Heavy Noble Gases Research Articles

Solvation of Silver Ions in Noble Gases He, Ne, Ar, Kr, and Xe.

We use a novel technique to solvate silver cations in small clusters of noble gases. The technique involves the formation of large, superfluid helium nanodroplets that are subsequently electron ionized, mass-selected by deflection in an electric field, and doped with silver atoms and noble gases (Ng) in pickup cells. Excess helium is then stripped from the doped nanodroplets by multiple collisions with helium gas at room temperature, producing cluster ions that contain no more than a few dozen noble gas atoms and just a few (or no) silver atoms. Under gentle stripping conditions, helium atoms remain attached to the cluster ions, demonstrating their low vibrational temperature. Under harsher stripping conditions, some of the heavier noble gas atoms will be evaporated as well, thus enriching stable clusters of NgnAgm+ at the expense of less stable ones. This results in local anomalies in the cluster ion abundance, which is measured in a high-resolution time-of-flight mass spectrometer. On the basis of these data, we identify specific "magic" sizes n of particularly stable ions. There is no evidence, however, for enhanced stability of Ng2Ag+, in contrast to the high stability of Ng2Au+ that derives from the covalent nature of the bond for heavy noble gases. "Magic" sizes are also identified for Ag2+ dimer ions complexed with He or Kr. Structural models will be tentatively proposed. A sequence of magic numbers n = 12, 32, and 44, indicative of three concentric solvation shells of icosahedral symmetry, is observed for HenH2O+.

Atomic Gold Ions Clustered with Noble Gases: Helium, Neon, Argon, Krypton, and Xenon.

High-resolution mass spectra of helium droplets doped with gold and ionized by electrons reveal HenAu+ cluster ions. Additional doping with heavy noble gases results in NenAu+, ArnAu+, KrnAu+, and XenAu+ cluster ions. The high stability predicted for covalently bonded Ar2Au+, Kr2Au+, and Xe2Au+ is reflected in their relatively high abundance. Surprisingly, the abundance of Ne2Au+, which is predicted to have zero covalent bonding character and no enhanced stability, features a local maximum, too. The predicted size and structure of complete solvation shells surrounding ions with essentially nondirectional bonding depends primarily on the ratio σ* of the ion-ligand versus the ligand-ligand distance. For Au+ solvated in helium and neon, the ratio σ* is slightly below 1, favoring icosahedral packing in agreement with a maximum observed in the corresponding abundance distributions at n = 12. HenAu+ appears to adopt two additional solvation shells of Ih symmetry, containing 20 and 12 atoms, respectively. For ArnAu+, with σ* ≈ 0.67, one would expect a solvation shell of octahedral symmetry, in agreement with an enhanced ion abundance at n = 6. Another anomaly in the ion abundance at Ar9Au+ matches a local maximum in its computed dissociation energy.

Reactivity of heavy noble gases under high pressure

Reactivity of heavy noble gases under high pressure

Heavy Noble Gas Isotopes as New Constraints on the Ventilation of the Deep Ocean

AbstractPast studies of noble gas concentrations in the deep ocean have revealed widespread, several percent undersaturation of Ar, Kr, and Xe. However, the physical explanation for these disequilibria remains unclear. To gain insight into undersaturation set by deep‐water formation, we measured heavy noble gas isotope and elemental ratios from the deep North Pacific using a new analytical technique. To our knowledge, these are the first high‐precision seawater profiles of 38Ar/36Ar and Kr and Xe isotope ratios. To interpret isotopic disequilibria, we carried out a suite of laboratory experiments to measure solubility fractionation factors in seawater. In the deep North Pacific, we find undersaturation of heavy‐to‐light Ar and Kr isotope ratios, suggesting an important role for rapid cooling‐driven, diffusive air‐to‐sea gas transport in setting the deep‐ocean undersaturation of heavy noble gases. These isotope ratios represent promising new constraints for quantifying physical air‐sea gas exchange processes, complementing noble gas concentration measurements.

