cosmochemistry Cosmochemistry is concerned with experimental, observational, and theoretical studies of the chemical, isotopic, and mineralogical composition of extraterrestrial materials such as cometary dust particles (e.g., the Giotto mission to comet P/Halley), interplanetary dust particles (known as IDPs) collected using high-flying aircraft), lunar samples, and meteorites. The seminal work by Harold Urey, Hans Suess, and Harrison Brown on the chemical processes involved in the origin and evolution of the Solar System and the abundances of the elements led to the emergence of cosmochemistry as a separate sub-discipline in the late 1940s.

Historically, two major areas in cosmochemistry research have been: (1) the determination of the solar-system abundances of the elements, and (2) the chemical behaviour of the elements in a solar composition (i.e. hydrogen-rich) environment. These two topics are interwoven because the observed elemental abundances in primitive meteorites (chondrites) are generally correlated with the volatility of the elements, or their compounds, in material of solar composition.

Before proceeding some commonly used terms need to be defined. Chondrites are stony meteorites that contain small, melted beads known as chondrules and finer-grained material known as matrix. Some chondrites also contain calcium, aluminium-rich inclusions (CAIs), which are dominated by calcium, aluminium, and titanium-bearing refractory oxides and silicates. Observational studies indicate that the chondrules, mineral fragments, inclusions, and matrix in chondrites formed in the solar nebula and have been little altered since that time by planetary processes (e.g., aqueous alteration, igneous differentiation). The chondrites are divided into three groups; carbonaceous, ordinary, and enstatite chondrites, on the basis of their major element composition and mineralogy. These three major classes are further subdivided into different petrographic types. The most primitive chondrites, in the sense of most closely reproducing the elemental abundances in the photosphere of the Sun, are the CI (or Cl) carbonaceous chondrites.

Solar abundances of the elements

To a very good approximation, the abundances of most elements in the CI carbonaceous chondrites and in the Sun are identical. The few exceptions to this generalization are: light elements such as lithium, which are destroyed by thermonuclear reactions in the Sun; atmophile elements such as hydrogen, oxygen, carbon, nitrogen, and the noble gases, which are incompletely condensed in meteorites; and some rare elements such as mercury, germanium, lead, and tungsten, which are difficult to analyse in the Sun or in meteorites. To a lesser extent there is also a good correspondence between elemental abundances in the Sun and in all chondrites. This close relationship has led cosmochemists to believe that chondrites are relatively unaltered samples of material that condensed in the solar nebula. Over 30 years of meteorite studies and chemical equilibrium modelling of nebular chemistry demonstrate that evidence of nebular chemical reactions is preserved, with varying degrees of alteration by subsequent processes such as thermal metamorphism and aqueous alteration, in chondritic meteorites.

Cosmochemical behaviour of the elements

Chemical analyses of chondrites and interplanetary dust particles, and thermochemical equilibrium calculations of chemistry in hydrogen-rich solar material, show that the elemental fractionations in chondrites and IDPs are primarily the result of volatility controlled processes such as gas Æ solid condensation and solid Æ gas evaporation in the solar nebula. The occurrence of some characteristic mineral morphologies, such as enstatite (MgSiO₃) whiskers with screw dislocations, in some IDPs shows that gas Æ solid condensation was directly responsible for growing some minerals.

Cosmochemists therefore classify the elements according to their chemical behaviour in hydrogen-rich solar material. Refractory elements are the first elements to condense from solar composition gas. Both lithophiles (preferentially found in oxides or silicates, or both) and siderophiles (preferentially found in metals) with low vapour pressures, or that form compounds with low vapour pressures, fall into this category. The condensation of iron metal alloy and magnesian silicates (MgSiO₃, enstatite, and Mg₂SiO₄, forsterite) divides the refractory elements from the moderately volatile elements. In turn, troilite (FeS) condensation divides the moderately and highly volatile elements. Finally, water-ice condensation separates the highly volatile elements (e.g., lead, indium, bismuth, and thallium) from the atmophile elements (hydrogen, carbon, nitrogen, and the noble gases). Table 1 summarizes the major condensation reactions in solar composition material.

