Cosmochemistry

1. Origin of the elements and Earth

Big Bang First determined by E Hubble, who measured the speed at which some galaxies were moving away from the Earth. He then recognized that the universe seemed to be expanding, and that he could use the measured velocities of expansion to figure out the time of origin.

Radiometric dating of meteorites indicates that the solar system began to form about 4.56 Ga BP as the solar nebula, consisting mostly of molecular H₂ plus He and minor Be and Li (the only products of the Big Bang). About 2% estimated to be heavier dust created by nuclear synthesis reactions in early stars and supernovae.

The first 100,000 years: The nebular cloud began to collapse because of gravitational interactions. Also flattened into a disk because of rotation (centrifugal forces), with 1-10% of mass constituting the central disk. Most of the mass gradually lost angular momentum and collapsed into the center to form the sun. Planetesimals (kilometer-size bodies) began to form by accretion, and gravitational collapse provided heat that eventually permitted nuclear synthesis (fusion) of hydrogen to helium.

The next 10 million years: known as the T-Tauri stage; during this stage the solar wind began to emanate radially outward from the and the nebula lost about 1/2 of its initial mass during this stage. Of the remaining material, 99.9% of the mass collapsed to form the sun; the other 0.1% remained in the disk. The disk material had sufficient mass to contract to the median plane, where it aggregated into the planets. Planet formation took place under conditions of strong gradients in T and P generated by the early sun. As a result, the more volatile elements evaporated in the inner, hotter portion, were stripped off by the solar winds, and condensed to solids further outward where the temperatures were sufficiently low. The actual condensation temperatures depended on the elements/compounds involved. Only the most refractory elements condensed in the innermost zone; with the more volatile elements condensed in the outermost zone. As a result, the nebula experienced chemical differentiation, with refractory oxides (Al2O3, CaO, TiO2) concentrated in the innermost portions of the solar system, and Fe-Ni alloys, Fe-Mg-Ni silicates, alkali metals and silicates, sulfides, hydrous silicates, H2O, and solids of ammonia & methane concentrated progressively outward.

Condensed solids then began to accrete as planetesimals. The terrestrial (Earth-like) planets (Mercury, Venus, Earth, Mars) formed from the more refractory materials, as well as the parent bodies that produced asteroids and meterorites. In the outer portions, the large gaseous planets formed (Saturn, Jupiter, Uranus, Neptune). [Pluto is anomalous in orbit and probably composition; may actually be an escaped moon]

IF YOU'D LIKE TO KNOW MORE ABOUT THE SOLAR SYSTEM...

THUS the Earth's composition is a product of its accretion history. However, as the process of chemical differentiation was not perfectly efficient, the Earth contains some of every stable element (not just those elements that were condensable at our distance from the sun). That said, only 7 elements comprise 97% of Earth: O (50.7%), Mg (15.3%), Fe (15.2%), Si (14.4%), S (3.0%), Al (1.4%), Ca (1.0%), consistent with solar abundances and condensates anticipated for Earth's position.

2. Differentiation of the Earth

Differentiation may have started during accretion; continued afterward because of intense heating by gravitational collapse, impacts, and radioactive heat. Eventually part of the Earth melted, which increased mobility such that dense melts moved inward and light melts moved to the surface. Gravitational energy released by this process may have melted the entire Earth (magma ocean), with the possible exception of the outermost surface. The result was a layered Earth structure.

Goldschmidt (1937) proposed that Earth's elements separate into different phases; this concept gave rise to the terms:

lithophile (stone-loving) elements form the light silicate phases

chalcophile (copper-loving) elements form an intermediate sulfur phase

siderophile (iron-loving) elements form a dense metallic phase

However, although we still use this classification it has limited applicability. For example, Fe, a siderophile element, is found in all three phases. This is partly dictated by the availability of the anions (particularly S).

These three layers do not correspond to the three layers of the Earth. The core is siderophile, but chalcophile component likely dissolved in siderophile core and was never a separate phase.  Mantle is the lithophile phase; Earth's crust had not yet formed.

O is the most common element, and the dominant anion. It combined with silica to form the outer lithophile layer, with neutrality provided by the addition of other cations. Thus the abundance of O determined the thickness of the upper layer. Most common lithophile elements of early Earth:

olivine (Mg,Fe)₂SiO₄

orthopyroxene (Mg, Fe)SiO₃

clinopyroxene Ca(Mg,Fe)Si₂O₆

The inner siderophile layer comprised the excess of siderophile cations (mostly Fe) that were left over after all the O, S depleted. All other elements (remaining 3% of Earth's mass) went into one of these layers in accordance with the atom's affinity. Again, differentiation not perfect, thus we find gold (siderophile) and copper (chalcophile) etc. at the Earth's surface.

3. How do we know this?

Best interpretations of available data; scenario presented is consistent with physical laws of celestial mechanics, gravity, nuclear synthesis, etc., as well as with the seismic probes and samples (mantle xenoliths) we have of the Earth's interior.

What evidence do we have?

gravitational constant - use to calculate Earth's mass (average density), which is 5.52 g/cm³. Density of surface rocks rarely > 3, therefore the Earth must contain a large proportion of very dense material.

nebular composition e material the comprises the solar system can be measured using various types of spectroscopy (emission of characteristic light spectra). H by far the most abundant, as it made up most of original nebula. All other elements except He, the next most abundant, were synthesized from H in the sun and other stars. Together H & He comprise > 99% of all atoms in solar system. Decrease in abundance with increasing Z reflects increasing synthesis difficulty. Also evident is

(1) relatively low abundance of some elements such as Li, Be, B, Sc is a consequence of their formation only by spallation by cosmic rays, supernova explosions and because of their consumption in subsequent fusion processes;

(2) sawtooth pattern – "Oddo-Harkins rule", which says that atoms with even numbers are more stable because their nuclei are more tightly bound.

(3) Fe is particularly stable because its nucleus is tightly bound

Note abundance of Fe (plus Mg, Ni) in solar system relative to Earth's crust; used to infer that these components must constitute much of the Earth's core. Fe is also dense enough to satisfy density requirement.

seismic studies locities of P and S waves in various materials can be measured and compared with known seismic velocities. Reflection and refraction of seismic waves at discontinuities provides direct evidence for layered structure, while absence of shear wave transmission indicates the liquid nature of the outer core.

mantle rocks ophiolites, xenoliths

3. Meteorites

Solid extraterrestrial objects that strike the Earth; many are likely fragments derived from collisions of larger bodies (particularly asteroid belt between Mars and Jupiter). Believed to represent early stages in development of the solar nebula, and thus provide information about state of early solar system. Classification:

irons  - composed of Fe-Ni alloy

stones - composed of silicates (can be difficult to tell from terrestrial rocks, but comprise about 94% of meteorites)

chondrites contain chondrules (spherical silicate inclusions that appear to have formed as droplets of glass); considered to be "undifferentiated" meteorites – most primitive

achondrites do not contain chondrules

stony-irons - contain subequal amounts of each

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Chondrite Earth Model (CEM) - average composition assumed to represent original composition of the Earth.  However, Earth is denser, and has a higher Fe/Si ratio, than provided by CEM.