The early history of the solar system recorded in dust grains
Recent astrophysical observations of young stars and of their accretion disks demonstrate that the first few million years is a key period for the formation of the first grains and first planets. Samples of this early period, when the Sun was not yet a nuclear furnace, are fossilised in meteorites. The aim of the CEMYSS (Cosmochemical exploration of the first two million years of the solar system) project was to investigate the early evolution of the solar system using high-precision and high-spatial resolution ion microprobe measurements of the isotopic composition of meteorites and other extraterrestrial samples. Measurement with nanometric spatial resolution allowed determination for the first time of the isotopic composition of solar wind nitrogen implanted in the collectors of the NASA GENESIS mission. This revealed that the Sun, and by inference the nebular gas, is different (14N-rich) in isotopic composition from terrestrial planets and meteorites. This indicates large isotopic variations in the accretion disk, a result of processes that are not yet understood. Short-lived radioactive nuclides (SLRs) are additional powerful tracers for the birth of the solar system, of the dynamics of the disk, and of the interactions among the active early Sun, the nebula gas and the first solids. Measurements of trace isotopes by large-radius high mass-resolution ion microprobes made possible determination of the abundance and distribution in the accretion disk of several key SLRs. 60Fe, for example, is a tracer of the incorporation in the forming solar system of presolar stellar products, and 10Be is a tracer of the irradiation of the accretion disk by the cosmic rays emitted by the young active forming Sun. A major effort of the CEMYSS project was devoted to study of the distribution of 26Al, which is the most important of the SLRs, since it can provide the most precise chronology of all the processes that convert gas to planets. Multi-collector large-radius ion microprobe measurements of the Mg isotopic composition (26Mg is the decay product of 26Al) suggest that accretion of large objects started very early, just a few hundred thousand years after the start of the solar system through nebula collapse.
Keywords
Solar system, dust grains, meteorites, young stars, accretion discs, CEMYSS
