Physics and Astronomy Colloquium

Comets are frozen, largely unaltered reservoirs of dust and gases present in the early solar nebula. They are likely to contain well-preserved records of the chemical, mineralogic, and isotopic character of primordial solar-system matter. On January 15, 2006, the Stardust Mission returned to Earth with a cargo of particles collected from the coma of comet Wild 2, the first samples of indisputably cometary matter available for laboratory study. Among these investigations, the noble gases provide unique data on contributions to comets from various solar-system volatile reservoirs, and of physical processing of gases acquired from these reservoirs. In this talk we discuss the first measurements of helium and neon in Stardust material.

One of the surprises in samples collected from an icy object forming, and, for most of its lifetime, residing in the cold outer reaches of the solar system was the discovery in other laboratories that many of its constituent particles are igneous, refractory “rocklets” formed at very high temperatures, presumably close to the early Sun, which were then somehow transported to the trans-Neptunian Kuiper Belt and incorporated into Wild 2 at about the time of the solar system’s birth 4.5 billion years ago. (In retrospect the “rocklets” shouldn’t have been all that astonishing: Ed Ney, Louis Rose and others at Minnesota argued three decades ago from IR spectroscopic data that igneous grains were present in comets). A second and completely unanticipated feature of Stardust matter is the finding here of enormous concentrations of He and Ne that, of known gas acquisition mechanisms, only intense ion irradiation seems able to explain. These two observations, together with isotopic data pointing to Ne similar to that found in primitive meteorites, suggest that gases in Stardust grains were implanted from an ancient, energetic nebular reservoir near the young evolving Sun.