Earth

Development of Earth's structure and composition

The origin of Earth in its present form has long been the subject of intellectual interest, but since the mid-20th century scientists have made particularly significant advances both in concepts and in measurements. Analysis of the isotopes in meteorites and, in particular, of rocks brought back from the Moon by U.S. Apollo astronauts have produced some of the major contributions. Other gains have come from geochemical research on terrestrial samples combined with the new understanding of internal processes brought on by the recognition of plate tectonics, study of the terrestrial planets as a group, and advances in numerical modeling of the physical processes that lead to planetary formation.

The starting point in tracing planetary evolution is nucleosynthesis, the formation of the chemical elements on a cosmic scale. This includes the nuclear processes by which the lightest elements—mostly hydrogen and helium—were produced at the explosive birth of the universe (see big-bang model), some 14 billion years ago, and the subsequent formation of the heavier elements within stars (see chemical element: Origin of the elements). By analogy with what astronomers presently observe to happen in regions of star formation, it is thought that the solar system began as a cloud of gas and dust comprising such preexisting elements. Under its own gravitational attraction, the cloud collapsed into a rotating disk of matter, called the solar nebula. The collapse could have been initiated by a shock wave emanating from a nearby supernova, a violently exploding star, or by random density fluctuations in the cloud itself. Once sufficiently high pressures and densities were achieved in the compacted nebular core, nuclear fusion reactions within it could begin, giving birth to a star. The outer part of the rotating disk—the matter not incorporated into the new Sun—became the raw material for the planets and other orbiting bodies of the solar system. The birth of the Sun, which makes up more than 99.9 percent of the mass of the entire solar system, is taken to be the time at which the planets started to form, approximately 4.56 billion years ago.

Accretion of the early Earth

As the gas making up the solar nebula beyond the Sun cooled with time, mineral grains are thought to have condensed and aggregated to form the earliest meteoritic material. In addition, as is suggested by the finding of anomalous concentrations of isotopes in a few meteorites, solid material from outside the solar system, apparently existing prior to the formation of the Sun, was occasionally incorporated into these developing small bodies.

The concentrations of isotopes that decay radioactively and of isotopes that are produced by radioactive decay provide scientists the information required to determine when meteorites and the planets formed. For example, the concentrations of rubidium-87 and the strontium-87 into which it decays, or those of samarium-147 and its decay product neodymium-143, indicate that the oldest meteorites formed some 4.56 billion years ago. Other isotope studies demonstrate that Earth formed within, at most, a few tens of millions of years after the birth of the Sun.

The most abundant elements in the Sun, hydrogen and helium, are severely lacking in the inner, terrestrial planets but are still abundant constituents of the large, gaseous, outer planets such as Jupiter and Saturn. It is thought that only at the distance of Jupiter and beyond—in the colder regions of the solar system, including the zone beyond Neptune in which comets originated (see Kuiper belt and Oort cloud)—could the more volatile substances, which also include water, carbon dioxide, and ammonia, condense and be retained in appreciable amounts during the formation of the planets. Nevertheless, when the relative abundances of the less volatile elements are compared for the Sun, for a class of primitive, largely chemically unaltered meteorites called CI carbonaceous chondrites (considered by many researchers the most pristine samples of original solar system material), and for the estimated composition of Earth, their values are all in close agreement. This is the basis for the chondritic model, which holds that Earth (and presumably the other terrestrial planets) was essentially built up from bodies made of such meteoritic material. This idea is corroborated by isotopic studies of rocks derived from interior regions of Earth considered to be little changed throughout the planet's history. Thus, it appears that the composition of Earth is roughly what would be expected given the observed elemental abundances in the Sun and accounting for the loss of the more volatile elements.

The dust and grains that condensed out of the cooling nebular gas aggregated gravitationally to form larger fragments of rock. The chondritic meteorites (see chondrite) observed today are basically just such collections of grains and fragments that were compacted together into larger pieces. Through continued accretion, the smaller pieces formed boulders and asteroid-size bodies (planetesimals) and, ultimately, bodies the size of the Moon and Mars. The larger the planetesimals grew, the greater their gravitational attraction and the more effectively they swept up additional particles and rock fragments while circling the Sun. Growth slowed when most bodies were lunar- and Mars-size because they were limited in number and hence effectively isolated one from another in their orbits. As Jupiter increased to its giant size, its powerful gravity perturbed these “embryos” of the terrestrial planets, elongating their orbits and allowing their incremental growth to approximately the mass of Earth to proceed over tens of millions of years.

Stony meteorites and iron meteorites (those composed largely of iron alloyed with nickel and sulfur) both fall on Earth today, and both types are thought to have been present during the formation of the planetesimals that would accrete to become Earth. In other words, Earth seems to have accreted only after most, if not all, solid matter had already condensed. Thus, a wide range of minerals was included in the grains, the larger fragments, and even the planetesimals that were accumulated by the growing planet. Apparently, such an aggregation of dense metallic fragments and less dense rocky fragments is not very stable. Calculations based on the measured strengths of rocks indicate that the metallic fragments probably sank downward as Earth grew. Although the planet was relatively cold at this stage—less than 500 K (440 °F; 230 °C)—the rock was weak. This is an important point because it leads to the conclusion that Earth's metallic core began to form during accretion of the planet and probably before the planet had grown to one-fifth of its present volume.

