Thematic Articles

The planets of the Solar System grew by collisions, starting with the aggregation of tiny dust particles within the solar nebula and culminating in giant collisions between large planetary bodies. These giant impacts occasionally caused the formation of satellites such as the Earth’s Moon. Our understanding of planet formation is based on information from various sources, including meteorites – leftovers from the earliest stages of planet formation – and samples from the Earth and Moon. By combining results from isotopic dating of these materials with dynamic modelling of the solar nebula and planet formation, researchers can reconstruct the accretion and early evolution of planetary bodies during the first ~100 million years of Solar System history.

The early Solar System contained a wide range of abiotic organic compounds. As the Solar System evolved, these organic molecules were incorporated into planetesimals and eventually planetary bodies, such as the parent bodies of meteorites. One particular class of meteorites, the carbonaceous meteorites, contains a large variety of extraterrestrial organic compounds. These compounds represent a record of the chemical reactions and conditions in the early Solar System. Different formation mechanisms and sources (interstellar, nebular or parent body) contributed to the inventory of meteoritic organic molecules. Their subsequent delivery to the early Earth may have contributed the first prebiotic building blocks of life.

Recent results from the Stardust comet sample-return mission have confirmed the idea that there is a continuum between primitive small bodies in the outer main asteroid belt and comets. Indeed, the mineralogy as well as the chemical and oxygen isotope compositions of the dust from comet Wild 2 are very similar to those of carbonaceous chondrites, a class of meteorites allegedly derived from primitive, dark asteroids. Comets no longer represent extremely primitive samples of the early Solar System that are radically different from dark asteroids. We enter a new era in which comets and their siblings, the dark asteroids, are seen as a collection of individual objects whose geology can be studied. The most primitive of these objects, i.e. the ones that escaped thermal metamorphism or hydrothermal alteration, can help us decipher physicochemical processes in the interstellar medium and in the protoplanetary disk from which our Solar System formed.

Stable isotopes record the evolution of planetary systems, beginning with stars coalescing from molecular clouds, followed by the nucleosynthesis of elements in stars, and proceeding to the accretion and differentiation of planets. Current stable isotope measurements range in scale from isotopic mapping of the Milky Way Galaxy with spectrographs on telescopes to the analysis of stardust with ion probes

Extraterrestrial samples include a rich variety of materials with different histories. Among the array of Solar System materials are tiny grains with extremely anomalous isotopic compositions—records of nucleosynthetic processes that occurred deep within their now extinct parent stars. The isotopic and mineralogical characterization of these presolar grains in the laboratory provides unprecedented insight into stellar and galactic evolution, nucleosynthesis, and dust formation and processing. The discovery of presolar grains has opened up a pivotal new dimension in the field of astrophysics. Coupled with astronomical observations and astrophysical studies, stardust analyses bring nanometer-scale detail to the history of our immense Galaxy.

Cosmochemistry is the study of extraterrestrial materials aimed at understanding the nature of Solar System bodies, including the planets, their natural satellites, and small bodies. An important goal is to increase our understanding of the chemical origin of the Solar System and the processes by which its planets and small bodies have evolved to their present states. Research in cosmochemistry covers an enormous range of disciplines and techniques, including mineralogy, petrology, major and trace element chemistry, isotope compositions, radiometric ages, magnetism, and radiationexposure effects. These studies provide a wealth of data about the processes of stellar evolution, planetary-system formation, alteration in asteroidal and cometary interiors, and the accretion history of the Earth, including the origin of Earth’s volatile and organic materials.