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>.