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METEORITES are the best evidence we have of how the Solar System began.
Some types of meteorite have remained almost unchanged since then, unlike
rocks on Earth which have been melted and recrystallised into entirely different
forms. For many years, cosmologists relied on meteorites to help them reconstruct
the complex processes which took place in the earliest part of Solar System
history. More recently, researchers have begun to come across puzzling anomalies
in the chemical make up of some types of meteorite. Now they have realised
that components of these meteorites are the remnants of much older processes
that went on even before the Solar System existed.

Meteorites come in all shapes and sizes, from iron meteorites weighing
several tonnes to micrometre-sized particles of cosmic dust. Early work
concentrated on identifying minerals in the meteorites and how their compositions
differed from those of rocks on Earth. Later, cosmologists developed more
sophisticated methods of analysis, such as mass spectrometry, which enabled
them to measure even tiny amounts of gases.

Cosmologists divide meteorites into two basic types: iron meteorites
consist mainly of the metals iron and nickel; stony meteorites contain mainly
silicate minerals, such as olivine and pyroxene. The most primitive of all
meteorites are stony meteorites called chondrites, after the rounded droplets
of silicates they contain (from khondros, Greek for grain). Most of the
elements in chondrites are in similar proportions to those of the Sun, implying
that they have altered little since they, and the Solar System, were formed.

One group of chondrites, the carbonaceous chondrites, are rich in carbon
and contain up to four or five per cent carbon by weight. Of this carbon,
more than 97 per cent is in the form of organic matter that was produced
during the early history of the Solar System. A very small proportion of
the carbon has survived from before the Solar System existed. This ancient
carbon carries minute quantities of other atoms, in particular atoms of
the inert gases neon, krypton and xenon. And since it has the chemical hallmarks
of its parent, cosmologists can find out what produced it – a nova or a
supernova, for example.

The first clue that meteorites were not quite what they seemed came
to light in 1969, when two cosmologists at the University of Minnesota,
David Black and Robert Pepin, studied the chemical make-up of some carbonaceous
chondrites. For years, astronomers and astrophysicists believed that the
Solar System – the Sun, Earth and other planets and the asteroids – was
formed when a hot, dense cloud of gas and dust collapsed to form the solar
nebula. They thought that the heat and turbulence of this process would
have mixed and homogenised all the starting materials, so that the Solar
System no longer bears any resemblance to them. (A simple analogy would
be baking a cake, where the cake bears no resemblance to the original ingredients.)
Black and Pepin were looking at proportions of different isotopes of inert
gases in chondrites by heating samples of meteorite and using a mass spectrometer
analysing the gases given off. Based on the current theory of how the solar
nebula was formed, they expected to find the element neon present in the
chond rites as a mixture of its three isotopes: neon-20, neon-21 and neon-22,
since this is the mixture we find elsewhere in the Solar System .

To their surprise, a small fraction of the neon was different from any
found previously: it was made up almost entirely of neon-22. But there was
very little of this unusual neon (about 5 parts per million), and Black
and Pepin did not immediately realise the significance of their discovery.

Then, in 1973, Robert Clayton and his colleagues at the University of
Chicago uncovered another isotopic anomaly. They found that the oxygen in
a carbonaceous chondrite called Allende has a very different mixture of
isotopes from the proportions we find in the well-mixed bodies of the Solar
System, including the Earth. Clayton concluded that this oxygen must have
survived from a period before the formation of the Solar System. This discovery
turned upside down the idea that all the elements and their isotopes had
been well mixed in the solar nebula. It also set the stage for detailed
studies of pre-solar materials.

The particular mixture of neon isotopes in carbonaceous chondrites was
the first hint that material formed before the Solar System survives. (To
continue the cake analogy, it was rather like finding a currant in the cake,
which remains unchanged by the mixing and baking, and is distinct from the
‘matrix’ of sponge.) But how had the unusual neon – called neon-E – managed
to survive for so long? Researchers suspected it might have been trapped
inside some sort of carbon-based material. This seemed plausible: because
it is inert neon would not have formed compounds but could still be carried
by some other material.

When, in 1974, a group of Swiss meteoriticists led by Peter Eberhardt
at the University of Berne looked more closely for neon-E in carbonaceous
chondrites they found it in low-density material, rich in carbon, which
released the neon when it was heated to low temperatures; and in high-density
material which released it at high temperatures. Two other inert gases,
krypton and xenon, also appeared as unusual mixtures of isotopes which could
not be explained by normal Solar System processes.

