Solar Wind Ne Research Articles

Similarities and differences between the solar wind light noble gas compositions determined on Apollo 15 SWC foils and on NASA Genesis targets

AbstractWe compare the solar wind (SW) He, Ne, and Ar compositions collected during the Apollo Solar Wind Composition (SWC) experiments (1969–1972; Al‐ & Pt‐foils) and the Genesis mission (2002–2004; so‐called DOS targets considered here). While published SW 20Ne/22Ne and 36Ar/38Ar ratios of both data sets agree, differences exist in the 4He/3He, 4He/20Ne, and 20Ne/36Ar ratios. However, 20Ne/36Ar ratios from Apollo‐16 Pt‐foils, exclusively adopted as SW values by the SWC team, are consistent with the Genesis results. We investigate if the differences indicate a variability of the SW over the course of about 30 yr, or systematic biases of the two data sets, which were collected in different environments and measured several decades apart in different laboratories (University of Bern; ETH Zurich). New measurements of Apollo‐15 SWC aluminum foils in Zurich generally agree with the original measurements performed in Bern. Zurich samples show slightly lower 4He concentrations suggesting a few percent of diffusive loss of 4He during storage of the foils. A 3% difference between the He isotopic ratios measured in Bern and in Zurich possibly represents an analytical bias between the laboratories. The low SW 4He/20Ne and 20Ne/36Ar ratios in Apollo‐15 Al‐foils compared to Genesis data are consistent with a mixture of Genesis‐like SW and noble gases from small amounts of lunar dust. Our data suggest that the mean SW He, Ne, and Ar isotopic and elemental compositions have not significantly changed between the overall Apollo and Genesis mission collection periods.

Cosmogenic nuclides in the Košice meteorite: Experimental investigations and Monte Carlo simulations

AbstractResults of nondestructive gamma‐ray analyses of cosmogenic radionuclides (7Be, 22Na, 26Al, 46Sc, 48V, 54Mn, 56Co, 57Co, 58Co, and 60Co) in 19 fragments of the Košice meteorite are presented and discussed. The activities varied mainly with position of fragments in the meteoroid body, and with fluxes of cosmic‐ray particles in the space affecting radionuclides with different half‐lives. Monte Carlo simulations of the production rates of 60Co and 26Al compared with experimental data indicate that the pre‐atmospheric radius of the meteoroid was 50 ± 5 cm. In two Košice fragments, He, Ne, and Ar concentrations and isotopic compositions were also analyzed. The noble‐gas cosmic‐ray exposure age of the Košice meteorite is 5–7 Myr, consistent with the conspicuous peak (or doublet peak) in the exposure age histogram of H chondrites. One sample likely contains traces of implanted solar wind Ne, suggesting that Košice is a regolith breccia. The agreement between the simulated and observed 26Al activities indicate that the meteoroid was mostly irradiated by a long‐term average flux of galactic cosmic rays of 4.8 particles cm−2 s−1, whereas the short‐lived radionuclide activities are more consistent with a flux of 7.0 protons cm−2 s−1 as a result of the low solar modulation of the galactic cosmic rays during the last few years before the meteorite fall.

Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician sediments at Lynna River, Russia

We measured the He and Ne concentrations of 50 individual extraterrestrial chromite grains recovered from mid-Ordovician (lower Darriwilian) sediments from the Lynna River section near St. Petersburg, Russia. High concentrations of solar wind-like He and Ne found in most grains indicate that they were delivered to Earth as micrometeoritic dust, while their abundance, stratigraphic position and major element composition indicate an origin related to the L chondrite parent body (LCPB) break-up event, 470Ma ago. Compared to sediment-dispersed extraterrestrial chromite (SEC) grains extracted from coeval sediments at other localities, the grains from Lynna River are both highly concentrated and well preserved. As in previous work, in most grains from Lynna River, high concentrations of solar wind-derived He and Ne impede a clear quantification of cosmic-ray produced He and Ne. However, we have found several SEC grains poor in solar wind Ne, showing a resolvable contribution of cosmogenic 21Ne. This makes it possible, for the first time, to determine robust cosmic-ray exposure (CRE) ages in these fossil micrometeorites, on the order of a few hundred-thousand years. These ages are similar to the CRE ages measured in chromite grains from cm-sized fossil meteorites recovered from coeval sediments in Sweden. As the CRE ages are shorter than the orbital decay time of grains of this size by Poynting–Robertson drag, this suggests that the grains were delivered to Earth through direct injection into an orbital resonance. We demonstrate how CRE ages of fossil micrometeorites can be used, in principle, to determine sedimentation rates, and to correlate the sediments at Lynna River with the fossil meteorite-bearing sediment layers in Sweden. In some grains with high concentrations of solar wind Ne, we nevertheless find a well-resolved cosmogenic 21Ne signal. These grains must have been exposed for up to several 10Ma in the regolith layer of the pre-break-up L chondrite parent body. This confirms an earlier suggestion that such regolith grains should be abundant in sediments deposited shortly after the break-up of the LCPB asteroid.

