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 (albeit a bit ungracefully,
see http://www.genesismission.org/
for more details) for analysis in terrestrial laboratories. At UCLA,
we will analyze the GENESIS samples to determine the elemental and,
especially, the isotopic compositions of the captured solar
wind. The mission's highest priority regards the isotopic
composition of 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.
