Measuring Oxygen Isotopes with COSIMA

Abstract:

Oxygen isotopes in a variety of solar system solids show non-mass-dependent fractionation, i.e. are fractionated along a slope = 1 line in a three isotope plot, rather than the equilibrium fractionation line whose slope is close to 0.5 (Clayton, 1973). Many models have been put forward to explain this observation, such as galactic chemical evolution (Clayton, 1988), photochemical self-shielding (Thiemens and Jackson, 1987; Clayton, 2002; Yurimoto and Kuramoto, 2004; Lyons and Young, 2005), quantum chemical explanations (Hathorn and Marcus, 1999, 2000; Gao and Marcus, 2002; Marcus, 2004), the processing of solids via nebular lightning (Nuth et al, 2011), and others. Some of the models were invalidated when the Genesis results showed that the oxygen isotopic fractionation of solar wind (and hence of the Sun) was relatively much richer in ¹⁶O than such bodies as the Earth or the Moon. Whatever the process that produced non-mass-dependent fractionation in some chondrules and calcium aluminum inclusions, its signature may also be detectable in other solar system solids. If at least some cometary dust was produced in the inner nebula and only later transported outward to be incorporated into comets, then such dust may also show some degree of non-mass-dependent fractionation.
The COSIMA instrument on the Rosetta spacecraft (Kissel et al 2009) is a secondary ion mass spectrometer designed to measure the composition of cometary dust. Using calibration data from the COSIMA reference model and flight data if possible, measurement all three isotopes of oxygen will be attempted, and the results compared to other solar system bodies.