Refractory lithophiles include the alkaline earths (e.g. calcium and magnesium) the lanthanides (rare earth elements, or REE), the actinides, aluminium, and elements in groups 3b (scandium, yttrium), 4b (titanium, zirconium, and hafnium), and 5b (vanadium, niobium, and tantalum) of the periodic table. The refractory siderophiles are the platinum-group metals (except palladium), molybdenum, tungsten, and rhenium. As shown in Table 1, the refractory lithophiles and siderophiles constitute about 1 per cent by mass of the total condensable material (rock + ices) in the solar nebula. Extensive studies of the chemical composition of stony meteorites show that these refractory elements behave as a group in most meteorites: that is, their abundances in different types of meteorites are either enriched or depleted by about the same factor. Large enrichments, which are 20 times solar elemental abundances on average, of the refractory lithophile and siderophile elements are found in CAIs in the Allende meteorite and other carbonaceous chondrites. The CAIs have a mineralogy dominated by minerals rich in calcium, aluminium, and titanium such as hibonite, CaAl₁₂O₁₉, melilite, a solid-solution of gehlenite, Ca₂Al₂SiO₇, and åkermanite, Ca₂MgSi₂O₇, spinel, MgAl₂O₄, and perovskite, CaTiO₃.

Table 1. Condensation sequence for the 20 most abundant elements in the solar nebula (10⁻⁴ bar total pressure)

Bruce Fegley

Bibliography

Kerridge, J. F. and Matthews, M. S. (eds) (1988) Meteorites and the early Solar System. University of Arizona Press, Tucson.
Lewis, J. S. and and Prinn, R. G. (1984) Planets and their atmospheres: origin and evolution. Academic Press, New York.
Weaver, H. A. and Danly, L. (eds) (1989) The formation and evolution of planetary systems. Cambridge University Press.

Astrochemistry

In the night sky, the expanses of space between the stars of the Milky Way appear to be empty. In fact this space is occupied by a very thin gas that is mostly hydrogen and that has mere traces (less than 0.1% by number of atoms) of other elements such as oxygen, carbon, and nitrogen. The gas is also dusty; it contains grains of dust (particulate matter) that, like an interstellar fog, impede one's view of the stars. This gas is not evenly spread in space, but is clumpy. Although on average there is approximately one hydrogen atom for every cubic centimeter of interstellar space, a clump may be one thousand or more times as dense as a comparable volume of average density. Since about 1970 astronomers have been finding that these denser regions contain a great variety of molecules; about 120 different molecular species have been identified in the interstellar medium. A few of them are listed in the accompanying table. The study of these molecules in the Milky Way and in other galaxies is called astrochemistry.

How Do Astronomers Detect These Molecules?

Astronomers identify interstellar atoms and molecules via spectroscopy . For example, interstellar sodium atoms that happen to be in a line of sight going from a point on Earth's surface toward a bright star absorb light emitted by that star at a wavelength that is characteristic of sodium atoms (about 589 nanometers; 2.3 × 10⁻⁵ inches). Most interstellar molecules are detected by spectroscopic analysis that measures absorption or emission at radio wavelengths rather than those corresponding to visual light. Astronomers use large radio telescopes to detect radiation emitted by interstellar molecules. These emissions arise because the molecules are set to rotating when they collide with each other. The molecules lose energy and slow down in their rotations by emitting radiation at wavelengths that are specific for them, such that each emission is a "signature" of one type of molecule. For example, the molecule carbon monoxide, CO, may emit at various radio wavelengths, including 2.6 millimeters (0.1 inches), 1.3 millimeters (0.05 inches),0.65 millimeters (0.03 inches), and 0.32 millimeters (0.01 inches). Interstellar gas is usually very cold (around 10 degrees above absolute zero), but even under these conditions the molecular collisions are energetic enough to keep the molecules rotating and, therefore, emitting radiation.

Sometimes these interstellar molecules may be located in warmer regions. If the gas of which they are a part is close to a star, or becomes heated

because one clump collides with another, the temperature of the molecules may rise considerably, perhaps to several thousand degrees above absolute zero. In these cases, the collisions between gas molecules are correspondingly more energetic, and molecules may be set to vibrating as well as rotating. For example, a carbon monoxide molecule, CO, vibrates to-and-fro as if the two atoms are connected by a coiled spring. A vibrating molecule also eventually slows down and loses energy (unless it is involved in further collisions) by emitting radiation that is again specific to that particular molecule. In the example of CO, that radiation has a wavelength of about 4.7 micrometers (18.5 × 10⁻⁵ inches), the detection of which necessitates the use of large telescopes that are sensitive to infrared radiation.

How Are Interstellar Molecules Formed?

