Early Solar System Chronology
One of the main themes of my research is to understand the timing of events in the early solar system through the investigation of short-lived radionuclides. Short-lived radionuclides were present in the early solar system, but because of their short half-lives have completely decayed away. Evidence of their former presence is found as excesses of the stable daughter nuclides in various early solar system objects. The nuclides that are currently most useful for chronology are 26Al and 53Mn. The relative chronologies inferred from these nuclides can be put on an absolute time scale by comparing with the 207Pb-206Pb long-lived chronometer.
A major current effort is to develop 60Fe as an early solar system chronometer. This nuclide seems promising for studies of the timing of chondrule formation and the timing of aqueous alteration of meteorite parent bodies.
The presence of 60Fe in primitive meteorites also permits us to study the environment in which the solar system formed. 60Fe is only produced in significant quantities by stellar nucleosynthesis, and type II supernovae are a prolific source of 60Fe. By comparing initial abundances of 60Fe, 26Al, 53Mn and other nuclides, we can potentially infer the stellar source for these nuclides and the environment in which the solar system formed.
Presolar Grains: The Raw Materials for Solar System
Another theme of my research is to investigate the materials that came together to form the solar system. Presolar grains are micron-sized dust particles that existed in interstellar space before the solar system formed. Some of them survived solar system formation to be incorporated into meteorites and other primitive solar system materials. Most of the known types of presolar grains (including silicon carbide, aluminum oxide, spinel, graphite, forsterite, enstatite, and diamond) formed in the ejecta of dying stars. Each grain carries a snapshot of the isotopic composition of the stellar ejecta and thus provides a probe of nucleosynthesis within the parent star.
Because presolar grains have a wide range of resistance to chemical and thermal processing, they can also be used as tracers of processes in interstellar space and in the early solar system. One of the more far-reaching conclusions from work or relative abundances of presolar grains is that chemical variations between different classes of meteorites that arose via volatility-based fractionations apparently reflect processes that operated on the dust inherited from the Sun’s parent molecular cloud, not on a gas reservoir produced by completely evaporating the presolar dust.
Processes and Material Reservoirs in the Early Solar System
The chemical, physical, and isotopic properties of primitive solar system materials provide a variety of clues to the conditions and processes in the early solar system. For example, oxygen isotopes provide a powerful tracer of material reservoirs, although we do not yet fully understand the nature of the material reservoirs and how they interacted. Trace elements are an important tracer of both volatility controlled and igneous fractionation processes. We investigate a wide variety of different materials in an attempt to understand their history.
Samples from the Stardust Mission
The Stardust Mission was sent to comet WILD2 to collect samples of comet dust and return them to Earth. The Cameca ims 1280 ion microprobe at the University of Hawaii can make isotopic measurements on grains as small as a few microns across while they are still embedded in other materials. We have developed procedures to measure oxygen isotopes in Stardust samples and are working on measuring other isotopic systems. These data will help us to understand the nature of the material that makes up comets and the processes that affected that material.
Solar Wind from the Genesis Mission
The Genesis Mission was carried out to collect clean samples of solar wind and return them to Earth for analysis. We have been working on the bulk solar wind collectors and have measure the hydrogen fluence and the nitrogen isotopic composition of the solar wind trapped in these collectors. We will be working to acquire data for a variety of additional elements in the coming years.
UH W. M. Keck Cosmochemistry Laboratory