ÐÓ°ÉÔ­´´

FOR the past 70 years, astronomers have known that the space between
the stars in our Galaxy is not entirely empty. It is filled with a tenuous
gas, which contains a sprinkling of small grains of ‘dust’. Much of the
hydrogen and helium in the gas was formed in the big bang, although stars
have turned some of the hydrogen into helium. The other elements in the
gas and dust grains have been produced in stars where nuclear reactions
change hydrogen and helium into carbon, oxygen, iron and still more complex
nuclei. When stars die, some of their matter is spread into interstellar
space, eventually becoming incorporated into future generations of stars
and planets.

Although elements made in stars exist in space as individual atoms or
inert grains that contain large numbers of atoms, they also form molecules,
some of them quite unusual in that they do not form readily on Earth. Indeed,
dense regions of gas and dust – interstellar clouds – contain a remarkable
array of different molecules, including complex organic molecules containing
many carbon atoms as both chains and rings.

By studying these molecules we may not only learn some interesting new
chemistry, but we may also find some clues to the origin of life.

Interstellar chemistry also provides useful astronomical data. Interesting
chemical reactions are happening in these clouds. These reactions are affected
by the formation of stars in the cloud, so we can use the chemical composition
of a cloud to look at its structure and investigate how new stars form.

Astronomers first realised in the 1920s that interstellar space is not
entirely empty, when they found that distant stars seem progressively redder
in colour than nearby stars. Small solid grains of graphite or silicates,
with diameters of about the wavelength of light, absorb and scatter light
of shorter wavelengths, so that the light travelling through the cloud ends
up with a higher proportion of the longer, redder, wavelengths. These grains
are produced in the dense atmospheres of stars and expelled into interstellar
space by the pressure of radiation from stars.

Astronomers can detect the most abundant of the atoms in space, hydrogen,
by tuning into the radiation that hydrogen atoms emit at the radio wavelength
of 21 centimetres. When optical astronomers look at the visible and ultraviolet
spectra of distant stars, they see narrow absorption lines caused by atoms
of sodium, calcium and other heavy atoms in interstellar space absorbing
radiation in that part of the spectrum. The amounts of grains and atoms
are low, however, and the average density of the interstellar material is
orders of magnitude smaller than in any vacuum that can be created in a
terrestrial laboratory.

This tenuous interstellar medium is a hostile place, pervaded by short-wavelength
radiation from the stars and energetic cosmic rays from supernovae. At first
sight, we would expect these radiations and cosmic rays to prevent atoms
forming into molecules, and to break up any that formed. Yet radio astronomers
have discovered emission lines at short radio wavelengths – generally around
a millimetre – that are characteristic of molecules.

These observations show that our Galaxy, the Milky Way, contains an
extraordinary diversity of types of molecule, ranging in complexity from
the diatomic molecule carbon monoxide (CO), to the long linear molecule
cyano-octatetra-yne (HC₉N). There are many organic molecules,
including aldehydes, alcohols, ketones, ethers, nitrates, and derivatives
of acetylene.

There is room in interstellar space for highly reactive molecules, once
formed, to survive: they are unlikely to meet and react with other species.
Astronomers discovered several such molecules in interstellar space before
chemists had recognised them in laboratory experiments. One example is the
ring molecule cyclopropenylidene (C₃H₂), where the
three carbon atoms lie at the vertices of a triangle. It was the first interstellar
organic molecule to be detected in which the carbon atoms do not lie in
a straight line. This molecule is ubiquitous in our Galaxy and astronomers
have also observed it in another galaxy, Centaurus A.

Not all molecules in interstellar space are electrically neutral. Several
are positive ions, formed by attaching a positively charged proton (a hydrogen
nucleus) to a neutral molecule, as in HCO+, HN₂+ and HCS+. These
molecular ions indicate that processes causing ionisation, the production
of charged atoms or molecules, are important in interstellar chemistry.
The most likely cause are the high-speed protons and other atomic nuclei
that astronomers call cosmic rays.