Breakdown of the heavy noble gases in a focused beam of powerful sub-THz gyrotron

The results of experimental investigations of heavy noble gas breakdown thresholds in a focused beam of powerful subterahertz radiation are presented. The gas discharge, maintained by the powerful radiation of the terahertz frequency band, is a new specific object of gas discharge physics. This paper presents the experiments with pulsed gyrotrons capable of generating a power of 250 kW at a frequency of 250 GHz. The gyrotron wave beam was focused by means of quasioptical mirrors into the spot with a diameter less than 3 mm that provided intensity into the focal spot up to 3.5 MW/cm2. The achieved electrical field intensity was enough for gas breakdown into the range of pressure values of 1–1500 Torr for heavy noble gases (argon and krypton). The boundary values of field intensity for discharge existence were measured. Experimental data were compared to the analytical model of Raizer and Vyskrebentsev for monatomic gases.

Nebular atmosphere to magma ocean: A model for volatile capture during Earth accretion

The origin and abundance of mantle volatiles present major questions for Earth's evolution. Here we quantify volatile capture from an atmosphere derived from the solar nebula during accretion, using a boundary layer model of magma ocean dynamics coupled to a nebular atmosphere model adapted to Earth formation. Key elements include (i) nebular atmosphere winds based on scaling laws for deep rotating fluids; (ii) water production at the magma surface; and (iii) gas transfer between magma and atmosphere based on the systematics of air-sea gas exchange by wind and diffusion. Provided the Earth accreted to 30% or more of its final mass in the presence of the solar nebula, the mantle is expected to have ingassed several ocean mass equivalents of water plus hydrogen, along with hundreds of petagrams of helium-3 and other light noble gases. In contrast to light gases, nebular ingassing does not provide the mantle with enough heavy noble gases to account for their present-day atmosphere abundances. Our model also predicts that thermal insulation by the nebular atmosphere led to very hot conditions in Earth's interior during accretion, with peak temperatures above 6000 K at the core-mantle boundary and possible dynamo conditions in the Early Hadean.

Quantifying the helium and hydrocarbon accumulation processes using noble gases in the North Qaidam Basin, China

Limited reserves and insecure resource supply have led to global shortage crises in helium, a vital gas for cryogenic engineering and other countless industrial manufacturing processes. Despite the attention drawn by global supply disruptions, the helium accumulation mechanism in natural gas fields remains poorly understood. Noble gases are excellent tracers for studying migration and accumulation processes of fluids in the subsurface and can be used to investigate the influence of subsurface fluids on helium accumulation. We present noble gas isotope and abundance data as well as major gas compositional data from 10 producing wells in three gas fields in the North Qaidam Basin, China. Helium is more concentrated in the Mabei and Dongping gas fields (2.06–48.4 × 10−4 cm3 STP/cm3) than in the Niudong gas field (1.15–1.42 × 10−4 cm3 STP/cm3). The helium is mainly radiogenic, with 3He/4He ratios of 0.01–0.05 Ra, where Ra is the atmospheric value of 3He/4He, and lacks significant contribution from the mantle (0.03–0.67%). The noble gases derived from air-saturated water (20Ne, 36Ar, 84Kr and 130Xe) can be explained by an oil-modified groundwater-exsolution model with excess heavy noble gases. The calculated Voil/Vwater and Vgas/Vwater ratios indicate that the Mabei region is the most oil-rich area and the Dongping region has the driest natural gas, which is consistent with the geological context. These ratios further support the fractionation models. The strong linear relationship between 4He and 20Ne (R2 = 0.98) suggested that 4He was dissolved into groundwater before migrating into the oil or gas phase. The initial 4He concentrations in groundwater can accumulate within 0.31–2.78 Myr assuming a 4He flux from the entire crustal section. According to the fractionation model, helium in groundwater partitions into the gas phase when contacting hydrocarbons. The different volume ratios among oil, gas and water during the equilibration process cause much greater variability in the helium concentrations in the gas phase (e.g., 6.08 × 10−4 to 2.01 × 10−3 cm3 STP/cm3 in Mabei) than those in the groundwater phase (e.g. 9.18 × 10−3 to 1.39 × 10−2 cm3 4He STP/cm3 H2O in Mabei). Hydrocarbons play a critical role in helium accumulation and dilution. Helium-rich natural gas fields are characterized by old groundwater systems and moderate hydrocarbon abundance. This study has succeeded in quantitatively assessing the helium accumulation process in natural gas fields in the North Qaidam Basin and revealed that both groundwater and hydrocarbon phases control the helium accumulation in the subsurface environment. This outcome has broad implications for the prediction of hydrocarbon and helium as resources.

