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1.
The Genesis Discovery mission returned solar matter in the form of the solar wind with the goal of obtaining precise solar isotopic abundances (for the first time) and greatly improved elemental abundances. Measurements of the light noble gases in regime samples demonstrate that isotopes are fractionated in the solar wind relative to the solar photosphere. Theory is required for correction. Measurement of the solar wind O and N isotopes shows that these are very different from any inner solar system materials. The solar O isotopic composition is consistent with photochemical self‐shielding. For unknown reasons, the solar N isotopic composition is much lighter than essentially all other known solar system materials, except the atmosphere of Jupiter. Ne depth profiling on Genesis materials has demonstrated that Ne isotopic variations in lunar samples are due to isotopic fractionation during implantation without appealing to higher energy solar particles. Genesis provides a precise measurement of the isotopic differences of Ar between the solar wind and the terrestrial atmosphere. The Genesis isotopic compositions of Kr and Xe agree with data from lunar ilmenite separates, showing that lunar processes have not affected the ilmenite data and that solar wind composition has not changed on 100 Ma time scales. Relative to Genesis solar wind, ArKrXe in Q (the chondrite noble gas carrier) and the terrestrial atmosphere show relatively large light isotope depletions.  相似文献   

2.
We 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.  相似文献   

3.
NASA's Genesis Mission returned solar wind (SW) to the Earth for analysis to derive the composition of the solar photosphere from solar material. SW analyses control the precision of the derived solar compositions, but their ultimate accuracy is limited by the theoretical or empirical models of fractionation due to SW formation. Mg isotopes are “ground truth” for these models since, except for CAIs, planetary materials have a uniform Mg isotopic composition (within ≤1‰) so any significant isotopic fractionation of SW Mg is primarily that of SW formation and subsequent acceleration through the corona. This study analyzed Mg isotopes in a bulk SW diamond‐like carbon (DLC) film on silicon collector returned by the Genesis Mission. A novel data reduction technique was required to account for variable ion yield and instrumental mass fractionation (IMF) in the DLC. The resulting SW Mg fractionation relative to the DSM‐3 laboratory standard was (?14.4‰, ?30.2‰) ± (4.1‰, 5.5‰), where the uncertainty is 2? SE of the data combined with a 2.5‰ (total) error in the IMF determination. Two of the SW fractionation models considered generally agreed with our data. Their possible ramifications are discussed for O isotopes based on the CAI nebular composition of McKeegan et al. (2011).  相似文献   

4.
Abstract– We have measured the isotopic composition and fluence of solar‐wind nitrogen in a diamond‐like‐carbon collector from the Genesis B/C array. The B and C collector arrays on the Genesis spacecraft passively collected bulk solar wind for the entire collection period, and there is no need to correct data for instrumental fractionation during collection, unlike data from the Genesis “Concentrator.” This work validates isotopic measurements from the concentrator by Marty et al. (2010, 2011) ; nitrogen in the solar wind is depleted in 15N relative to nitrogen in the Earth’s atmosphere. Specifically, our array data yield values for 15N/14N of (2.17 ± 0.37) × 10?3 and (2.12 ± 0.34) × 10?3, depending on data‐reduction technique. This result contradicts preliminary results reported for previous measurements on B/C array materials by Pepin et al. (2009) , so the discrepancy between Marty et al. (2010, 2011) and Pepin et al. (2009) was not due to fractionation of solar wind by the concentrator. Our measured value of 15N/14N in the solar wind shows that the Sun, and by extension the solar nebula, lie at the low‐15N/14N end of the range of nitrogen isotopic compositions observed in the solar system. A global process (or combination of processes) must have operated in interstellar space and/or during the earliest stages of solar system formation to increase the 15N/14N ratio of the solar system solids. We also report a preliminary Genesis solar‐wind nitrogen fluence of (2.57 ± 0.42) × 1012 cm?2. This value is higher than that derived by backside profiling of a Genesis silicon collector ( Heber et al. 2011a ).  相似文献   

5.
Abstract– We report concentrations and isotopic compositions of He, Ne, and Ar measured with high spatial resolution along a radial traverse of a silicon carbide (SiC) quadrant of the Genesis mission concentrator target. The Ne isotopic composition maps instrumental fractionation as a function of radial position in the target: the maximum observed isotopic fractionation is approximately 33‰ per mass unit between the center and periphery. The Ne fluence is enhanced by a factor of 43 at the target center and decreases to 5.5 times at the periphery relative to the bulk solar wind fluence. Neon isotopic profiles measured along all four arms of the “gold cross” mount which held the quadrants in the concentrator target demonstrate that the concentrator target was symmetrically irradiated during operation as designed. We used implantation experiments of Ne into SiC and gold to quantify backscatter loss and isotopic fractionation and compared measurements with numerical simulations from the code “stopping and range of ions in matter.” The 20Ne fluence curve as a function of radial distance on the target may be used to construct concentration factors relative to bulk solar wind for accurate corrections for solar wind fluences of other light elements to be measured in the concentrator target. The Ne isotopic composition as a function of the radial distance in the SiC quadrant provides a correction for the instrumental mass‐dependent isotopic fractionation by the concentrator and can be used to correct measured solar wind oxygen and nitrogen isotopic compositions to obtain bulk solar wind isotopic compositions.  相似文献   