We find interstellar molecules in local condensations of material in
space. Although astronomers call these regions of enhanced density ‘interstellar
clouds’, they have few of the characteristics of terrestrial clouds. While
our clouds consist of mainly one material, water, that has condensed out
of an atmosphere of different gases, the interstellar clouds consist of
large clumps of the same kind of material as the rest of the interstellar
medium. Inside the clouds, these atoms react chemically.

Astronomers classify interstellar molecular clouds into three major
types, depending on the amount of material they contain. The more tenuous
classes, the ‘diffuse’ and ‘translucent’ molecular clouds only partly obscure
the light from the stars that lie behind them. Astronomers have studied
these clouds mainly by looking at the stars behind them, and investigating
the absorption lines that the gases in the cloud have imprinted on their
spectra at ultraviolet, visible and infrared wavelengths. Astronomers have
also detected a few molecules in diffuse clouds, and more in translucent
clouds, by their radio emission lines.

The third kind of cloud, the ‘dense’ molecular clouds, appear as dark
patches in the sky, because they obscure almost totally the light from the
stars that lie behind them. Astronomers have detected the molecules in dense
clouds by their emission and absorption lines in the radio, millimetre and
– in special circumstances – the infrared regions.

The average density of gas in space is equivalent to about one hydrogen
molecule per cubic centimetre. The densities of molecular clouds range from
about 100 atoms or molecules per cubic centimetre in the diffuse and translucent
clouds up to 1000 or 10,000 per cubic centimetre in the dense clouds, where
we also find small clumps of much higher density. The diffuse clouds are
warmer, with temperatures of 70 K or more. The dense clouds are usually
at about 10 K, although the temperature may be higher in localised regions
that have been disturbed by new stars evolving and forming.

The chemistry that we find in the clouds depends on how dense they are.
Ultraviolet radiation from stars in the Galaxy can penetrate diffuse clouds,
so reactions result from atoms or molecules absorbing a high energy ultraviolet
photon. These photons have so much energy that they can break molecules
down into separate atoms. This limits the complexity of the molecules that
can grow: the molecules so far detected in diffuse clouds consist of only
two atoms.

In dense clouds, the dust grains absorb ultraviolet light, so shielding
the gas atoms and molecules from the destructive radiation. Here, very complex
molecules can survive. Although the dust blocks ultraviolet radiation, cosmic
rays can easily penetrate the dense clouds, and they are responsible for
driving the chemical reactions in the centres of the dark clouds. Translucent
clouds form an intermediate class, sharing the characteristics of diffuse
and dense clouds.

The gas in interstellar space consists almost entirely of hydrogen and
helium, with about 10 hydrogen nuclei to one helium nucleus. For every 1000
hydrogen nuclei, there are fewer than one nucleus of each of the other,
heavier, elements.

Because hydrogen is the most common element, we would expect molecular
hydrogen (H₂) to be the most abundant molecule in dense clouds.
Molecular hydrogen does not, however, emit radiation at the wavelengths
that astronomers can readily detect – the radio and millimetre waves. It
is much easier to detect radiation from the rarer molecules that emit more
powerfully at these wavelengths. Astronomers can easily detect the radiation
from carbon monoxide, which is only one ten-thousandth as abundant as hydrogen
molecules in dense clouds. They have also found almost 100 other molecules,
ionised molecules and active fragments of molecules called radicals, which
contain many light elements: hydrogen, carbon, oxygen, nitrogen, sulphur,
silicon, chlorine and phosphorus. Some of these are less than one-billionth
as abundant as hydrogen.

Astronomers have detected how much molecular hydrogen there is in diffuse
clouds from the absorption lines that the hydrogen molecule imprints on
the ultraviolet spectra of more distant stars. In these clouds, we can also
measure the abundance of hydrogen atoms. This helps us to understand the
chemistry of the simplest reaction, between two hydrogen atoms to form a
molecule and the reverse reaction, a hydrogen molecule dissociating into
atoms (see ‘The simplest chemical reaction’, New ÐÓ°ÉÔ­´´, 12 May 1990).