Partly as a result of this research, by 1980 most researchers agreed
that the Solar System had not formed from a homogeneous cloud of gas and
dust, in which there was a fixed ratio of isotopes for each element, but
from one that contained traces of material still ‘labelled’ according to
where they had originated: supernova, nova, or red giant star. (This would
be akin to finding out the countries of origin of the currants in the cake.)

It seemed likely that neon, krypton and xenon were trapped inside mineral
grains. Once researchers had found inert gases in carbonaceous chondrites,
they focused their efforts on the mineral carriers of the gases. If they
could find out the chemical and isotopic make-up of the carrier they would
be able to identify its origin – and that of the unusual isotopes of the
gases. This would tell them how many different types of stellar process
went into the formation of the Solar System.

The first carrier identified was that of xenon-HL (HL means ‘enriched
in both heavy and light isotopes’). In 1975, Edward Anders and his colleagues
at the University of Chicago recognised that xenon-HL’s carrier must be
made largely of either carbon or the mineral spinel. In 1983, Colin Pillinger
and his colleagues, first at the University of Cambridge and later at the
Open University in Milton Keynes, confirmed that the carrier of xenon-HL
was carbon-based and slightly enriched in carbon-12, although within the
range of carbon isotopes on Earth and from other meteorites. This implied
that the carrier could have formed in the Solar System. But the carrier
had another unexpected property – it contained a high concentration of the
isotope nitrogen-15, well outside anything the researchers had seen in material
either from Earth or from meteorites. It seemed that the grains of mineral
that carried xenon-HL were pre-solar remnants. Researchers at the University
of Chicago did more work and finally, in 1987, they identified the carbon
– tiny grains of diamond, just 3 millionths of a millimetre across.

Meanwhile, meteoricists in several laboratories in Europe and the US
were still trying to isolate the carriers of two other unusual components
in chondrites: a rare isotope of xenon, called s-xenon – and the puzzling
neon-E. They already knew the xenon must be locked in an extremely resistant
mineral, which they suspected might be spinel. But when they analysed the
Murchison meteorite (a carbonaceous chondrite available in the large quantities
needed for this research), which they knew to be rich in s-xenon, they again
found the xenon in carbon-based material. The isotopic mixture of this carbon
was well outside the range of normal Solar System materials, with more than
twice as much carbon-13 as normal. In 1987, Ernst Zinner at Washington University
revealed that the carbon-rich material was in fact grains of silicon carbide
(SiC).

Neon-E turned out to be trapped in two different carriers: a low-temperature
form, either amorphous carbon or poorly crystalline graphite, and a high-temperature
one, silicon carbide. These grains, like the silicon carbide that carries
s-xenon, are enriched in carbon-13, but in this case by up to 30 times the
usual amount for the Solar System.

Unusual mixtures of isotopes of inert gases, carbon and nitrogen are
not the only clues that make researchers think that some components of meteorites
were formed outside the Solar System. They have found strange isotopes of
several other elements such as magnesium, titanium, samarium, and calcium.
One light element which turns up in components of meteorites also rich in
carbon and nitrogen is hydrogen – its isotopes are also present in mixtures
that are well outside the ranges characteristic of the Solar System. Researchers
have found components rich in deuterium (hydrogen-2) in two types of meteorites:
carbonaceous and ordinary chondrites, although they have not isolated individual
components. In carbonaceous chondrites the deuterium is carried as part
of the organic material which accounts for most of the carbon. They think
the organic carrier took up the deuterium during reactions between ions
and molecules in interstellar clouds, before the Solar System was formed.

But sometimes the evidence gives cosmochemists even more problems: this
deuterium-rich water might be a truly pre-solar component; or it might have
been formed later by the breakdown of organic compounds in primitive parent-bodies
in our Solar System, such as asteroids. This is still unresolved.

Next, cosmologists tried to find out how the unusual mixtures of isotopes
and their carriers were formed. The particular mixture of isotopes in a
material contains a lot of hidden information about the formation process
and the prevailing conditions when elements were formed (see ‘The birth
of elements’, New ÐÓ°ÉÔ­´´, 16 December 1989). Carbon-12, for example,
is produced by helium-burning and is made in primitive stars, whereas carbon-13
can only be formed from a carbon-12 seed nucleus, for instance during hydrogen-burning
in the outer zones of large stars.

