The discovery of crystals of titanium carbide in an interstellar graphite spherule is reported. The new species is particularly interesting in that it came in a protective wrapping (the graphite spherule) which eliminated the possibility of chemical alteration during its residence in the interstellar medium and in the meteorite in which it was discovered

Amari, Sachiko

Bernatowicz, Thomas J.

Lewis, Roy S.

Zinner, Ernst K.

English

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LPSC XXII Press Abst iiN91-19998
INTERSTELLAR GRAINS WITHIN INTERSTELLAR
GRAINS; Thomas J. Bernatowicz I, Sachiko Amari2,1, Ernst K. Zinner I and Roy S.
Lewis2; 1McDonnelI Center for the Space Sciences & Physics Department, Washington
University, St. Louis MO 63130-4899, USA ; 2Enrico Fermi Institute & Dept. of Chemistry,
University of Chicago, Chicago IL 60637-1433, USA.
In this paper we report the discovery of crystals of titanium carbide in an
interstellar graphite sphemle. The titanium carbide is another species in the
growing list of interstellar grains which have been discovered in chemically
processed samples of primitive meteorites. The new species is particularly
interesting in that it has come to us in a protective wrapping (the graphite
sphemle) which has eliminated the possibility of chemical alteration during its
residence in the interstellar medium and in the meteorite in which it was
discovered. It thus looks today just as it did when it formed in the atmosphere
of some carbon-rich star, at a time before the sun and the planets came into
existence.
For more than a haft century astronomers have been aware of the
presence of interstellar dust from its obscuring effects on the light from stars.
From various kinds of studies they have also been able to divine some notion of
its composition and panicle sizes. Large clouds of interstellar dust and gas can
gravitationally collapse to form new stars. Nuclear reactions within these new
stars alter the composition of the original gas and dust constituents, and stellar
winds spew this altered material back into the interstellar medium. Thus, the
chemical composition of the galaxy evolves with time.
Cosmochemists have long known that the sun and the planets owe their
particular compositions to a mixture of interstellar gas and dust, but it was
thought until fairly recently that all vestiges of the original constituents had
been erased by complete homogenization of the dust component very early in
the formation of our solar system. The evidence for this idea was based on the
observation that the isotope composition of the more abundant elements was
the same regardless of whether one was making measurements on a terrestrial
rock, a moon rock or a meteorite. Any differences that were observed were
usually explicable as the result of well-understood nuclear and chemical
processes that occur within the solar system. However, some poorly
understood variations in isotope composition occurred in the noble gases,
which are generally present in very low abundances in solid materials. It was
in an effort to isolate the mineral carriers of these noble gases that Edward
Anders and his colleagues at the University of Chicago began nearly two
decades ago to chemically treat primitive meteorites in order to dissolve away
unwanted minerals and concentrate the carriers of unusual neon and xenon.
These chemical processing procedures eventually led to the discovery of
meteorite grains which could unambiguously be pronounced as stardust --
grains which condensed in the atmospheres of diverse types of stars, were
https://ntrs.nasa.gov/search.jsp?R=19910010685 2020-03-19T18:27:19+00:00Z
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INTERSTELLAR GRAINS; Bernatowicz et al.
expelled from these atmospheres and existed for a time as interstellar grains,
and which f'mally were incorporated into the interstellar cloud from which our
solar system formed, but nonetheless had survived all of these tumultuous
events.
To date, small interstellar grains of diamond, silicon carbide and graphite
have been found. It is fair to ask how we can be confident that these grains
really are stellar condensates, and not simply minerals that formed in our own
solar system. Because of advances in microanalytical techniques, it is possible
to measure the isotope composition of some abundant elements in individual
grains, many of which are so small as to be invisible to the naked eye. We find
that the compositions of some are dramatically different from the average solar
system composition. For example, in typical solar system material, variations
of a few percent in the proportions of the two carbon isotopes 12C andl3c
would normally be considered large; but in individual interstellar carbon grains
the proportions can be as much as fifty times the solar system average value.
This difference is comprehensible if we consider that carbon in the solar
system represents an average of carbon from many different stars, while an
individual interstellar graphite grain represents only the carbon from one
particular star, which need not have carbon isotope abundances similar to this
average.
In our current work we have combined isotope studies of individual
interstellar graphite grains from the Murchison meteorite with studies of the
interiors of these grains with the transmission electron microscope (TEM).
These grains are very small (only a few thousandths of a millimeter in size),
yet after isotope measurements it is nonetheless possible to pick up a particle
with delicate apparatus, imbed it in a special hard resin, and slice it, using a
microtome with a diamond blade, into dozens of wafers, each only several
hundred atoms thick. This is necessary because, even though the particles are
very small, they are too thick to be studied in the TEM. Once such slices have
been obtained, it is possible to view features of the grain even down to the size
of individual layers of atoms.
The graphite particles are roughly spherical in shape, and have two basic
kinds of intemal structure. In one type, the graphite is arranged in circular
layers, much like the inside of an onion. In the other type of interstellar
graphite, the structure is that of a ball of small scales. Both kinds of structure
are peculiar, and not observed in terrestrial graphite. An unexpected but very
exciting discovery was the presence of small crystals, a hundred or so atomic
diameters in size, within one of the scaly graphite particles. In addition to
getting pictures of the crystal lattice structure of the included crystals, we
could also study their composition and crystal structure in the TEM, and from
these studies we learned that the crystals are titanium carbide, a mineral not
previously found in meteorites.
LPSC XXII Press Abst i3
INTERSTELLAR GRAINS; Bernatowicz et al.
Chemical equilibrium calculations made in 1978 by Lattimer, Schramm
and Grossman at the University of Chicago had in fact predicted that these
two minerals, graphite and titanium carbide, would be the first to condense in
the atmosphere of a carbon-rich star. According to their calculations, graphite
should condense first, followed by titanium carbide, at temperatures of 1200-
1700 degrees Celsius. The fact that we observed titanium carbide crystals
inside of the graphite must mean that the growth of graphite was somewhat
delayed from what calculations based on chemical equilibrium suggest--an
effect which had also been predicted by some astronomers. From studying the
internal structure of the interstellar graphite sphemles and the included titanium
carbide we can make some educated guesses about the formation of the
spherules. First, they probably formed at high temperatures and were the first
grains to condense in their particular stellar atmospheres; second, they may
have formed relatively rapidly (possibly in times measured in days, for
example), since the graphite layers are often not very well developed, as they
would be expected to be if condensation proceeded slowly; third, the grains
may have been expelled from their stars to regions of substantially lower gas
pressure (interstellar space?), since we don't find the other minerals which
would have condensed after graphite and titanium carbide.
No one has yet observed titanium carbide by conventional astronomical
measurements of stars or the interstellar medium. This is perhaps not
surprising, since this mineral comprises only a few parts per million of one of
the graphite spherules we studied. But it points out that the laboratory study of
interstellar grains extracted from primitive meteorites may yield a far richer
and more complete picture of how such grains form than can be gotten from
astronomical study alone. The laboratory observations also pose interesting
challenges to theoreticians who model the chemistry of stars and the formation
of solid grains in their atmospheres.



Instellar grains within interstellar grains

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