Hydrogen molecules are readily destroyed by ultraviolet photons. The
molecule first absorbs a photon (a light particle), to produce an excited
molecule,

H₂ + h&ugr; -> H₂^*

where hn represents the energy of a photon. The excited molecule has
more energy than two individual atoms, so it breaks up spontaneously, emitting
a photon of lower energy:

H₂^* -> H + H + h&ugr;

Because we know the rate at which hydrogen molecules are destroyed to
form hydrogen atoms, we can calculate the rate at which a hydrogen molecule
forms from atoms, in order to produce the ratio of atoms to molecules that
we observe. We believe that hydrogen molecules are formed on the surfaces
of dust grains, rather than in space. If a hydrogen atom collides with a
grain and meets another hydrogen atom in the process, they can come off
the grain together as a hydrogen molecule. When we do the calculation for
the diffuse clouds, it works out that every collision of a hydrogen atom
with a grain must produce an hydrogen molecule.

Photons capable of breaking up molecular hydrogen do not penetrate the
interior of dense clouds. Our calculations show that molecular hydrogen
forms so readily on grains that almost all the hydrogen in the dense clouds
is in this form. Cosmic rays and chemical reactions do destroy some of the
hydrogen molecules, but not enough to reduce the overwhelming preponderance
of molecular hydrogen in the cloud. These processes are, however, important
sources of hydrogen atoms, which can then take part in other reactions.

Some molecules undoubtedly form when atoms other than hydrogen meet
on the surfaces of grains. These molecules, however, will not be released
back into space, but will tend to stick to the grains to form a mantle of
molecular material. The molecules that we detect in the gas must, therefore,
have formed by reactions between atoms and molecules in the gas phase. The
density in interstellar clouds is so low that we would not expect to find
reactions that involve three atoms or molecules colliding. But the formation
of hydrogen is enough to cause other molecules to form.

Most chemical reactions between neutral atoms or molecules go slowly
in the cold interstellar clouds. When ultraviolet radiation or cosmic rays
bombard atoms or molecules, they knock out an electron to form a positively
charged ion. These molecular ions react readily with neutral atoms or molecules.
Astronomers have detected molecular ions in interstellar clouds. One of
the most important is carbon monoxide with a proton added (HCO⁺). Astronomers
were initially puzzled by the radio emission from this ion, which they could
not identify. As a result, it was called ‘X-ogen’ until researchers confirmed
its identity by measuring the spectrum of HCO⁺ molecular ion on the Earth.

To make HCO⁺ we must first make carbon monoxide, and there are a variety
of ways to produce this molecule in interstellar clouds . In fact, calculations
show that these processes should convert almost all the carbon in a dense
interstellar cloud to carbon monoxide. This leaves little carbon for the
formation of the complex hydrocarbons that have been discovered. Photons
can, however, break up carbon monoxide to give free carbon. Although ultraviolet
photons from stars cannot penetrate the centres of dense clouds, there is
a process that can produce photons locally. When cosmic rays collide with
atoms and molecules, they knock out fast electrons producing ions. The electrons
collide with hydrogen molecules and produce photons as they slow down.

When we try to think of ways to form complex molecules, we are confronted
by the problem that we are uncertain about the rates of many of the reactions,
especially at the low temperatures in these clouds. One potentially powerful
mechanism is called radiative association. Here, two atoms or molecules
combine to form an excited molecule, which then loses energy by emitting
a photon .

The infrared spectra of nebulae have led astronomers to believe that
the interstellar medium may contain large molecules with rings of carbon
atoms, for example, coronene (C₂₄H₁₂) and naphthalene (C₁₀
H₈). The compounds are called polycyclic aromatic hydrocarbons. They may
be created, together with graphite dust grains, in the cool atmospheres
of red giant stars. We have begun to explore their role in interstellar
chemistry, but little is known about their reactivity. Laboratory investigations
are just beginning (see Science, this issue).