The Milky Way, like all other galaxies, was formed from intergalactic gas that was essentially atomic. So where do the molecules come from? One can deduce that they are not left over from the processes that formed the Milky Way because scientists can detect molecules in regions in which they are (currently) being rapidly destroyed; therefore there must be a formation process in operation now. For example, the hydroxyl molecule, OH, can be observed in rather low density interstellar gas regions (containing about 100 H atoms per cubic centimeter) in which it is being destroyed by stellar radiation in a time frame, typically, of ten thousand years. This seems a long time but because the Galaxy has been in existence for a much longer time (about 15 billion years), the OH radicals (and many other species) must have been formed relatively recently in the Galaxy's history.

Simple collisions between O and H atoms do not lead to the formation of OH molecules, because the atoms bounce apart before they are able to form a chemical bond. Similarly, low temperature collisions between O atoms and H₂ molecules are also unreactive.

Astronomers have now determined that much of the chemistry of interstellar space occurs via ion-molecule reactions. Cosmic rays (fast-moving protons and electrons pervading all of interstellar space) ionize molecular hydrogen (H₂) and the resulting ions (H₂⁺) react quickly with more H₂ to form other ions (H₃⁺). The H₃⁺ ions drive a chemistry that consists of simple two-body reactions. The extra proton in H₃⁺ is quite weakly bound (relative to the bonding of one proton to another in H₂); in a collision an H₃⁺ molecule easily donates its proton to some other species, creating a new molecule. For example, an H₃⁺ ion reacts with an O atom to give OH⁺, a new species:

O + H₃⁺ → OH⁺ + H

and the OH⁺ then reacts with H₂ molecules to make, successively, H₂O⁺ and H₃O⁺ ions

OH⁺ → H₂O⁺ → H₃O⁺

This process of H abstraction finishes here, because the O⁺ ion in H₃O⁺ has saturated all its valencies with respect to H atoms. However, the H₃O⁺ ion has a strong attraction for electrons because of its positive charge, and the ion-electron recombination leads to dissociation of the ion-electron complex into a variety of products, including OH (hydroxyl) and H₂O (water).

Other exchange reactions occur; for example, CO may be formed through the neutral exchange

C + OH → CO + H

Similar ion-molecule reactions drive the chemistries of other atoms, such as C and N, to yield ions such as CH₃⁺ and NH₃⁺. These ions can then react with other species to form larger and more complex molecules. For example, methanol (CH₃OH) may be formed by the reaction of CH₃⁺ ions with H₂O molecules, followed by recombination of the product of that reaction with electrons

CH₃⁺ + H₂O → CH₃OH₂⁺

CH₃OH₂⁺ + e⁻ → CH₃OH + H

Ion-molecule reactions, followed by ion-electron recombinations and supplemented by neutral exchanges, are capable of forming the majority of the observed interstellar molecular species. Very large gas-phase reaction networks, involving some hundreds of species interacting in some thousands of chemical reactions, are routinely used to describe the formation of the observed interstellar molecules in different locations in models of interstellar chemistry.

Does the Dust Play a Role in Astrochemistry?

The dust has several important chemical roles. Obviously, it may shield molecules from the destructive effects of stellar radiation. It also has more active roles. We have seen that free atoms in collision may simply bounce apart before they can form a chemical bond. By contrast, atoms adsorbed on the surface of a dust grain may be held together until reaction occurs. It is believed that molecular hydrogen is formed in this way (i.e., through heterogeneous catalysis ) and is ejected from dust grain surfaces into the gas volume with high speed and in high states of vibration and rotation. Other simple molecules, such as H₂O, CH₄, and NH₃, are also likely to form in this way.

In the denser clumps where the gas is very cold, the dust grains are also at a very low temperature (around 10 degrees above absolute zero). Gasphase molecules colliding with such grains tend to stick to their surfaces, and over a period of time the grains in these regions accumulate mantles of ice: mostly H₂O ice, but also ices containing other molecules such as CO, CO₂, and CH₃OH. Astronomers can detect these ices with spectroscopy. For example, water ice molecules absorb radiation at a wavelength about 3.0 micrometers (11.8 × 10⁻⁵ inches), having to do with the O–H vibration in H₂O molecules; the molecules do not rotate because they are locked into the ice. In instances in which such ice-coated dust grains lie along a line of sight toward a star that shines in the infrared, this 3.0 micrometer (11.8 × 10⁻⁵ inch) absorption is very commonly seen.

Interstellar solid-state chemistry can occur within these ices. Laboratory experiments have shown that ices of simple species such as H₂O, CO, or NH₃ can be stimulated by ultraviolet radiation or fast particles (protons, electrons) to form complex molecules, including polycyclic aromatic hydrocarbons (PAHs) containing several benzene-type rings. The detection by astronomers of free interstellar benzene (C₆H₆) in at least one interstellar region suggests that this solid-state chemistry may be the route by which these molecules are made.

