The Treasure in Meteorites

American-German research team finds interstellar organic materials in meteorites

Once again, meteorites are turning out to be scientific treasure chests. Primitive meteorites do not only contain stellar dust that formed at high temperatures- they also contain traces of pristine organic substances, often in their original state, or sometimes just slightly altered. They built up at low temperatures in the interstellar gas and dust cloud that formed our solar system billions of years ago. These are the results of a study by researchers at the Carnegie Institute in Washington, Harvard University in Cambridge, and the Max Planck Institute for Chemistry in Mainz, Germany (Science, May 5, 2006).

Some of these interplanetary dust particles seem to come from comets. They contain comparatively large amounts of stellar dust and organic material of interstellar origin. That material could be an important source of prebiotic molecules, building blocks for life on earth. We can also find presolar material in primitive meteorites, which originate from the asteroid belt. However, in the case of meteorites, it seemed until now that pristine organic material was not preserved as it was in interplanetary dust particles. Rather, the parent bodies and the original solar nebula went through thermal change, and we have lost information about the carriers of the pristine organic material.

The international research team from the Carnegie Institute in Washington, Harvard University and the Max Planck Institute for Chemistry in Mainz has identified largely unaltered organic material of presumably interstellar origin in meteorites. The scientists were looking at specific hydrogen and nitrogen isotope signatures in a kind of primitive meteorites called carbonaceous chondrites.

The magazine Science reports that the researchers found strong, localised concentrations of the rare deuterium isotope, plus nitrogen-15, in insoluble organic materials from carbonaceous chondrites. These isotopes had only until now been found in such quantities in interplanetary dust particles. Peter Hoppe from the Max Planck Institute for Chemistry explains that "we may be able to explain these signatures qualitatively through ion-molecule reactions at low temperatures in interstellar clouds." The researchers used two special ion microprobes in Washington and Mainz. The one in Mainz, the NanoSIMS ion microprobe, (a "secondary ion mass spectrometer") lets scientists look at isotopes at a spatial resolution finer than 100 millionths of a millimetre (see image). The measurements showed that the deuterium and nitrogen-15 concentrations are not spatially correlated - and this suggests they were created differently and had different organic carriers.

The results suggest that interstellar organic material found its way to the asteroid belt at a time when its temperature was relatively low. We can find pristine organic materials not only in interplanetary dust particles but also in meteorites, and this means that much larger amounts of this valuable cosmic material is available for laboratory research. This allows scientists to look in new ways at how organic substances are created - and other chemical processes - in the interstellar medium and in protoplanetary systems.