The Sulfur Isotopic Composition of the Sun

The Solar Wind Ion Mass Spectrometer (SWIMS) on the Advanced Composition Explorer (ACE) has successfully measured the isotopic composition of sulfur in the solar wind. Preliminary analysis yields ³⁴S/³²S ≈ (4.3 +/- 0.6)% which is in good agreement with the meteoritc value of ³⁴S/³²S = 4.43%. As opposed to elemental composition which can be very accurately measured spectroscopically, the isotopic composition of sulfur on the Sun cannot be directly measured. Solar wind measurements are thus the only means of determining solar isotopic abundances. For refractory elements the meteoritic isotopic composition is thought to represent the solar composition very well. This has been verified with measurements of Mg and Si in the solar wind. In contrast to Mg and Si, sulfur is a volatile element, and as such is especially susceptible to processes that alter its isotopic composition, be it during the formation of the solar system, or during the history of the sample being studied. Thus the determination of its isotopic composition in the Sun can yield valuable information on the original sulfur isotopic composition in the protosolar nebula and on possible fractionation mechanisms active during the formation of the early solar system. We will compare the solar values of ³³S/³²S and ³⁴S/³²S with values derived from meteoritic samples, for the GCR, the interstellar medium, and from SEPs.