Nonlocal control of plasma conductivity

A study of the influence of the chemoionization processes on nonstationary gas-discharge conductivity has been performed. Due to energetic, suprathermal electrons, which appear in the chemoionization reactions, the highly nonequilibrium and time dependent nonlocal electron distribution function is formed. In such gas-discharge plasma, thermal electrons always have positive conductivity, while suprathermal, energetic electrons may have negative conductivity in heavy (argon, krypton, and xenon) noble gases dependent on conditions, which may lead to the nonmonotonic temporal behavior of gas-discharge plasma conductivity and potentially may create the absolute negative conductivity.

Recycling of heavy noble gases by subduction of serpentinite

Isotopic analyses of natural samples suggest that the heavy noble gases (HNG; Ar, Kr, and Xe) in the mantle are dominated by components recycled by subduction (Holland and Ballentine, 2006; Kendrick et al., 2011; Parai and Mukhopadhyay, 2018; Smye et al., 2017). However, the mechanism of recycling is not clear: are they primarily trapped in pore spaces and inclusions, are the gases dissolved in the lattices of mineral phases, or are they located along grain boundaries? Serpentinite is a dominant volatile-bearing lithology in subducting plates, and experimental studies suggest that HNG may be highly soluble in serpentinite (Jackson et al., 2015; Zaikowski and Schaeffer, 1979). To understand its role in HNG recycling, the solubility of noble gases in natural serpentinite has been experimentally measured at pressures from 0.15 to 1.13 kbar. Measured solubilities for He, Ne, Ar, Kr, and Xe are 1.78 ⋅ 10−9, 1.35 ⋅ 10−10, 3.03 ⋅ 10−10, 1.25 ⋅ 10−9, and 3.29 ⋅ 10−9 mol ⋅ g−1 ⋅ bar−1 with an average relative error of 9%. The solubilities are relatively high, indicating that even if less than 1% of the initial HNG estimated to be in subducting serpentinite survives slab dehydration, there would be enough remaining to dominate the mantle HNG budget. That the MORB and OIB mantle sources retain any primordial Kr or Xe in the face of such a large potential slab flux suggests subduction zones extract most (but not all) HNG from the slab at shallow depths. Serpentinite fractionates the HNG, with solubilities increasing from Ar to Kr to Xe. Kr/Ar and Xe/Ar ratios in MORB are quite similar to those predicted for subducted serpentinite, consistent with most HNG in the MORB-source coming from subducted serpentinite, possibly in combination with marine sediments. HNG ratios in most EM1- and EM2-type OIBs can also be explained primarily by serpentinite recycling. A prominent exception is Iceland, which appears to require dominantly sediment recycling, with little or no serpentinite. HIMU OIBs have highly fractionated HNG ratios that cannot be explained by serpentinite recycling but are consistent with subduction of HNG in amphibole (Chavrit et al., 2016).

The upper atmosphere of Venus: Model predictions for mass spectrometry measurements

Venus is the closest and most similar planet to Earth in the whole solar system, yet, basic measurements are missing. The atmospheric composition has been studied to some extent with the Pioneer Venus Orbiter mission and the more recent Venus Express, however, the concentration of many species and their isotopic ratios are not yet known with satisfying accuracy. Especially when it comes to the heavy noble gases as Kr and Xe, there is little to no collected data in this field. These elements form a key link to the reconstruction of the atmospheric evolution, which would deliver crucial information on the formation of Earth-like bodies. Knowing the abundances of the noble gases may allow us to answer the question why Venus did not evolve to a potentially habitable planet like its neighbour Earth.This paper presents a meta-study of existing observations of Venus' upper atmosphere and exosphere and an exospheric model of the thermal and hot atmosphere of Venus above the homopause. Mass spectra for possible future Venus missions are predicted, as for example the EnVision mission of ESA. The main focus are the heavy noble gases and requirements for the trajectories are proposed. In addition, mass spectra and recommended integration times for the Venus flyby of ESA's JUICE mission are derived.