Noble gas studies in CAIs from CV3 chondrites: No evidence for primordial noble gases

Abstract— Calcium‐aluminum‐rich inclusions (CAIs) were among the first solids in the solar system and were, similar to chondrules, created at very high temperatures. While in chondrules, trapped noble gases have recently been detected, the presence of trapped gases in CAIs is unclear but could have important implications for CAI formation and for early solar system evolution in general. To reassess this question, He, Ne, and Ar isotopes were measured in small, carefully separated and, thus, uncontaminated samples of CAIs from the CV3 chondrites Allende, Axtell, and Efremovka.The 20Ne/22Ne ratios of all CAIs studied here are <0.9, indicating the absence of trapped Ne as, e.g., Ne‐HL, Ne‐Q, or solar wind Ne. The 21Ne/22Ne ratios range from 0.86 to 0.72, with fine‐grained, more altered CAIs usually showing lower values than coarse‐grained, less altered CAIs. This is attributed to variable amounts of cosmogenic Ne produced from Na‐rich alteration phases rather than to the presence of Ne‐G or Ne‐R (essentially pure 22Ne) in the samples. Our interpretation is supported by model calculations of the isotopic composition of cosmogenic Ne in minerals common in CAIs. The 36Ar/38Ar ratios are between 0.7 and 4.8, with fine‐grained CAIs within one meteorite showing higher ratios than the coarse‐grained ones. This agrees with higher concentrations of cosmogenic 36Ar produced by neutron capture on 35Cl with subsequent β−‐decay in finer‐grained, more altered, and thus, more Cl‐rich CAIs than in coarser‐grained, less altered ones.Although our data do not strictly contradict the presence of small amounts of Ne‐G, Ne‐R, or trapped Ar in the CAIs, our noble gas signatures are most simply explained by cosmogenic production, mainly from Na‐, Ca‐, and Cl‐rich minerals.

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 Sun's 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.

Fractionation of nitrogen isotopes in solar energetic particles

Abstract In a further step to assess processes leading to the complicated secular trend of the isotopic composition of N implanted in lunar regolith, we investigate mechanisms fractionating solar energetic particles (SEPs). We conclude that such mechanisms are likely to occur, most probably producing an enrichment of 15N over 14N in SEPs over the photospheric abundance ratio. Simultaneously, 22Ne is enriched over 20Ne but to a lesser extent. An enrichment of the heavy Ne isotope is observed in the suprathermal solar particles, implanted in the lunar regolith. Hence, the now well‐established difference between the isotopic composition of suprathermal Ne and solar wind Ne in the lunar regolith might be taken as evidence for the validity of this model. The present‐day fluxes of energetic particles produced in impulsive flare events, capable to produce such isotopic fractionations are, however, orders of magnitude below the required amounts to explain the lunar observations. The details of the secular variation of the N isotopic composition remain an enigma.

Implanted solar helium, neon, and argon in individual lunar ilmenite grains: Surface effects and a temporal variation in the solar wind composition

Solar implanted helium, neon, and argon were extracted from individual ilmenite grains from lunar soil 71501 and soil breccia 79035 using laser vaporization and analyzed by static mass spectrometry. Clear differences were observed for these two grain populations, one from a contemporary soil and the other from an ancient soil exposed on the lunar surface approximately 1 Ga ago. The different trends between the 20Ne 22Ne ratios and the He Ne ratios do not simply reflect differences either in regolith gardening or in diffusive losses but rather suggest a greater relative helium abundance in the ancient solar wind by a factor of about 1.8. The majority of the grains are enriched in solar energetic particle (SEP) Ne relative to solar wind (SW) Ne in a manner that increases with surface exposure. The progressive enrichment in retained SEP Ne relative to SW Ne is explained by a combination of diffusion and nonfractionating losses of the less deeply implanted SW component. The neon isotopic differences observed in various analyses of ilmenite separates from these two soils and previously attributed to a secular variation of either the SW SEP flux ratio or the SW Ne isotopic composition may alternatively be a natural consequence of greater SW losses which accompany an enhanced helium flux in the ancient solar wind.