The solid grains in the clouds not only shield the molecules from ultraviolet
starlight but they also provide ‘sinks’ that remove molecules from the gas
phase. The grain surfaces are so cold that the molecules freeze onto them.
In the space of a million years – a short time in the life of a cloud –
most of the molecules should have frozen solid. Yet we do observe molecules
in the gas phase. There must be some way in which the molecules frozen onto
the grain surfaces can escape again.

Researchers have suggested two possible mechanisms. When a cosmic ray
hits a grain, its energy may loosen some molecules, or alternatively the
molecules may be released when two grains collide. These processes tend
to add to the gas molecules such as water (H₂O), methane (CH₄),
ammonia (NH₃), hydrogen sulphide (H₂S), phosphine
(PH₃) and silane (SiH₄), which then participate in
the sequences of chemical reactions.

The chemistry is also modified by flows of gas from stars. Many stars
lose a large amount of mass from their surfaces, as a ‘stellar wind’, while
the explosion of a supernova produces much faster gas flows. In 1987, astronomers
discovered infrared spectral lines of carbon monoxide in the expanding,
ejected material of the supernova 1987A, which exploded in the nearby galaxy,
the Large Magellanic Cloud.

High-speed flows of gas can evaporate the mantle of molecules from the
grains, depositing molecules into the gas phase. The flows also drive shock
waves into the interstellar gas, so compressing and heating it.

In the warm gas, a different chemistry operates. The chemistry of molecular
ions is replaced by a chemistry involving neutral particles, controlled
by reactions with molecular hydrogen. A warm gas may explain why there is
so much of the molecular ion CH⁺, which cannot be explained by a cold chemistry.
In this case, the ion would be made in the reaction of ionised carbon with
molecular hydrogen:

C⁺ + H₂→ CH⁺ + H

This reaction absorbs energy and goes very quickly at temperatures above
2000 K. Although this explanation is attractive, measurements of the speed
of the gas containing CH⁺ do not fit with what we would expect from a shock.
The origin of the warm gas is therefore still uncertain.

There are still many other questions in interstellar chemistry to be
answered. Their solution will require research in different branches of
science, particularly in the study of molecular processes in plasmas.

We have probably identified the essential ideas governing the molecular
processes that control interstellar chemistry, but we do not yet have a
comprehensive view of the chemical structure of the interstellar medium
to describe how stars interact with the interstellar gas as they are born,
evolve and die. We can use measurements of chemical composition to learn
about the radiation between stars, the densities and temperatures of interstellar
clouds and the flux of cosmic rays but we cannot yet derive the ages of
molecular clouds. Nor can we use chemical composition to define how molecular
clouds evolve. Astrochemists are making progress, however, with the development
of new receivers and the construction of new telescopes, we can look forward
to studying the interstellar chemistries of other galaxies and compare them
with our own.

The discovery of organic molecules in our own galaxy raises the fundamental
question of what their role is in the evolution of life on planets formed
in stellar systems like the Solar System. The interstellar molecules may
be destroyed when pre-solar or pre-stellar nebulae condense, but the formation
and survival of complex organic species in the harsh conditions of interstellar
space shows that they are remarkably durable. It may be that these molecules
do survive and play a crucial role in the chemistry that led to the beginning
of life on Earth and possibly elsewhere.

* * *

Brewing up molecules in interstellar clouds

PROTONATED carbon monoxide, HCO+, is a secondary product of cosmic rays
which traverse the interstellar gas and ionise it. The cosmic rays ionise
hydrogen molecules to create positively charged ions, H₂+. These
ions react very rapidly with hydrogen molecules. Indeed, it is difficult
to make H₂+ in a hydrogen plasma in the laboratory for this reason.
The reaction extracts a hydrogen atom from the neutral H₂ molecule
to produce another molecular ion, H₂+:

H₂+ + H₂ -> H3+ + H

The H3+ molecular ion reacts readily with the abundant carbon monoxide
in the cloud to form HCO+ by transferring a proton from H3+ to carbon monoxide:

H3+ + CO -> HCO+ + H₂

The HCO+ ion eventually captures an electron to form a neutral molecule,
which then falls apart:

HCO+ + e- -> H + CO

This kind of reaction with electrons is very rapid, so there are not
many electrons left free to roam in dense clouds.