Neon highlights the link

The unusual neon-E in the chondrites is pure neon-22. Researchers knew
that this type of single-isotope neon is produced very rarely, mainly from
the decay of sodium-22 from novae. From the short half-life of sodium-22
(2.6 years), they thought the sodium must have condensed into grains very
rapidly, probably under explosive conditions, and once trapped, decayed
to neon-22. Since sodium-22, and therefore neon-22 (neon-E), was produced
from a nova, they thought the same nova could also have produced the carbon
grains that carry neon-E. The amorphous carbon that carries the low-temperature
form of neon-E contains nitrogen enriched in nitrogen-15, and novae are
the main producers of nitrogen-15 in the galaxy. Novae also have a high
carbon to oxygen ratio, which favours the early formation of elemental carbon.
So it is quite feasible, from the isotopic evidence, that the low temperature
neon-E and its carbon carrier were produced at the same time, in the same
nova.

There is not always such a clear link between inert gases and their
carriers. The carbon in the silicon carbide which carries the high temperature
form of neon-E is rich in carbon-13 (in some grains the ratio of carbon-12
to carbon-13 is as low as 2.8). This carbon-13 could have built up as the
result of a group of competing nuclear reactions – the CNO cycle – that
takes place in large stars, such as red giants. The CNO cycle is dominated
by three elements: carbon, nitrogen, including nitrogen-14, oxygen and hydrogen.
One way of finding out if the carbon in silicon carbide is produced in this
cycle would be to measure its ratio of isotopes of nitrogen. This nitrogen
is trapped, like neon, or replaces atoms of the mineral itself.

But this creates more question than answers. The carrier of high-temperature
neon-E turns out to have a very variable ratio of nitrogen isotopes, with
some grains rich in nitrogen-14 and others in nitrogen-15. Grains that are
rich in nitrogen-15 were more likely to have been formed in a nova than
as a result of reactions in the CNO cycle.

Some more clues to this puzzle come from the ratio of silicon isotopes,
which also varies. Last year, Edward Anders and Ernst Zinner measured individual
large grains of silicon carbide from the Murchison chondrite. They showed
that there must be at least five different stars that produce the mineral
grains, each one having its own particular balance of isotopes of carbon,
nitrogen and silicon. So it now seems that the silicon carbide, with its
high temperature neon-E, was formed in a complex way, perhaps under different
conditions, at different times, and in different stars.

Of the other inert gases, the formation of s-xenon is fairly straightforward
to explain. Cosmologists know it is produced by the slow capture of neutrons
in red giant stars. Silicon carbide that is rich in carbon-13, combined
with nitrogen rich in nitrogen-14, can also readily be produced in such
an environment, so the s-xenon in silicon carbide was probably produced
in a carbon-rich star.

The xenon-HL in diamonds, on the other hand, seems to have got there
by pure coincidence, since there is no single process that can enrich xenon
in both its heavy and its light isotopes to give xenon-HL. The process of
explosive nucleosynthesis in a supernova will enrich both isotopes – but
only as separate layers. The heavy isotopes (xenon-H) are made in the outer,
cooler carbon shell of a supernova, whilst the lighter xenon-L is produced
deeper inside the star, in a hotter environment.

One theory is that the two components must have become intimately mixed
during the explosions of the star, and as the gases expanded outwards from
the supernova, xenon-HL became trapped in grains of diamond. The diamonds
were formed earlier, during a relatively quiescent step in the star’s evolution
when gases leaked away slowly to outer space, leaving behind carbon-containing
particles. Nitrogen rich in nitrogen-14, which can replace some carbon atoms
in diamond (carbon and nitrogen atoms are of similar size) must have been
trapped as an intrinsic part of the lattice structure, as the diamond grains
were formed.

There are problems with this model. First, the type of star that would
generate carbon and nitrogen in the correct ratios of isotopes – a red giant
– would not be massive enough to form the type II supernova needed to produce
xenon-HL. Exactly how the diamonds form, and how they then incorporate xenon,
is not clear either. One idea is that the diamonds may have formed by collisions
between grains of amorphous carbon or crystalline graphite when they met
the supernova shock wave, xenon becoming trapped within the grains during
condensation of the diamonds. But there is a snag. For this to work, amorphous
carbon or crystalline graphite must be present as a collision nucleus, so
that some mixtures of diamond and either amorphous carbon or graphite would
also form. Neither mixture is found in meteorites and researchers now favour
a different theory called chemical vapour deposition, or CVD .

If this theory is correct, CVD diamonds were formed in the outer shell
of a carbon star, then were either swept away by the stellar wind or thrown
out by the star as it passed through its planetary nebula phase. Xenon-HL
produced in a supernova was travelling faster than the expanding cloud of
diamonds, and had enough energy to become implanted in them. The minute
quantities of diamond in meteorites give us a fascinating insight into the
ancient processes that went on in the pre-Solar Universe.