Using noble gases to trace groundwater evolution and assess helium accumulation in Weihe Basin, central China

Abstract The severe shortage of helium resources is an impending global problem. However, the helium accumulation processes and conditions favorable for helium enrichment in reservoirs remain poorly understood, which makes helium exploration challenging. Noble gases are good tracers of subsurface fluids provenance, migration and storage, as well as indicators of the nature and quantity of associated phases. In this study the variation of major gases and noble gases data in Weihe Basin provide us with an excellent opportunity to understand the groundwater evolution and helium accumulation processes. Twelve gas samples collected from wellheads of geothermal wells can be classified into three groups, in which Group A has high concentrations of N2 (58.57–91.66%) and He (0.32–2.94%); Group B has high contents of CH4 (52.94% and 69.50%) and low concentrations of He (0.057% and 0.062%); Group C has a high content of CH4 (71.70%) and He (2.11%). Helium isotopic ratios are predominantly radiogenic in origin and therefore crustally derived. Measured elemental ratios of noble gases are compared with multiple fractionation models for Group A and B samples, implying that open system heavy oil-water fractionation with excess heavy noble gases has occurred in the basin with Voil/Vwater ratios of 0.06–0.18. The amount of helium in Group A and B samples requires the release of all 4He produced in the crust since 0.30–1.98 Ma into the groundwater. The Group C sample requires an additional He flux from adjacent granitic bodies. The accumulation of helium and hydrocarbon in the Weihe Basin can be explained by a 4-stage process. Accumulation of commercially viable helium requires high He flux from source rocks, the existence of a free gas phase of major gas components (CH4 in most cases, N2 or CO2) and minimal major gas addition after formation of the free gas phase.

Noble gases in micrometeorites from the Transantarctic Mountains

The bulk of extraterrestrial matter currently accreted by the Earth is in the form of micrometeorites (MMs) and interplanetary dust particles (IDPs), thus they may have collectively made a substantial contribution to the volatile inventory of the Earth and the other terrestrial planets. We have performed a complete noble gas study, accompanied by a complete petrographic characterization, of MMs from the Transantarctic Mountain (TAM) collection in the size range ∼300 to ∼1000 µm that fell over an extended time period during the last ∼1 Ma. Our noble gas study includes krypton and xenon, which have been largely missing in previous work. Helium and neon are dominated by a solar component, with generally lower abundance in scoriaceous MMs than in unmelted ones, and also generally lower in abundance than in previously studied MMs, which may be explained by the larger particle size (surface/volume ratio) of the MMs we studied. Considering an enhanced MM flux in the early Solar System, such MMs may have supplied a significant fraction of Earth’s neon. A number of MMs have kept what was probably their pre-terrestrial He/Ne ratio, from which we infer that the observed solar component is retained in a tiny surface region not affected by atmospheric entry. The abundances of (volume-correlated) heavier gases are similar to what was found in previous studies of smaller MMs. While Ar contains both solar and “planetary” contributions, the heavy noble gases (Kr, Xe) generally show “planetary” patterns but are often also compromised by terrestrial contamination as evidenced by an enhanced Kr/Xe ratio. Kr and Xe in a subset of scoriaceous MMs are dominated by isotopically fractionated air, possibly acquired during the passage through Earth’s ionosphere. Those not obviously affected by air show isotopic ratios similar to primitive meteorites (the Q component), thus primordial heavy gases supplied to the Earth by MMs are likely as those found in macroscopic meteorites. There is no evidence for the presence of a “cometary” Xe component as identified in the coma of comet 67P/Churyumov-Gerasimenko, hence a cometary source for a significant fraction of MMs in the studied size range is unlikely. Cosmogenic helium, neon and argon were detected in several cases. Cosmic ray exposure ages were calculated based on cosmogenic 21Ne in combination with the Poynting-Robertson effect, but depend on assumptions about atmospheric entry loss. Still, several cases are consistent with an origin from the asteroid belt (even assuming no loss) and one scoriaceous MM (#45b.17) would have to originate from beyond Jupiter. In at least two cases, including #45b.17, the isotopic composition of cosmogenic Ne appears to be inconsistent with predominant production in small particles free-floating in space, however; much of the irradiation of these MMs may have occurred when they were part of larger parent bodies.

