The isotopic composition of oxygen in the fast solar wind: ACE/SWIMS

Oxygen is the most abundant element in the solar system after H and He, and the lightest element with three stable isotopes. Oxygen is a volatile elem ent. it was not retained completely in meteorites or in their predecessors during their formation in the early solar system. The solar oxygen isotopic composition is important because the Sun is by far the largest reservoir o f oxygen in the solar system. Oxygen occurs in highly volatile phases of th e solar system, such as water. as well as in refractory phases, for example in silicates. Because of this, the differences between the isotopic compos ition of oxygen in solar-system bodies and the Sun are a sensitive measure of the gas to dust ratio in the early solar system. We report the first mea surements of the oxygen isotopic composition in the fast solar wind with th e Solar Wind Isotope Mass Spectrometer (SWIMS) on the Advanced Composition Explorer (ACE). The fast solar wind is the least fractionated type of solar wind. We obtain an isotopic abundance ratio O-16/O-18 of 446 +/- 90 which is (within the quoted uncertainties) consistent with the terrestrial value of 498. However, in combination with previously published values for the ph otosphere (O-16/O-18 = 440 +/- 50 [Harris et al., 1987]) and the isotopical ly more fractionated slow solar wind (O-16/O-18 = 450 +/- 130 [Collier et a l., 1998]), this new determination may suggest that the Sun is isotopically heavier than terrestrial.