Comets are frozen reservoirs of dust and gases that were present in the early solar nebula. Specimens of cometary material, especially those from Comet Wild 2 -- collected by NASA's Stardust mission, the first to return samples of a comet directly to Earth from outer space -- preserve records of the primordial solar system's chemicals, minerals, and elemental isotopes, for those who have the tools to interpret them. Many Wild 2 samples have been analyzed at the Advance Light Source by Berkeley Lab scientists and users from other institutions, including UC Berkeley's Space Sciences Laboratory and Lawrence Livermore's Institute of Geophysics and Planetary Physics.
The latest analyses of Wild 2 samples to benefit from access to the ALS were performed by groups in the United States and France, led by physicist Robert Pepin of the University of Minnesota. They published their results in the Jan 4 issue of Science.
Pepin is an expert in the origin and early history of the solar system's volatile elements and compounds, as revealed by the noble gases in meteorites and lunar samples. In the Wild 2 samples, he and his colleagues set out to measure abundances of helium and neon to see what processes the cometary material had undergone before the comet settled into its long, cold orbit. As he explained in an e-mail to co-author Saša Bajt, formerly of Livermore's Geophysics Institute and now at DESY in Germany, "Many folks, including some of the [paper's] reviewers (and some of its authors), expected to find the gases sited in something like the meteoritic Q-phase."
The Q-phase, named for "quintessence," is a residue of organic molecules that remains after the minerals in a sample of a meteorite have been dissolved away; the Q-phase is rich in noble gases from very early in the history of the solar nebula and possibly predating it. By determining the isotope ratios of these rare, ancient gases, scientists hope to learn how they were formed and thus distinguish among competing models of the formation of the solar system.
Fourier Transform Infrared (FTIR) measurements were carried out by Bajt at beamline 1.4 at the Advanced Light Source, where she has worked with Michael Martin on many previous researches into the organic constituents of Comet Wild 2. Her task was to find out what organics were present in the samples tested by Pepin and colleagues. The answer: none.
"Thanks to your FTIR measurements," Pepin said to Bajt, "we're forced to the conclusion that the carriers must be the refractory metal-metal sulfide-metal carbide grains.... The Q-carrier of the gases in meteorites is known to be organic, and there are essentially no organics in our samples, so the only possibility left is the grains themselves. That's the first-order finding of the paper, and it's a rather startling one."
"Helium and Neon Abundances and Compositions in Cometary Matter," by Bernard Marty et al, appears in the January 4 issue of Science. For more about research into the composition of Comet Wild 2 at the ALS, go here. For more about Comet Wild 2's organics, go here.
-- Paul Preuss
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