Element Distribution in the Solar System

The abundance and distribution of chemical elements in the solar system resulted from many events: the creation of hydrogen and helium in the Big Bang, the synthesis of heavier elements by nuclear fusion reactions in earlier generations of stars, differential condensation in the early solar nebula during the accretion of the planets, and physical and chemical processes within solar-system bodies after their formation. Elemental abundance and distribution within the solar system therefore point to the origin and history of the solar system and its constituent bodies.

Overview

The solar system consists of three rather distinct parts. At the center is the Sun, composed mostly of hydrogen and helium and containing most of the solar system’s mass. Next comes an inner system of four small planets (the terrestrial planets, Mercury, Venus, Earth, and Mars), three moons, and many asteroids, all composed mainly of rock and metal. Last there is an outer system of four large gas/liquid/ice planets (the Jovian or “gas giant” planets, Jupiter, Saturn, Uranus, and Neptune), many small ice-rock moons and other bodies, and lots of icy cometary nuclei. The four inner planets are made predominantly of magnesium and iron silicates, with cores rich in iron, nickel, and sulfur. The four gas giants have thick envelopes of hydrogen, helium, and hydrogen compounds in the form of gases, liquids, ices, or a combination of these, with cores of silicates and metals; these large planets generally approximate the Sun in composition. The small ice-rock bodies may have rocky Silicate cores with outer layers of various ices, or they may be homogeneous mixtures of ice and rock.

Slightly over 91 percent of the atoms in the solar system are hydrogen. The remaining-almost 9 percent is helium. Together they account for almost 99.9 percent of all atoms. In terms of number of atoms, the next two most abundant are oxygen (about 0.08 percent) and carbon (about 0.04 percent). These are followed by nitrogen, silicon, magnesium, neon, iron, and sulfur. Each of the other elements accounts for less than 0.001 percent of the atoms, and their abundance generally falls off rapidly with an increasing atomic number. There also is a general alternation in abundance, with atoms of even-numbered elements more abundant than odd-numbered elements.

The overall abundance of chemical elements in the solar system was established by events before the solar system formed. Hydrogen and most of the helium were created in the first few minutes after the Big Bang. The event also created a trace of lithium, small amounts of beryllium, and boron. Most of the atoms in our solar system heavier than helium were formed by nuclear fusion reactions in earlier generations of stars that went through their “life cycles” before the solar system began to form. The heavier atoms were dispersed throughout interstellar space when these stars ended their energy-producing “lives,” either discarding their outer layers as planetary nebulae or exploding violently as supernovae. The heavier atoms enriched the interstellar clouds of gas from which new generations of stars and their accompanying systems of planets formed. Measurements of the solar system abundances for the heavier nuclei roughly match the expected abundances based on the types of nuclear reactions that are expected to occur in stars.

The solar system formed when part of a large cloud of interstellar gas and dust began to contract inward under its own gravity. As the cloud contracted, it began to rotate faster to conserve angular momentum; eventually it spun off an equatorial disk of matter. Most of the mass of the contracting cloud collected in the center to form the protosun, while the rest of the solar system formed in the equatorial disk. Chemical reactions governed the events that occurred in the “leftover” matter of the disk. Close to the developing Sun, only a few refractory materials condensed to form small grains of minerals and metals; far from the Sun, even gases such as ammonia and methane condensed to form grains of ice.

The first solids to condense in the early solar nebula, at temperatures around 1,600 kelvins, were oxides of calcium, aluminum, titanium, and some rare Earth elements. At about 1,300 kelvins, iron-nickel metal condensed, followed at about 1,200 kelvins by enstatite (MgSiO₃), the magnesium variety of the silicate mineral pyroxene. At about 1,000 kelvins, sodium, potassium, calcium, and aluminum reacted with silicate grains to create the feldspar minerals. At 680 kelvins, metallic iron reacted with hydrogen sulfide gas to create the iron sulfide troilite (FeS). Also, between 1,200 and 500 kelvins, metallic iron reacted with oxygen to create ferrous iron oxide (FeO), which reacted in turn with enstatite to create a variety of the silicate mineral olivine (FeMgSiO₄). Between about 600 and 400 kelvins, water reacted with calcium silicates and Olivine to form various hydrous minerals. This sequence accounts for all the most common constituents of meteorites. A class of meteorites called chondrites matches the element abundance in the Sun, minus those elements that would have remained gases. Chondritic meteorites are thus considered to be the most primitive surviving materials in the solar system.

