrial planets shared in their early and late evolution have been due to their common origin from the protoplanetary gas-and-dust nebula and plume magmatism widespread on all the planets of the terrestrial group. The characteristic features of the structure and evolution of Venus are most brightly manifested in the specific composition of its atmosphere and of plume magmatism. Venus, with its surface as hot as 450 °C and the near-surface pressure of 92-93 bars, has a hot and dense atmosphere 93 times that of the Earth in mass. Most of its atmospheric mass (99%) belongs to the 65-km thick troposphere consisting of CO₂ (96.5%) and N₂ (3.5%). The upper troposphere includes a 25-30 km thick cloud layer composed mainly of sulfuric acid droplets, water vapor, and SO₂. At a height of 49.58 km, the clouds approach the conditions of the terrestrial surface and might be hospitable to bacterial life. Volcanism, the most active and widespread process of Venusian geology, maintains continuous SO₂ emission. There are diverse volcanic edifices on Venus, which are most often large and are similar to the Earth’s plume-related volcanoes. The evolution before 1 Ga, as well as the share and the role of alkaline rocks and carbonatites among its volcanics, are among the most debatable issues about Venus. Being located closer to the Sun, Venus cooled down more slowly and less intensely than the Earth after the primary accretion. In the Proterozoic, it began heating and reached its present state at ~ 1 or 2 Ga. In the future, as the Sun becomes a red giant, the Earth is predicted to begin heating up in 500-600 Myr to reach the temperature of present Venus in about 1.5 Gyr.