Veronika S. Heber

The oxygen isotopic composition of the sun inferred from captured solar wind

The Sun is highly enriched in the most abundant isotope of oxygen, oxygen-16, relative to most other planetary materials. All planetary materials sampled thus far vary in their relative abundance of the major isotope of oxygen, 16O, such that it has not been possible to define a primordial solar system composition. We measured the oxygen isotopic composition of solar wind captured and returned to Earth by NASA’s Genesis mission. Our results demonstrate that the Sun is highly enriched in 16O relative to the Earth, Moon, Mars, and bulk meteorites. Because the solar photosphere preserves the average isotopic composition of the solar system for elements heavier than lithium, we conclude that essentially all rocky materials in the inner solar system were enriched in 17O and 18O, relative to 16O, by ~7%, probably via non–mass-dependent chemistry before accretion of the first planetesimals.

Helium in Lunar Samples Analyzed by High-Resolution Stepwise Etching: Implications for the Temporal Constancy of Solar Wind Isotopic Composition

The 3He/4He ratio in the present and past solar wind (SW) is of interest because it provides an estimate for the protosolar deuterium abundance. SW He and Ne composition has been analyzed in ilmenites of lunar soils (71501, 12001, 74241) and regolith breccias (79135, 79035, 14301) by closed system stepwise etching. The samples have SW antiquities ranging from ~0.1 to ~3.7 Gyr. The experiments have a very high depth resolution; thus, possible artifacts that may compromise the precise determination of the SW composition can be controlled. The nominal average (3He/4He)SW ratios correlate with the SW antiquity, suggesting at first glance a (3He/4He)SW increase of 6% per Gyr. In addition, also the (20Ne/22Ne)SW ratio seems to have increased with time by about 2% per Gyr. However, there are reasons indicating that the apparent temporal increase of (3He/4He)SW is an artifact. First, there is no straightforward explanation for a concurrent temporal change of (20Ne/22Ne)SW to the extent observed. Second, the average nominal (3He/4He)SW and (20Ne/22Ne)SW ratios correlate with each other, and this correlation parallels the one displayed by the single etch steps, indicating a progressive admixture of the deeper sited, isotopically heavier solar energetic particles with ongoing etching. We suggest that the nominal temporal variation of (3He/4He)SW and (20Ne/22Ne)SW is the result of secondary processes that cause erosion of the outermost layers of the regolith grains and thus a partial removal of the very surface-sited SW. Correcting this partial grain surface loss, assuming a constant SW Ne isotopic composition throughout the Suns lifetime, leads to constant (3He/4He)SW at (4.47 ? 0.13) ? 10-4 within the last ~4 Gyr. The results suggest that the present-day (3He/4He)SW can directly be used to deduce the protosolar (D+3He)/4He composition and that a possible mixing between radiative interior and convective zone must be restricted to a thin boundary layer.

Determining the Elemental and Isotopic Composition of the Pre-solar Nebula from Genesis Data Analysis: The Case of Oxygen

We compare element and isotopic fractionations measured in solar wind samples collected by NASAs Genesis mission with those predicted from models incorporating both the ponderomotive force in the chromosphere and conservation of the first adiabatic invariant in the low corona. Generally good agreement is found, suggesting that these factors are consistent with the process of solar wind fractionation. Based on bulk wind measurements, we also consider in more detail the isotopic and elemental abundances of O. We find mild support for an O abundance in the range 8.75 - 8.83, with a value as low as 8.69 disfavored. A stronger conclusion must await solar wind regime specific measurements from the Genesis samples.