Solar wind helium, neon, and argon in Genesis aluminum collectors.

摘要

The Sun contains over 99% of the mass of the solar system and so, to fully develop a model of how the solar system formed and evolved what the starting composition was and how it evolved, it is crucial to know the isotopic composition of the sun. The Genesis mission collected samples of solar wind (SW) for 853 days and returned them to Earth for analysis. Making these measurements on earth-based instruments is currently the only way to get sufficient precision to differentiate between different solar system components, and SW is the only source of solar material available for sampling. However, there are several processes that have the potential to significantly alter the composition between the time when SW ions are accelerated away from the sun, to the time the laboratory measurements are made. This work attempts to constrain these sources of fractionation and present the best estimate of the isotopic composition of SW helium, neon, and argon implanted into two different aluminum SW collectors on board the Genesis Mission, Al on sapphire and polished Al.;First, during the collection phase of the Genesis mission, diffusion can alter the initial implantation profiles of the SW ions in the collector targets and cause losses of shallowly implanted species. These losses preferentially affect the lighter isotopes, which in turn means the measured ratios of the remaining reservoir will be heavier, both isotopically and elementally. I have conducted a diffusion experiment on a similar time scale as the Genesis mission to determine the diffusion parameters of the two different aluminum collector materials and to quantify the changes in the measured ratios due to diffusive losses for the light noble gases. The results of this experiment show that the polished Al collector is not sufficiently retentive of the light noble gases to be a reliable collector for the light gases, but that the composition of the light gases implanted in the Al on sapphire collector does not show a measurable effect due to thermal diffusion.;Isotopic fractionation can also occur even before implantation of the SW ions, if the processes which accelerate the SW away from the sun are mass-dependent. In an effort to quantify this effect, the Genesis mission collected separate samples of different types ('regimes') of SW: low-speed, high-speed, and coronal mass ejections, in addition to collecting bulk SW. Compositional differences between the different SW regimes (especially the low-speed and high-speed SW) are thought to provide a measure of this fractionation. By making high-precision isotopic measurements on collectors of the three SW regimes, we have put strict upper limits on the difference between the low-speed and high-speed SW regimes: 20Ne/22Ne 0.24 +/- 0.37% and 36Ar/ 38Ar 0.11 +/- 0.26%. Both of these differences are less than 1sigma statistical errors. Helium isotopes are much more susceptible to modification which prevents us from putting a strict upper limit as for Ne and Ar.;And finally we have made isotopic measurements of the light noble gases of the bulk SW (without selective collection of different SW regimes) from the aluminum collectors. Accounting for the sources of fractionation discussed above, I propose the following as the best current bulk SW isotopic values: 20Ne/22Ne = 13.75 +/- 0.02, 21Ne/ 22Ne = 0.0329 +/- 0.0002, and 36Ar/38Ar = 5.501 +/- 0.005 (all errors are 1sigma).