Kr environment in feldspathic glass and melt: A high pressure, high temperature X-ray absorption study

Noble gases are used to trace the evolution of the Earth's atmosphere and magmatic processes. However little is known about their solubility mechanisms in silicate melts or glasses, whereas it could be fundamental to interpret these data. We report here the first in situ X-ray absorption spectroscopy data at the Kr K-edge on a feldspathic Kr-bearing glass and melt (0.5 wt% Kr) up to 2.7 GPa and 1086 °C. Kr fitting of the EXAFS signal shows that Kr is surrounded by oxygen atoms. Two Kr-O distances are identified: 2.49 ± 0.1 Å and 3.32 ± 0.1 Å. Results can be interpreted as two Kr populations. Kr surrounded by O atoms at 3.32 ± 0.1 Å suggests a filling of the large cavities available in the silicate network, in agreement with previous studies. On the contrary, Kr surrounded by O atoms at 2.49 ± 0.1 Å, and the observed +0.6 eV shift of the edge position at high pressure, suggest Kr bonding to O atoms and Kr oxidation inside cages formed by the largest aluminosilicate rings (i.e. ~12-membered-rings). Present results show that heavy noble gases incorporation in silicate melts can no longer be considered as a passive filling of the voids.

New methods for measuring atmospheric heavy noble gas isotope and elemental ratios in ice core samples.

The global ocean constitutes the largest heat buffer in the global climate system, but little is known about its past changes. The isotopic and elemental ratios of heavy noble gases (krypton and xenon), together with argon and nitrogen in trapped air from ice cores, can be used to reconstruct past mean ocean temperatures (MOTs). Here we introduce two successively developed methods to measure these parameters with a sufficient precision to provide new constraints on past changes in MOT. The air from an 800-g ice sample - containing roughly 80mL STP air - is extracted and processed to be analyzed on two independent dual-inlet isotope ratio mass spectrometers. The primary isotope ratios (δ15 N, δ40 Ar and δ86 Kr values) are obtained with precisions in the range of 1 per meg (0.001‰) per mass unit. The three elemental ratio values δKr/N2 , δXe/N2 and δXe/Kr are obtained using sequential (non-simultaneous) peak-jumping, reaching precisions in the range of 0.1-0.3‰. The latest version of the method achieves a 30% to 50% better precision on the elemental ratios and a twofold better sample throughput than the previous one. The method development uncovered an unexpected source of artefactual gas fractionation in a closed system that is caused by adiabatic cooling and warming of gases (termed adiabatic fractionation) - a potential source of measurement artifacts in other methods. The precisions of the three elemental ratios δKr/N2 , δXe/N2 and δXe/Kr - which all contain the same MOT information - suggest smaller uncertainties for reconstructed MOTs (±0.3-0.1°C) than previous studies have attained. Due to different sensitivities of the noble gases to changes in MOT, δXe/N2 provides the best constraints on the MOT under the given precisions followed by δXe/Kr, and δKr/N2 ; however, using all of them helps to detect methodological artifacts and issues with ice quality.