When, in 1805, Jane Taylor wrote the rhyme: Twinkle, twinkle, little
star, how I wonder what you are / up above the world so high, like a diamond
in the sky, she cannot have known how well she was predicting the discoveries
which would be made over a century and a half later.

* * *

How to measure stable isotopes

ISOTOPES to most people mean radioactivity. By emitting radiation, unstable
isotopes of elements such as uranium decay to stable ones. Most elements
have stable isotopes – forms of the element with the same number of protons
and electrons, but a different number of neutrons, and therefore a different
mass.

Carbon-12, for example, is the most abundant stable isotope of carbon
(98.89 per cent), and has six electrons, six protons and six neutrons; carbon-13
is a minor stable isotope (1.11 per cent), with seven neutrons, and carbon-14
is a radioactive isotope (0.001 parts per billion) with eight neutrons.

Carbon-14 is well known as a dating tool in archaeology. It is unstable,
and decays, by emitting a beta particle, to nitrogen-14 with a half life
of about 5730 years. In nature, carbon is a mixture of the stable

carbon-12 and carbon-13 isotopes, and carbon-13, with its extra neutron,
is called the heavier isotope.

Actual abundances of carbon-12 and carbon-13 are difficult to measure
directly, but relative differences in the ratio of carbon-12 to carbon-13
can be measured very precisely indeed. Researchers usually measure the ratio
of isotopes by mass spectrometry. First, they must convert the element into
a gas.

For example, to find out the ratio of carbon isotopes in diamond (a
form of carbon) they would convert it to carbon dioxide. More recently,
researchers have developed a technique for determining the ratio of isotopes
in individual grains from meteorites.

With this method, they direct a finely-focused beam of ions at the meteorite.
The beam knocks off ions from the surface of grains and the ions are then
analysed by a mass spectrometer.

Astronomers detect molecules such as carbon monoxide, hydroxyl and hydrogen
cyanide in the atmospheres of planets, stars, galaxies and nebulae by an
indirect method called infrared absorption spectroscopy. Different isotopes
absorb different amounts of electromagnetic radiation, causing shifts in
line spectra when they are recorded.

In this way researchers can find out the ratio of carbon-12 monoxide
to carbon-13 monoxide, for example, in the gas surrounding a star. These
measurements are less accurate than those made on solid samples using mass
spectrometry, but they still prove that materials formed outside the Solar
System generally have a wider variation in their ratio of isotopes than
those formed inside it.

* * *

A twinkling of diamonds in the sky

DIAMONDS occur in many types of meteorite. These diamonds are not formed
in the same way as natural diamonds on Earth. The diamonds in some groups
of meteorites are there because of high-temperature, high-pressure shock
such as that caused by collisions between asteroids in space, or by impact
of the meteorite on Earth. Diamonds formed in this way are relatively large
– between 0.1 and 1.0 millimetres across – and are embedded in the graphite
(another form of carbon) from which they were generated by shock transformation.

In 1987, Ed Anders of the University of Chicago and his colleagues isolated
diamonds from the Allende carbonaceous chondrite. These diamonds are small
– usually less than 0.000005 millimetres in diameter. They could not have
been formed in the same way as diamonds in other types of meteorite, since
the Allende meteorite is of a type that is unshocked – it shows no evidence
of collisions in space. The Allende diamonds must have been formed outside
the Solar System, even before the mass which would produce Allende was formed,
and they entered the Solar System as fully formed grains.

Now some researchers think they have found out how these tiny diamonds
were made. They think it was by a low-pressure process called chemical vapour
deposition (CVD), coincidentally used by industry to make artificial diamonds,
highly valued by industrialists for their hard cutting edge. Briefly, the
process involves mixing a source of carbon, usually methane, with hydrogen
(99 per cent), over a surface which speeds up the subsequent chemical reaction.
This catalyst must have its atoms arranged in the same way as diamond, and
may be a small ‘seed’ diamond that promotes the growth of larger ones. Frequently
it is a layer of silicon carbide. When the gases are ionised to a plasma
(by heat or some other high-energy source), small crystalline diamonds form
on the catalyst surface.

All the right conditions for small dia monds to grow in this way – low
pressures, a high concentration of hydrogen relative to carbon, silicon
carbide and a plasma – are found in the outer regions of stars and supernovae,
and it seems likely that this is where diamonds in carbonaceous chondrites
were made.

Monica Grady and Ian Wright are cosmochemists working on extraterrestrial
material at the Open University, Milton Keynes.