Presolar grains in meteorites: matter from other stars

Meteorites are the best available witnesses from the birth of the sun and its planets. This is because meteorites are samples from asteroids, relatively small bodies which never got as hot as the planets, and which therefore largely retain their original composition. We have an extensive research program on noble gases in meteorites and their constituents, with the goal to better understand the early history of the solar system.

Our recent efforts concentrate on measuring noble gases in individual tiny "presolar grains" extracted from meteorites. While almost all matter in meteorites and planets must have been very well homogenized in the "solar nebula" out of which asteroids and planets formed, the presolar grains avoided this homogenisation. As their name implies, they condensed in the cooling envelopes of dying stars in the vicinity of the nascent sun, and were later incorporated into the meteorite parent bodies without having been molten again. We know this because presolar grains have very unusual and diverse isotopic compositions in all elements. For example, the ratio of carbon-12 to carbon-13 covers the entire range between 1 to 10'000 (carbon 12 is an atom containing 6 protons and six neutrons, whereas carbon 13 also contains 6 protons but 7 neutrons). Because chemical elements are produced in the very hot interior of stars (from lighter elements), each presolar grain therefore retains "fingerprints" of the element-forming process in its particular parent star. Presolar grains are thus very valuable to study the details of how elements formed.

Noble gases have been instrumental in detecting and isolating presolar grains in meteorites, but it is very difficult to measure the tiny amounts of noble gases present in one grain of about 1 micron (0.001 mm) size. We use a mass spectrometer with a very high sensitivity, and we melt the grains with a laser to extract the noble gases. Our goal is to study what fraction of the grains contain helium-4 or neon-22, and to look for correlations between the noble gas data and other isotopic ratios, such as 12C/13C, 18O/16O and 26Mg/24Mg. Such combined investigations help us to constrain the types of parent stars of the grains, for example whether a certain grain condensed in the cooling envelope of a so-called Red Giant Star or a Supernova. The elements carbon, oxygen and magnesium are measured by a sophisticated instrument called NanoSIMS by colleagues in two other research institutions, the Max Planck Institute for Chemistry in Mainz, Germany and the Washington University in St. Louis, USA. These colleagues also supply the grains.