Effects of planetesimal impacts

During its accretion, Earth is thought to have been shock-heated by the impacts of meteorite-size bodies and larger planetesimals. For a meteorite collision, the heating is concentrated near the surface where the impact occurs, which allows the heat to radiate back into space. A planetesimal, however, can penetrate sufficiently deeply on impact to produce heating well beneath the surface. In addition, the debris formed on impact can blanket the planetary surface, which helps to retain heat inside the planet. Some scientists have suggested that, in this way, Earth may have become hot enough to begin melting after growing to less than 15 percent of its final volume.

Among the planetesimals striking the forming Earth, at least one is considered to have been comparable in size to Mars. Although the details are not well understood, there is good evidence that the impact of such a large planetesimal created the Moon. Among the more persuasive indications is that the relative abundances of many trace elements in rocks from the Moon are close to the values obtained for Earth's mantle. Unless this is a fortuitous coincidence, it points to the Moon having been derived from the mantle. Computer simulations have shown that a glancing collision of a Mars-size planetary body could have been sufficient to excavate from Earth's interior the material that would form the Moon. Again, the evidence for such large collisions suggests that Earth was very effectively heated during accretion.

It is apparent, then, that many processes contributing to the early development of Earth occurred almost simultaneously, within tens to hundreds of million of years after the Sun was formed. Meteorites and Earth were formed within this time, and the Moon, which has been dated at more than four billion years in age, apparently was formed in the same time period. Simultaneously, Earth's core was accumulating and may have been completely formed during the planet's growth period. In addition to the possible accretional heating caused by planetesimal impacts, the sinking of metal to form the core released enough gravitational energy to heat the entire planet by 1,000 K (1,800 °F; 1,000 °C) or more. Thus, once core formation began, Earth's interior became sufficiently hot to convect. Although it is not known whether or in what form plate tectonics was active at the surface, it seems quite possible that the underlying mantle convection began even before the planet had grown to its final dimensions. Only later in Earth's development did radioactivity become an important heat source as well. 

Planetary differentiation

Once hot, Earth's interior could begin its chemical evolution. For example, outgassing of a fraction of volatile substances that had been trapped in small amounts within the accreting planet probably formed the earliest atmosphere. Outgassing of water to Earth's surface began before 4.3 billion years ago, a time based on analysis of ancient zircons that show the effects of alteration by liquid water. In Earth's deepest interior, chemical reactions between the mantle and the core became possible. Perhaps the most important event for Earth's surface, however, was the formation of the earliest crust by partial melting of the interior. This chemical separation by partial melting and outgassing of volatiles is termed differentiation. As the interior differentiated, less-dense liquids rose from the melt toward the surface and crystallized to form crust.

Uncertainty exists over when and how the continental crust began to grow, because the record of the first 600 million years has not been found. The oldest known rocks date to only about 4 billion years. Because these are metamorphic rocks—i.e., because they were changed by heat and pressure from preexisting crustal rocks at the time of their dated age—it can be inferred that crust was present earlier in Earth's history. In fact, two tiny grains of zircon from Australia have been dated at 4.28 billion and 4.4 billion years, but their relation to the formation of continental crust is uncertain.

Although direct evidence is not available, indirect evidence derived from the compositions of rocks indicates that continental crust formed early. Isotopic analyses suggests that the average age of the present continental crust is about 2.5 billion years. Thus, in all probability, repeated partial melting of the upper mantle formed successively more refined, continent-like crustal rocks starting before 4 billion years ago. Over the first billion years, however, much of the continental crust that was formed appears to have been reincorporated into the mantle—the isotopic data infers that on average about one-third of the continental crust was recycled every billion years. As a result, only a few fragments of crust older than 3.5 billion years remain, virtually none older than 4 billion years.

The process of partial melting and formation of crust, especially continental crust, leads to a depletion of certain elements (e.g., silicon and aluminum) from the mantle. Undepleted and thus relatively primitive regions still exist, making up about one-third to one-half of the mantle, according to the isotopic models. The distribution of depleted and undepleted regions, however, is uncertain. Although much (perhaps all) of the upper mantle appears to be depleted, it is not known whether depleted rocks also exist in the lower mantle.

What is recognized is that Earth is still differentiating into chemically distinct layers or regions. This is most evident in the processes of plate tectonics that involve ongoing production of crust at divergent plate boundaries such as the midocean ridges. As this material is cycled back down into the mantle at subduction zones and then upward again, it continues to undergo chemical processing from basaltic to andesitic and eventually to granitic (continental) composition. Thus, chemical and thermal evolution of the interior, intimately connected through mantle convection, is still vigorously in progress some 4.56 billion years after the formation of the planet.

Raymond Jeanloz

Jonathan I. Lunine

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