Dielectric functions E1 and E1 + Δ in near region of critical points and chemical composition of near surface layers of ions implanted GaAs

The studies were carried out using the semi-insulating SI semiconductor. The surfaces (100) of GaAs were irradiated by the noble gases Ne+ and Kr+ ions. The fluences were in the range from 1 × 1013 cm−2 to 3 × 1016 cm−2. The dielectric functions were obtained for the virgin and implanted semiconductors. The investigations were carried out for the near surface layers applying the spectroscopic ellipsometry (SE). The chemical composition was determined using the X-ray photoelectron spectroscopy (XPS). It was found that the dielectric functions values and chemical compositions of the native oxides are different for the samples implanted with light and heavy noble gases ions.

Noble gas signatures in snow

Abstract Noble gas concentrations in water are ideal probes to study surface and groundwater dynamics by providing indications of flow paths, connectivity between aquifers, and water residence times. Recent studies have pointed out anomalies in noble gas concentrations derived from groundwater in fractured systems, likely due to the presence of rapid infiltration and preferential flow paths. It has been suggested that such anomalies originate from conditions at high altitude when rainwater has had insufficient time to equilibrate with surface conditions. Potential sources also include snow, never previously investigated for its noble gas composition. In order to document the noble gas signature in snow, noble gas concentrations and isotopic ratios were measured in samples collected between 2013 and 2016. Here, we outline a methodology for measuring noble gases in collected snow samples that involves a two-step procedure where He and Ne are measured independently from Ar, Kr and Xe. Our results show that snow has elevated He concentrations with depleted concentrations of other noble gases with respect to air-saturated water (ASW). However, samples collected in 2013 show significant He and Ne depletion compared to those collected in 2014, 2015 and 2016. We suspect that, despite the well-controlled conditions of storage, the 2013 batch sample might have significantly re-crystalized, leading to a reduction in the characteristic diffusion length scale of the snow crystal structure. In addition, He and Ne concentrations display relatively low variability among all measured samples ( 40%). Our results confirm that He and Ne, which have small atomic radii, are likely dissolved within the ice/snow crystal lattice itself while the heavy noble gases (Ar, Kr and Xe) are likely accommodated by fluid inclusions, including air and quenched liquid water inclusions. Consequently, the smaller variability recorded in light noble gases may be due to the fact that He and Ne are hosted within plentiful host sites within the snow crystal lattice structure, whereas heavy noble gases rely on the presence of comparatively rare fluid inclusions.

Noble gases, nitrogen, cosmic ray exposure history and mineralogy of Beni M'hira (L6) chondrite

The concentrations and isotopic composition of noble gases helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon(Xe) and nitrogen were measured in the Beni M'hira L6 chondrite. The cosmic ray exposure age of Beni M'hira is estimated of 15.6 ± 3.7 (Ma). The radiogenic age, of around 485 ± 64 Ma, derived from 4He, and of around 504 ± 51 Ma from 40Ar, suggests an age resetting indicating the event impact. The heavy noble gases (Ar, Kr and Xe) concentrations imply that the gas is a mixture of trapped component Q and solar wind. The measured nitrogen abundance of 0.74 ppm and the isotopic signature of δ15N = 14.6‰ are within the range of ordinary chondrites. The homogeneous chemical composition of olivine (Fa:26 ± 0.25) and low-Ca pyroxene (Fs:22.4 ± 0.29) suggest that the Beni M'hira meteorite is an equilibrated chondrite. This is further corroborated by strong chondrule-matrix textural integration (lack of chondrules, except a few relict clast). Shock metamorphism generally corresponds to S5 (>45 GPa), however, locally disequilibrium melting (shock-melt veins) suggests, that the peak shock metamorphism was at ∼75 GPa, 950 °C.