Ice sublimes quickly into gas in a vacuum. Only at and beyond the distance of Jupiter, about 5 astronomical units from the Sun, was it cold enough (about 175 kelvins) for ice to form in a vacuum and survive billions of years. At 150 kelvins, methane hydrate (a solid mixture of methane and water) condensed, followed by ammonia hydrate at 120 kelvins. Argon and pure methane solidified at about 65 kelvins.

The condensation of various solid grains at different distances from the early Sun explains much of the present distribution of materials in the solar system, and the accretion of the planets further modified their chemistry. Computer simulations of solar system accretion indicate that the small grains orbiting the Sun coalesced first into many small planetesimals (up to several hundred kilometers across) and then into protoplanets (2,000 to 3,000 kilometers across). As these bodies grew, their increasing gravity swept in matter from wider and wider swaths in the disk. The protoplanets became massive enough to attract one another, disturbing their orbits. Thus, the protoplanets swept up matter from rather wide bands in the disk, mixing materials from a variety of temperature zones. Late in the accretion process, the protoplanets collided to create larger planets. The collisions would have vaporized substantial parts of the impacting bodies, while perhaps mixing materials from two protoplanets of rather different composition.

The Earth-Moon system may be the result of such a collision. The Moon has a much lower overall density than the Earth, suggesting that it has a lower proportion of metal to rock than the Earth. Also, the Moon is poorer in volatile materials than is the Earth, and the more volatile a material is, the lower is its abundance on the Moon. Thus, it appears that the Moon formed in a hotter region of the solar disk than did the Earth. Many of the problems explaining the formation of the Moon can be resolved if the early Earth collided with a Mars-sized protoplanet. Much of the material of the impacting protoplanet, along with part of the Earth’s crust and mantle, could have sprayed off into orbit around the Earth, later coalescing to form the Moon.

The large rocky and metallic objects possess a concentric-shell structure consisting of a dense metallic central core, a less dense silicate mantle, and a thin silicate crust. This layering reflects internal changes after the body formed. Gravitational separation of a dense metallic core occurred during or shortly after the accretion of a planet; this process may have been well underway during accretion. The collision of two large protoplanets with metallic cores might account for the unusually large core of the planet Mercury; the collision would have vaporized much of the outer rocky shells of the protoplanets, resulting in a large core but only a thin, rocky mantle and crust.

Deep within planets, high pressures crush minerals into new and denser crystal lattice structures. At depths below about 700 kilometers in the Earth, for example, magnesium silicates are crushed into densely packed cubic crystal structures. Water ice undergoes a remarkable series of changes in crystal structure as pressure increases; some of these high-pressure forms of ice are surely present in the interiors of large satellites in the outer solar system.

Internal processes may also concentrate materials on the surface of a planet or Satellite to form a crust. For example, the Moon’s crust formed by the melting of chondritic material. Early in the history of the Moon, its outermost 100 kilometers melted to produce a “magma ocean,” probably from heat generated by the final impacts of the accretion process. As the molten material solidified, magnesium and iron atoms accumulated in dense minerals such as pyroxene and olivine, which settled to the bottom. Calcium and aluminum mostly went to form less dense feldspar, which was neutrally buoyant, neither rising nor sinking. Sinking of the olivine and Pyroxene would have created a lower layer of peridotite, leaving behind an upper layer of anorthosite, matching the observed makeup of the ancient lunar crust.

When chondritic material is melted and then cooled, basalt or gabbro forms. Basalt is created from rapid cooling and has small mineral crystals, while gabbro forms when the cooling is slower, allowing the mineral crystals to grow larger. Thus, it is reasonable to expect basalt and gabbro to be very common rocks in the solar system. They form the crust of the ocean basins on Earth, where they are derived from rocks of the underlying mantle. Basalt also forms the dark lava plains, or maria, on the Moon, and lava flows on Venus. The shield volcanoes on Earth are made of basalt, and similar shield volcanoes are present on Venus and Mars. The reflection spectra of some asteroids indicate that they probably have basalt on their surfaces.

In contrast to the simplicity of ocean crust, the continental crust of the Earth is chemically more complex, consisting of granite, a rock type relatively rare in the solar system. The only other possible identification of it in the solar system came from one of the Venera landers on Venus (Venera 8). Granite consists of quartz, potassium and plagioclase feldspar, micas, and amphibole. Compared with basalt and gabbro, it is greatly enriched in silica (SiO₂), potassium, and sodium, and depleted in iron and magnesium. The granitic crust of the Earth is also enriched in some less abundant elements, notably those whose atoms are unusually large (rubidium and some rare-Earth elements) and those with large atomic numbers (uranium, thorium, and lead). These elements do not fit easily into the dominant minerals of the mantle, where the principal metallic elements (iron, magnesium, and calcium) have moderate-sized atoms and atomic numbers. Granite has formed by chemical differentiation during repeated melting of the Earth’s mantle and crust. Granite found on another solar-system object would be clear evidence that it had a high degree of internal activity.