6.
Abstract— We report mass‐spectrometric measurements of light noble gases pyrolytically extracted from 28 interplanetary dust particles (IDPs) and discuss these new data in the context of earlier analyses of 44 IDPs at the University of Minnesota. The noble gas database for IDPs is still very sparse, especially given their wide mineralogic and chemical variability, but two intriguing differences from isotopic distributions observed in lunar and meteoritic regolith grains are already apparent. First are puzzling overabundances of 3He, manifested as often strikingly elevated 3He/4He ratios—up to >40x the solar‐wind value—‐and found primarily but not exclusively in shards of some of the larger IDPs (“cluster particles”) that fragmented on impact with the collectors carried by high‐altitude aircraft. It is difficult to attribute these high ratios to 3He production by cosmic‐ray‐induced spallation during estimated space residence times of IDPs, or by direct implantation of solar‐flare He. Minimum exposure ages inferred from the 3He excesses range from ～50 Ma to an impossible >10 Ga, compared to Poynting‐Robertson drag lifetimes for low‐density 20–30 μm particles on the order of ～0.1 Ma for an asteroidal source and ～10 Ma for origin in the Kuiper belt. The second difference is a dominant contribution of solar‐energetic‐particle (SEP) gases, to the virtual exclusion of solar‐wind (SW) components, in several particles scattered throughout the various datasets but most clearly and consistently observed in recent measurements of a group of individual and cluster IDPs from three different collectors. Values of the SEP/SW fluence ratio in interplanetary space from a simple model utilizing these data are ～1% of the relative SEP/SW abundances observed in lunar regolith grains, but still factors of approximately 10–100 above estimates for this ratio in low‐energy solar particle emission.  相似文献   

7.
3He is an intermediate product in the proton-proton chain, and standard models of the Sun predict a large bulge of enhanced 3He abundance near M r /M 0 = 0.6 in the contemporary Sun. The relatively low abundance of 3He at the solar surface, which is derived from solar wind observations, poses severe constraints to non-standard solar models.Direct measurements of the 3He abundance in the solar atmosphere are extremely difficult, whereas indirect measurements, e.g., in the solar wind, have been performed with considerable precision. The interpretation of solar wind observations with respect to solar surface abundances has been greatly improved in recent years. Abundance measurements have been performed under a large variety of solar wind conditions and refined models have been developed for the transport processes in the chromosphere and the transition region and for the processes occurring in the solar corona. From these measurements we estimate the present isotopic number ratio 3He/4He to be (4.1 ± 1.0) × 10–4 at the solar surface, corresponding to the weight abundance X 3 = (9.0 ± 2.4) × 10–5. The zero-age Main-Sequence abundance of 3He (after burning of D) might have been slightly lower (by about 10 to 20%) than the present-day value.Non-standard solar models involving mild turbulent diffusion (Lebreton and Maeder, 1987) could account for a slow secular increase of the 3He/4He ratio in the solar atmosphere. On the other hand it is difficult to reconcile models with severe mass loss as proposed by Guzik, Willson, and Brunish (1987) with this constraint. The slowing down of the solar rotation during the early Main-Sequence evolution was accompanied by stronger differential rotation probably implying a more effective mixing of the inner parts. Again, the surface abundance of 3He imposes severe limits on the evolution of the distribution of momentum within the early Sun.  相似文献   

8.
NASA's Genesis mission revealed that the Sun is enriched in 16O compared to the Earth and Mars (the Sun's Δ17O, defined as δ17O–0.52×δ18O, is –28.4 ± 3.6‰; McKeegan et al. 2011). Materials as 16O‐rich as the Sun are extremely rare in the meteorite record. Here, we describe a Ca‐Al‐rich inclusion (CAI) from a CM chondrite that is as 16O‐enriched as the Sun (Δ17O = –29.1 ± 0.7‰). This CAI also has large nucleosynthetic anomalies in 48Ca and 50Ti (δ‐values are –8.1 ± 3.3 and –11.7 ± 2.4‰, respectively) and shows no clear evidence for incorporation of live 26Al; (26Al/27Al)0 = (0.03 ± 0.11) × 10–5. Due to their anomalous isotopic characteristics, the rare CAIs consistent with the Genesis value could be among the first materials that formed in the solar system. In contrast to the CAI studied here, the majority of CAIs formed in or interacted with a reservoir characterized by a Δ17O value near –23.5‰. Combined with 26Al‐26Mg systematics, the oxygen isotopic compositions of FUN (fractionation and unidentified nuclear effects), UN, and normal CAIs suggest that nebular conditions were favorable for solids to inherit this value for an extended period of time. Many later‐formed materials, such as chondrules, planetesimals, and terrestrial planets, formed in reservoirs with Δ17O near 0‰. The distribution could be easier to explain if the common CAI value of –23.5‰, which is consistent with the Genesis value within 3σ, represented the average composition of the protoplanetary disk.  相似文献   