Characterisation of Q-gases and other noble gas components in the Murchison meteorite

Noble gases in several HF/HCl resistant residues of the CM2 chondrite Murchison were measured by closed-system stepped etching, in order to study the planetary gases in their major carrier “Q”—an ill-defined minor phase, perhaps merely a set of adsorption sites. Neon, Ar, Kr, Xe, and probably also He in “Q” of Murchison have the same isotopic and nearly the same elemental abundances as their counterparts in Allende (CV3). The isotopic composition of Ne-Q is consistent with mass-dependent fractionation of either solar wind Ne or Ne from solar energetic particles. Unlike Allende, Murchison during HNO 3 attack releases, besides Q-gases, large amounts of two other Ne-components, Ne-E and Ne-A3, a third subcomponent of Ne-A. This work confirms that Q-gases of well-defined composition were an important noble gas component in the early solar system and are now found in various classes of meteorites, such as carbonaceous chondrites, ureilites, and ordinary chondrites. Ne-Q may have played a role in the formation of noble gas reservoirs in terrestrial planets.

Noble gases in lunar meteorites Yamato-82192 and -82193 and history of the meteorites from the moon

We analyzed the isotope abundances of He, Ne, Ar, Kr, and Xe in bulk samples and grain size separates of the lunar meteorites Yamato-82192 and -82193 collected in Antarctica. The history of exposure to cosmic rays and the terrestrial age is the same for both meteorites, confirming earlier suggestions that the two stones are fragments of the same meteorite fall. The terrestrial ages were determined using the radioisotope 81Kr in combination with stable cosmic-ray-produced noble gases. For Y-82192 and Y-82193 we obtain 83,000 ± 35,000y and 75,000 ± 30,000y, respectively. The Moon-Earth transit time lasted about 8 ± 3My. The upper limit of this time, 10.7 ± 0.6My, is obtained if we assume that Y-82192/3 was not exposed to cosmic rays in the lunar regolith, i.e. that it was excavated from a depth completely shielded from cosmic rays by the same impact that launched it into Earth orbit. The lower limit for the Moon-Earth transit time, about 5 My, is given by the fact that 1.6 My 10Be is present in equilibrium activity for a 4π exposure. If the Y-82192/3 material was ever exposed to cosmic rays in the lunar regolith this exposure must have lasted less than 11 My at shallow shielding of ⩽ 25 g/cm 2. Abundances of trapped solar wind particles are extremely low. Only solar wind Ne and Ar could definitely be detected. Comparison of the histories of lunar meteorites Allan Hills A-81005 and Yamato-791197 studied earlier with the data for Yamato-82192/3 shows that at least two impacts on the Moon are responsible for ejecting the lunar meteorites which have so far been found on Earth.

Decrease of the solar flare/solar wind flux ratio in the past several aeons deduced from solar neon and tracks in lunar soil plagioclases

We determined the He, Ne, and Ar concentrations and isotopic compositions of mineral separates of six lunar subsurface samples and of two regolith breccias which were exposed to the sun as early as 2–3 b. y. ago. The results are compared with our noble gas data obtained previously on mineral separates of lunar surface soil samples most of which contain recently implanted solar gases. The mean solar flare track densities were determined on aliquots of several of the plagioclase separates analyzed for noble gases. Solar wind retentive mafic minerals and ilmenites show that a possible secular increase of the 20Ne/22Ne ratio in the solar wind during the last 2–3 Ga is <2%. In plagioclases, which are less retentive for solar wind Ne, a considerable fraction of the retained Ne is of solar flare origin. The data are compatible with values for 20Ne/22Ne of ∼11.3–11.8, reported for solar flare Ne retained in plagioclase separates from lunar soils. The solar flare track data and the Ne data independently show that plagioclases exposed to the sun over the last 108 years recorded a lower mean ratio of solar flare to solar wind intensities than samples exposed about 1–3 b.y. ago. On the basis of track data we estimate these ratios to differ by a factor ∼2.