Io, the innermost of Jupiter’s large Galilean satellites, is a remarkable example of a body whose surface was created by internal processes. Io, though slightly smaller than the Earth’s moon, is internally hot and volcanically active due to tidal flexing. Io has too little gravity to retain water vapor, the main propellant for terrestrial volcanic eruptions. In fact, Io is extremely dry, having lost most of its water to space. The only material that is cosmically abundant, volatile enough to power eruptions, and heavy enough to have been retained by such a small body is sulfur. Io has a spectacular white, yellow, and red surface, probably coated by sulfur dioxide frost and various crystalline forms of sulfur erupted onto its surface.

As protoplanets and planets grew larger, they eventually became massive enough to attract and hold gases. However, violent collisions such as those that appear to have occurred late in the accretion process would probably have driven off whatever atmospheres the protoplanets had, so smaller planets would have had great difficulty in accumulating atmospheres. In the inner solar system, the warm temperatures would have given gas molecules greater speeds, enabling them to escape more readily from small planets. Also, the early Sun probably underwent a time of intense activity (called the T-Tauri phase) during which it emitted intense streams of charged particles that swept the inner solar system free of its remaining gas. For these reasons, the inner planets have thin atmospheres or none. Earth and Venus are massive enough to have retained significant atmospheres, but Mars has only a thin atmosphere, and Mercury has only a bare trace.

In the outer solar system, the Jovian planets grew large enough to retain gases despite disruption by protoplanet collision and early solar activity. Jupiter and Saturn, which were massive enough to retain essentially all their gases, are quite close to the Sun in composition. Uranus and Neptune did not become massive enough to attract or retain hydrogen and helium quite as effectively as Jupiter or Saturn, and thus they contain somewhat less hydrogen and helium and a somewhat greater proportion of denser gases such as ammonia and methane. The heavy elements in all four Jovian planets probably accumulated into roughly Earth-sized solid cores at their centers.

Small bodies in the outer solar system accreted like those in the inner solar system, but with the addition of water ice as a major constituent. Except for Io, the small bodies of the outer solar system have silicate cores with icy mantles and crusts. Comets, whose orbits extend to great distances from the Sun, probably formed in the vicinity of Jupiter and Saturn and were expelled during close passages by the giant planets. They are mostly water ice, with other frozen gases in smaller amounts.

Methods of Study

The composition of many solar-system objects is known at least partially through spectroscopy. When materials are in the form of a diffuse gas, their atoms or molecules emit and absorb certain specific wavelengths of light and other forms of electromagnetic radiation. The specific wavelengths of electromagnetic radiation emitted or absorbed by a gas act like a chemical fingerprint of its composition. Solids have far more complex and less conclusive spectral patterns than do gases, but the spectra of light reflected from the solid surface of asteroids can be matched to laboratory measurements of the reflection spectra of various types of meteorites.

Since the generally accepted model is that the entire solar system formed during the same time from a homogeneous cloud of gas and dust, it would be expected that any solid grains that condensed from the Solar nebula would have a composition similar to that of the Sun, minus those elements that would have remained gases. In the inner solar system, where the temperature of the solar nebula would have been above the freezing point of water, most hydrogen, nitrogen, and carbon would have remained gases. Most oxygen would combine with hydrogen to form water vapor, but some would combine with various metals to form oxides or would combine with silicon to form silicate minerals. Some sulfur would be available for iron-sulfide minerals such as Troilite (FeS). The noble gases (helium, neon, argon, krypton, and xenon) do not chemically bond with other elements and would be nearly absent from any solid grains. This simple approximation predicts that the inner solar system bodies should consist of the most abundant elements in the Sun, minus the gases; thus the most abundant elements of the inner planets should include oxygen, magnesium, silicon, sulfur, and iron, and the actual compositions are consistent with the predictions just described.

Meteorites are particularly valuable because they provide samples of the actual materials that condensed in the early inner solar system. The type of meteorites called chondrites match the theoretical expected composition very closely and are believed to be samples of the primitive inner solar system.

