Main presolar grain page
How do we identify presolar grains in meteorites?
We recognize presolar grains on the basis of their unusual isotopic
ratios. Isotopes are different atoms of the same chemical element that
have slightly different mass. For example, the element carbon has two
stable (not radioactive) isotopes: carbon-12 (12C) and
carbon-13 (13C). Most of the isotopes of most of the elements
are made by nuclear fusion inside stars, a process known as
nucleosynthesis. Although different stars produce different
relative proportions of the isotopes, the material from these stars gets
mixed up in space. Therefore, when our solar system formed, the
contributions from the different stellar sources were almost entirely
homogenized and the isotopic ratios of the elements are almost identical
throughout the solar system. (There are small variations due to chemical
and physical processes which do not concern us here.) Thus, carbon on the
Sun, Earth, Moon, Mars, and the other planets has about 89 12C
atoms for every 13C atom. Presolar grains, on the other hand,
contain the original atoms from their parent stars. These atoms did not
get mixed up with the atoms from other stars, so the isotopic ratios in
presolar grains are remarkably different from solar system stuff, and can
span a huge range. The following plot illustrates this diverse range. In
contrast to the uniform 12C/ 13C ratio throughout
the solar system, presolar silicon carbide (SIC) and graphite grains have
C isotopic ratios
that range from about 3 to 10,000!
By comparing the isotopic ratios measured in presolar grains to those
measured in stars by astronomers and those predicted by theoretical
models, we can identify what types of stars the grains formed in, and
(hopefully) learn more about how the stars work.
Noble gases: the discovery of presolar grains.
Until the 1960s, most scientists believed that the early solar system got
so hot that no presolar grains could have survived. However, in the
mid-1960s, researchers started finding evidence that some presolar
material might have survived. This evidence was unusual isotopic ratios of
the noble gases neon and xenon in certain types of meteorites. Because
these gases are very volatile, they are extremely rare in meteorites (any
heating of the meteorites tends to lead to the gases escaping). The fact
that the anomalous component survived suggested that the gases were
trapped in very refractory mineral grains. Throughout the 1970s and 80s,
scientists, especially Ed Anders and his co-workers at the University of
Chicago, attempted to isolate the carrier grains of the anomlous gases.
They did this mostly by trial-and-error, dissolving the meteorites in a
particular acid and seeing if the remaining solid residue still had the
gases. If it did, the residue would then be attacked further. Finally, in
1987, in collaboration with Ernst Zinner and his colleagues at Washington
University in St. Louis, the Chicago group succeeded in identifying
diamond and silicon carbide as presolar carriers of the noble gases. In
addition to the isotopically unusual noble gases, these grains were found
to have unusual isotopic ratios in other elements as well. Since then,
several more types of presolar grains have been identified in the same
acid residues of meteorites (see Presolar grain
types). The Chicago method of isolating presolar grains is still used
today. However, it has often been described as "burning down the haystack
to find the needle." In fact, we are probably lucky that among the types
of dust grains that stars produce are very strong, acid-resistant phases
like diamond and SiC. Most likely, there are more fragile presolar grains
in meteorites that we dissolve away with our current methods ("hay"
grains) and we just have not yet figured out a good way to identify them.
Next (Presolar grain types)
Main presolar grain page.
© Larry R. Nittler Last modified February 3, 1999