An important goal of planetary science is to understand how the tremendous diversity of objects in the solar system, from asteroids and comets to moons and planets, developed from a relatively well-mixed primordial solar nebula. From a cosmochemical viewpoint, one would like to know, in detail, what the average composition (elemental and isotopic) of this material was in order to better model the chemical and physical processes that led todifferentiation. Since the Sun contains 99.8% of the mass of the solar system, it retains the average composition. In practice, "solar" elemental abundances are usually inferred from laboratory analyses of chondritic meteorites because such measurements can usually be made with higher precision and accuracy than spectroscopic observations of the solar photosphere. However, this approach is problematic for volatile elements which are known to be fractionated, both elementally and isotopically, in various classes of primitive meteorites. A major advance would be to base solar compositions on the analysis of solar material, and to do so with a precision and accuracy useful for cosmochemistry. This is the goal of NASA's GENESIS Discovery mission.
Launched in August, 2001, GENESIS cruised to the Lagrange point L1, outside the Earth's magnetosphere, where it collected solar wind for 2+ years by implantation into ultra-pure materials. It then returned these captured solar particles (total mass 400 g, most of it H) to the Earth for analysis in terrestrial laboratories. At UCLA, we will analyze the GENESIS samples to determine the isotopic composition of the captured solar wind. The mission's highest priority among elements is oxygen. Oxygen is the most abundant element in the rocky planets, yet our understanding of the solar oxygen isotopic composition is poorly constrained and highly model-dependent. Oxygen is known to exhibit isotopic anomalies of uncertain origin that are manifest in materials from the inner solar system on all spatial scales, from microns to planets. Knowledge of the bulk solar system oxygen isotope composition would be highly useful in constraining models for the evolution of dust and gas in the solar nebula to form planetary material.
A problem is that there is currently no instrument on Earth that can perform precise isotopic analyses of the small amount of implanted solar wind oxygen brought back by GENESIS. Aside from sensitivity requirements, analytical difficulties include contamination by ubiquitous oxygen on collector surfaces and the fact that most of what was collected is hydrogen, which leads to molecular ion (hydride) interferences in the mass spectrum. To solve this problem, professor Kevin McKeegan and his research colleagues Dr. Peter Mao, Dr. Tak Kunihiro, and ESS engineer George Jarzebinski are building a new mass spectrometer of a unique design. The new $4M instrument, dubbed the MegaSIMS, combines an ion microscope (the "SIMS" part) with a tandem million-volt particle accelerator (the "Mega" part) and a high energy multicollector mass spectrometer. It is an axiom that to measure a small sample, one needs a large instrument, and indeed the MegaSIMS fills the old ESS machine shop. After completion of initial tests and the anticipation by the coming of "a little bit of the Sun" to southern California (literally!), the MegaSIMS have been operational and have produced interesting results thus far. These reports and updates can be found in the Laboratory's Archive section.