Samples of rocks from the Earth’s crust are easily obtained (even from ocean basins). Samples of the Earth’s mantle are found in two geologic settings: as parts of ophiolite suites (fragments of displaced oceanic crust and underlying mantle) and as kimberlite pipes, or volcano-like vents that appear to have brought rocks (and occasionally diamonds) from the mantle to the surface with great speed and violence. These processes bring both shallow mantle rocks, made mostly of olivine and pyroxene, and deeper mantle rocks, or eclogites, to the surface.

Studies of the propagation of Seismic waves through the Earth provide information about both the internal structure of the Earth and the physical properties of the Earth’s interior. The observed physical properties match those of a magnesium and iron silicate mantle and a dense iron-nickel core.

One measure of solar system bodies can provide great insight into their chemistry, even for bodies not sampled directly. That measure is bulk density or average density, which is the mass of a body divided by its volume. The volume of a body can be readily computed once the diameter is known, and the diameter can be obtained with high precision with spacecraft imagery. The same spacecraft that obtains imagery can also provide an accurate mass determination, through the gravitational effect of the body on the path of the spacecraft. Most common rocks from the Earth’s crust have densities of about 2.7 to 3.0 grams per cubic centimeter. The rocks of the Earth’s mantle are denser, about 3.3 grams per cubic centimeter for rocks from the upper mantle. Compression within the interior of a planet would result in higher densities, but bodies with bulk densities between 3 to 4 grams per cubic centimeter, such as the Moon, Mars, and Io, are probably made mostly of silicates of the sort found in the Earth’s mantle. Denser bodies, such as Earth, Venus, and Mercury, with average densities between 5 and 6 grams per cubic centimeter, must have some denser material in addition to silicates. The most likely dense material is an iron/nickel core like that of the Earth.

The only solids that are abundant in the solar system and less dense than silicate rocks are various ices, with densities of about 1 gram per cubic centimeter. Small solid bodies with densities of about 1 to 3 grams per cubic centimeter are very likely made of varying proportions of silicate rock and ice. Most of the satellites in the outer solar system are of this composition.

The very large outer planets also have low bulk densities; Saturn has an average density of only 0.7 grams per cubic centimeter, and Jupiter, Uranus, and Neptune have average densities between 1 and 2 grams per cubic centimeter. These planets are known, from spectroscopic evidence as well as direct imaging by spacecraft, to have dense gaseous atmospheres, that, according to models of their interiors, liquefy with depth as a result of tremendous pressure. At their centers probably are solid cores of ice, rock, and/or metal.

Context

The principal value of knowing the chemical composition of the solar system is the insight it provides into its origin and evolution. For example, scientists know that the gases that formed in the early solar system did not include free oxygen. Free oxygen would have combined rapidly with other substances. Therefore, the present atmosphere of the Earth cannot be original but must have formed through chemical and biological processes on the Earth’s surface.

Many elements essential to technological society, such as silver, lead, gold, and uranium, are rare in the solar system (and the universe generally). They can be extracted economically only because geologic processes on Earth have concentrated them into ore bodies. The Earth’s diverse ore deposits evolved in many ways because the Earth is a dynamic planet both in its interior and on its surface. Chemical elements are concentrated in the Earth’s crust by plate tectonic processes; they are further redistributed and concentrated by weathering and erosion, driven by Earth’s oxygen-rich atmosphere and liquid water.

In scientific findings published in 2023, researchers at Carnegie Science describe how one element, potassium, came to be present on the Earth. This insight was made possible by the study of meteorites. The process first began with left-over materials that followed the formation of the Sun. This material would later coalesce into planets and other celestial bodies. Potassium was one of these original materials. Carnegie researchers believe that their study of potassium isotopes suggests this element was predominately formed in the outer reaches of the solar system. Some of the oldest meteorites discovered, called carbonaceous chondrites, contain potassium isotopes that match deposits found on the Earth. Carnegie researchers theorize therefore that meteorites were the transport mechanism that helped provide the Earth with its supply of potassium.

Understanding solar system chemistry also can offer a glimpse into a possible future. There has been speculation that extraterrestrial mining might someday contribute valuable resources to human civilization. Some asteroids are rich in iron and other metals; eventually it may be technologically feasible to mine these metals economically. On the other hand, the Moon lacks both the internal and surface activity of Earth, so many types of ore deposits are unlikely to form. Mining the Moon may be the most practical way to supply a future lunar colony, but it probably would involve extracting metals from common rocks, a very energy-intensive and expensive proposition. Thus it is unlikely we will mine the Moon to supply Earth.

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