What Role Do Molecules Play in Astronomy?

The primary role that interstellar molecules play is a passive one: Their presence in regions so obscured by dust that we cannot see into them using optical telescopes is used to probe these regions. The most dramatic example of this is the discovery of the so-called giant molecular clouds in the Milky Way and other galaxies via the detection of the emission of 2.6 micrometers (10.2 × 10⁻⁵ inches) wavelength radiation by CO molecules present in these clouds. The existence of these huge gas clouds, containing up to a million times the mass of the Sun, was not suspected from optical observations because these clouds are completely shrouded in dust. However, radio astronomy has shown that these clouds are the largest nonstellar structures in the Galaxy, and that they will provide the raw material for the formation of millions of new stars in future billions of years of the Galaxy's evolution.

The radiation from molecules that we detect can represent a significant loss of energy from an interstellar cloud. Some molecules are very effective coolants of interstellar gases and help to maintain the temperatures of these gases at very low values. This cooling property is very important in clumps of gas that are collapsing inward under their own weight. If such a collapse can continue over vast stretches of time, then ultimately a star will form. In the early stages, it is important that the clumps remain cool, otherwise the gas pressure might halt the collapse. In these stages, therefore, the cooling effect of the molecules' emission of radiation is crucial. The formation of stars like the Sun is possible because of the cooling effect of molecules. Interstellar chemistry is therefore one factor determining the rate of star formation in the Galaxy. Astrochemists have shown that it takes about one million years for the molecules of a collapsing cloud to be formed; this is about the same amount of time as that required for the collapse itself to become established. The accompanying image illustrates a region of star formation in the Galaxy.

Astrochemistry also has a role that is particularly significant to the human species here on planet Earth. The planet was formed as a byproduct of the formation of the star that is the Sun, and is in effect the accumulation of dust grains that were the debris of large chunks of matter that subsequently impacted and stuck together. Earth is still subject to the occasional impacts of debris left over from the formation of the solar system. These impacts, now seen as a source of potential danger, in fact once brought prebiotic material to Earth. The oceans arose from the arrival of icy comets, and carbon, nitrogen, and elemental metals were brought by asteroid impacts. These elements and others are necessary for life on Earth, and a new discipline, astrobiology, is coming into being: Its aim is to study the transport of prebiological material in the Galaxy and the development of life within suitable environments in the universe.

Conclusion

Astrochemistry extends chemistry into regimes of exceptionally low density and temperature; it involves gas-phase, surface, and solid-state processes. Its products, the molecules, have opened up a new approach to astronomy.

see also Spectroscopy.

David A. Williams

Bibliography

Bernstein, Max P.; Sandford, Scott A.; and Allamandola, Louis J. (1999). "Life's Far Flung Raw Materials." Scientific American 281(1):42–49.

Dyson, John E., and Williams, David A. (1997). The Physics of the Interstellar Medium. Philadelphia: Institute of Physics Publishing.

Greenberg, J. Mayo (2000). "The Secrets of Stardust." Scientific American 283(6):70–75.

Hartquist, Thomas W., and Williams, David A. (1995). The Chemically Controlled Cosmos: Astronomical Molecules from the Big Bang to Exploding Stars. New York: Cambridge University Press.

Internet Resources

Astrobiology. New Scientist. Available from <http://www.newscientist.com/hottopics/astrobiology>.

The Star Formation Newsletter. Science Magazine. Available from <http://www.eso.org/gen-fac/pubs/starform/star-form-list.html>.

astrochemistry The study of chemical reactions that occur naturally in space. Molecules are able to form in space at very low temperatures (e.g. 20 K) and at pressures far lower than are achievable in a laboratory on Earth. Many chemical species that are unstable on Earth exist in space, and are detectable by their spectral lines at radio, infrared, optical, and ultraviolet wavelengths. There are two broad types of formation process: chemistry in gas clouds (including photodissociation) and chemistry on the surfaces of dust grains. Many astrochemical reactions involve only gases. In some cases atoms or molecules in the gas may become ionized by the passage of cosmic rays, and the subsequent reactions between ions and molecules are more rapid as a result. In the other main process, dust particles act as catalysts, providing a surface on which atoms, ions, and molecules can stick and then react. The dust also acts as a shield and prevents starlight from breaking up the molecules again. Heating of the dust by newly formed stars may release complex molecules from the grains back into the gas cloud and drive a new series of chemical reactions. See also Interstellar Molecule.

cosmochemistry See astrochemistry.