After hydrogen, carbon monoxide is the most abundant interstellar molecule.
Unlike molecular hydrogen it produces a spectrum at wavelengths around a
millimetre. Astronomers commonly use the emission from carbon monoxide to
map the distribution of molecular clouds in galaxies because the conditions
favourable for forming hydrogen are also favourable for carbon monoxide.

There are several ways in which carbon monoxide can form. If there are
neutral carbon atoms, a proton can transfer from the molecular ion H3+

H3+ + C -> CH+ + H₂

This gives the molecular ion, CH+, which captures further protons from
molecular hydrogen:

CH+ + H₂ -> CH₂+ + H CH₂+ + H₂
-> CH₃+ + H

The ion CH₃+ does not react with the hydrogen molecule but
it does with atomic oxygen:

CH₃+ + O -> HCO+ + H₂

This produces HCO+, which recombines with an electron to make carbon
monoxide.

An additional route begins with the hydrogen molecular ion reacting
with atomic oxygen to form a hydroxyl ion and a hydrogen molecule:

H3+ + O -> OH+ + H₂

This is followed by two further reactions with hydrogen:

OH+ + H₂ -> H₂O+ + H H₂O+ + H₂
-> H3+ + H

Hydrogen is by far the predominant chemical species in dense clouds.
As a result, anything that can react with hydrogen does so. The ion, H3O+,
cannot. Instead, it recombines with an electron to produce water,

H3O+ + e- -> H₂O + H

and the hydroxyl radical OH,

H3O+ + e- -> OH + H₂

Carbon monoxide then forms by the following reactions:

C + OH -> CO + H C+ + OH -> CO + H+ C+ + H₂O -> HCO+ + H
HCO+ + e- -> H + CO

Carbon monoxide is a very stable molecule. In the interstellar gas,
it is destroyed only by reacting with positive helium ions produced by cosmic
rays. In the reaction

He+ + CO -> He + C+ + O

the He+ ion takes an electron from carbon monoxide to make CO+ in an
excited state that breaks into a positive carbon ion and atomic oxygen.

* * *

Possible pathways to prebiotic molecules

HOW are complicated molecules formed in interstellar space? Here are
some suggestions, although we do not know much about reaction rates at low
temperatures in space.

A reaction between a methyl ion and a water molecule can lead to a whole
series of chemical reactions, the first reaction producing a methylhydroxyl
cation:

CH₃+ + H₂O -> CH₃OH₂+ +
hn

This takes up an electron to form methyl alcohol:

CH₃OH₂+ + e- -> CH₃OH + H

We can produce ethyl alcohol from a reaction between ethene and a water
molecule that has an extra proton:

H3O+ + C₂H4 -> CH₃CH₂OH₂+
+ hn

followed by

CH₃CH₂OH₂+ + e- CH₃CH₂OH
+ H

The Orion Nebula is a cloud of hydrogen gas, complex molecules and dust
– a glowing nursery for newborn stars Similar processes lead to methane
from the reaction between a methyl radical and hydrogen:

CH₃+ + H₂ CH5+ + hn CH5+ + CO CH₄
+ HCO+

A useful process for adding carbon atoms involves reactions, such as

C+ + CH₄ C₂H3+ + H C₂H3+ + e-
C₂H₂ + H C+ + C₂H₂ C₃H+
+ H or CH₃+ + CH₄ C₂H5+ + H₂

We can obtain compounds that contain elements such as nitrogen by simple
exchange reactions. The sequence

C₂H3+ + N C₂H3N+ + H C₂H3N+ + e-
CH₂CN + H

produces the cyanomethyl radical CH₂CN that is observed in
interstellar clouds.

Alexander Dalgarno is Phillips Professor of Astronomy at Harvard University
and also directs the Harvard-Smithsonian Institute for Theoretical Atomic
and Molecular Physics.

Further reading Interstellar Chemistry, WW Duley and DA Williams, Academic
Press, 1984; Molecular Astrophysics, TW Harbquist ed, Cambridge University
Press, 1990.