Ion Probe Measurements of Comet Dust: Investigating Oxygen Isotope Heterogeneity in the Solar System
- Author(s): Snead, Christopher Jorge
- Advisor(s): McKeegan, Kevin D
- et al.
The abundances of the stable isotopes of oxygen vary in terrestrial materials in ways that can be explained by mass-dependent fractionation. Refractory inclusions and chondrules in meteorites, however, have oxygen isotopic compositions that are suggestive of a mixing between isotopically separated reservoirs. Understanding the processes that produced 16O-rich and 17,18O-rich reservoirs has been a major objective of cosmochemical research for several decades. One complication of investigations into the nature of oxygen isotope heterogeneity has been the alteration of chondritic components on asteroidal parent bodies, which modify the original isotopic signatures of primordial dust. Comets accreted in distal cold regions of the solar nebula, and dust from comets probably experienced minimal parent body processing relative to asteroidal samples. Much of the dust collected in the stratosphere likely has cometary origins, but until the return of samples from NASA’s Stardust spacecraft, definitive links to comets had not been established. Stardust successfully returned particles from a known comet 81P/Wild 2, but the silica aerogel collectors severely altered the oxygen isotope compositions of the fine-grained dust component. Impacts of Wild 2 dust into aluminum foils produced craters that retained material as a melt residue, providing an opportunity to measure the oxygen isotopic composition of coarse and fine-grained components of comet dust.
This dissertation describes oxygen isotope measurements of Wild 2 impact crater residues via Secondary Ion Mass Spectrometry (SIMS). Hypervelocity experiments that simulated the collection conditions of Wild 2 dust were preformed using minerals of known oxygen isotope composition; the resulting craters were used to develop analytical techniques, to assess modification to the oxygen isotope composition due to hypervelocity capture, and as standards for oxygen isotope measurements of Wild 2 craters. This dissertation also describes the oxygen isotope measurements of interplanetary dust particles with hydrated mineralogy, in an attempt to observe 17,18O-enriched water that is predicted to be a consequence of some proposed mechanisms for producing observed oxygen isotope heterogeneity. Relationships between interplanetary dust particles, comet dust, and carbonaceous chondrites are examined, and implications for models of comet formation and oxygen isotope heterogeneity are discussed.