Experimental determination of noble gases and SF6, as tracers of CO2 flow through porous sandstone

Abstract In order to label the CO2 injected underground for geological storage and allow it to be differentiated from natural sources, a panoply of additive chemical tracers have been proposed. Yet, the transport of these tracers relative to CO2 in pore space is currently poorly constrained. This leads to uncertainty as to whether tracers will act as an early warning of CO2 arrival, or be preferentially retained in the pore space making them ineffective. Here, we present the factors affecting transport of noble gases and SF6 relative to CO2 in a porous rock. Using a porous sandstone core, each of the tracers were loaded into a sample loop and injected as discrete gas pulses into a CO2 carrier stream at five different experiment pressures (10–50 kPag upstream to ambient pressure downstream). Tracer arrival profiles were measured using a quadrupole mass spectrometer. Significantly, our results show that peak arrival times of helium were slower than the other noble gases at each pressure gradient. The differences in peak arrival times between helium and other noble gases increased as the pressure gradient along the system decreased and the curve profiles for each noble gas differ significantly. The heavier noble gases (Kr and Xe) along with SF6 show an earlier arrival time and a wider curve profile compared to He and Ne curves through the CO2 carrier gas stream. This shows that Kr and Xe could be substituted for SF6, a potent greenhouse gas, in tracer applications. For comparison, CO2 pulses were passed through a N2 carrier gas resulting in significantly slower peak arrival times compared to those of noble gases and SF6. Hence, all investigated tracers when co-injected with CO2 could potentially act as early warning tracers of CO2 arrival, though we find that Kr, Xe and SF6 will provide the most robust advance warning. Analysis of our experimental results shows that they cannot be explained by a simple one dimensional flow model through a porous medium. We outline a conceptual model that incorporates different preferential flow paths depending on flow velocities of individual gas streams. This model can explain the observed dataset and shows that the flow of noble gases and SF6 tracers is influenced by pore-scale heterogeneity.

Charge States of Krypton and Xenon in the Solar Wind

We calculate charge state distributions of Kr and Xe in a model for two different types of solar wind using the effective ionization and recombination rates provided from the OPEN_ADAS data base. The charge states of heavy elements in the solar wind are essential for estimating the efficiency of Coulomb drag in the inner corona. We find that xenon ions experience particularly low Coulomb drag from protons in the inner corona, comparable to the notoriously weak drag of protons on helium ions. It has been found long ago that helium in the solar wind can be strongly depleted near interplanetary current sheets, whereas coronal mass ejecta are sometimes strongly enriched in helium. We argue that if the extraordinary variability of the helium abundance in the solar wind is due to inefficient Coulomb drag, the xenon abundance must vary strongly. In fact, a secular decrease of the solar wind xenon abundance relative to the other heavier noble gases (Ne, Ar, Kr) has been postulated based on a comparison of noble gases in recently irradiated and ancient samples of ilmenite in the lunar regolith. We conclude that decreasing solar activity and decreasing frequency of coronal mass ejections over the solar lifetime might be responsible for a secularly decreasing abundance of xenon in the solar wind.

Processes of noble gas elemental and isotopic fractionations in plasma-produced organic solids: Cosmochemical implications

The main carrier of primordial heavy noble gases in chondrites is thought to be an organic phase, known as phase Q, whose precise characterization has resisted decades of investigation. The Q noble gas component shows elemental and isotopic fractionation relative to the Solar, in favor of heavy elements and isotopes. These noble gas characteristics were experimentally simulated using a plasma device called the “Nebulotron”. In this study, we synthesized thirteen solid organic samples by electron-dissociation of CO, in which a noble gas mixture was added. The analysis of their heavy noble gas (Ar, Kr and Xe) contents and isotopic compositions reveals enrichment in the heavy noble gas isotopes and elements relative to the light ones. The isotope fractionation is mass-dependent and is consistent with a mn-type law, where n≥1. Based on a plasma model, we propose that the ambipolar diffusion of ions in the ionized CO gas medium is at the origin of the noble gas isotopic fractionation. In addition, the elemental fractionation of experimental and chondritic samples can be accounted for by the Saha law of plasma equilibrium, which does not depend on the respective noble gas masses but rather on their ionization potentials. Our results suggest that the Q noble gases were trapped into growing organic particles starting from solar gases that were fractionated in an ionized medium by ambipolar diffusion and Saha processes. This would imply that both the formation of chondritic organic matter and the trapping of noble gases took place simultaneously in the ionized areas of the protoplanetary disk.