9.
NASA's Genesis mission was flown to capture samples of the solar wind and return them to the Earth for measurement. The purpose of the mission was to determine the chemical and isotopic composition of the Sun with significantly better precision than known before. Abundance data are now available for noble gases, magnesium, sodium, calcium, potassium, aluminum, chromium, iron, and other elements. Here, we report abundance data for hydrogen in four solar wind regimes collected by the Genesis mission (bulk solar wind, interstream low‐energy wind, coronal hole high‐energy wind, and coronal mass ejections). The mission was not designed to collect hydrogen, and in order to measure it, we had to overcome a variety of technical problems, as described herein. The relative hydrogen fluences among the four regimes should be accurate to better than ±5–6%, and the absolute fluences should be accurate to ±10%. We use the data to investigate elemental fractionations due to the first ionization potential during acceleration of the solar wind. We also use our data, combined with regime data for neon and argon, to estimate the solar neon and argon abundances, elements that cannot be measured spectroscopically in the solar photosphere.  相似文献   

10.
Abstract Solar noble gases He, Ne, Ar and Kr implanted in the H3–6 meteorite regolith breccia Acfer 111 agree in their elemental composition with that in present-day solar wind and, except for a 25% deficit of 4He, also with adopted solar abundances. The presence of such unfractionated solar gases makes Acfer 111 unique (until now). Closed system stepped etching releases noble gases that can be explained as mixtures of two distinct types of He, Ne, and Kr of isotopic compositions as they have been derived previously from meteorites and lunar samples that contain heavily fractionated solar gases. Since the same putative end members, ascribed to the solar wind (SW) and supra-thermal solar energetic particles (SEP), are also present in Acfer 111, we argue that these end members represent two truly independent components. We discount the possibility that one isotopic composition derived from the other by diffusion of the gases within, or upon their release from, their host phases. The isotopic signatures of noble gases in Acfer 111 agree with those in a lunar ilmenite of young antiquity ?100 Ma) but are in disagreement with the noble gases in lunar ilmenite 79035 of 1–2 Ga antiquity. Systematic changes are discussed of the nuclide abundance ratios as etching proceeds; they are ascribed to differences in trapping efficiency and in penetration depth of the different noble gas ion species upon their implantation.  相似文献   

11.
Abstract— From November 1998 to January 1999, the 39th Japanese Antarctic Research Expedition (JARE) conducted a large‐scale micrometeorite collection at 3 areas in the meteorite ice field around the Yamato Mountains, Antarctica. The Antarctic micrometeorites (AMMs) collected were ancient cosmic dust particles. This is in contrast with the Dome Fuji AMMs, which were collected previously from fresh snows in 1996 and 1997 and which represent modern micrometeorites. To determine the noble gas concentrations and isotopic compositions of individual AMMs, noble gas analyses were carried out using laser‐gas extraction for 35 unmelted Yamato Mountains AMMs and 3 cosmic spherules. X‐ray diffraction analyses were performed on 13 AMMs before the noble gas measurement and mineral compositions were determined. AMMs are classified into 4 main mineralogical groups, defined from the heating they suffered during atmospheric entry. Heating temperatures of AMMs, inferred from their mineral compositions, are correlated with 4He concentrations and reflect the effect of degassing during atmospheric entry. Jarosite, an aqueous alteration product, is detected for 4 AMMs, indicating the aqueous alteration during long‐time storage in Antarctic ice. Jarosite‐bearing AMMs have relatively low concentrations of 4He, which is suggestive of loss during the alteration. High 3He/4He ratios are detected for AMMs with high 20Ne/4He ratios, showing both cosmogenic 3He and preferential He loss. SEP (solar energetic particles)‐He and Ne, rather than the solar wind (SW), were dominant in AMMs, presumably showing a preferential removal of the more shallowly implanted SW by atmospheric entry heating. The mean 20Ne/22Ne ratio is 11.27 ± 0.35, which is close to the SEP value of 11.2. Cosmogenic 21Ne is not detected in any of the particles, which is probably due to the short cosmic ray exposure ages. Ar isotopic compositions are explained by 3‐component mixing of air, Q, and SEP‐Ar. Ar isotopic compositions can not be explained without significant contributions of Q‐Ar. SEP‐Ne contributed more than 99% of the total Ne. As for 36Ar and 38Ar, the abundance of the Q component is comparable to that of the SEP component. 84Kr and 132Xe are dominated by the primordial component, and solar‐derived Xe is almost negligible.  相似文献   

12.
The radiogenic and primordial noble gas content of the atmospheres of Venus, Earth, and Mars are compared with one another and with the noble gas content of other extraterrestial samples, especially meteorites. The fourfold depletion of 40Ar for Venus relative to the Earth is attributed to the outgassing rates and associated tectonics and volcanic styles for the two planets diverging significantly within the first billion or so years of their history, with the outgassing rate for Venus becoming much less than that for the Earth at subsequent times. This early divergence in the tectonic style of the two planets may be due to a corresponding early onset of the runaway greenhouse on Venus. The 16-fold depletion of 40Ar for Mars relative to the Earth may be due to a combination of a mild K depletion for Mars, a smaller fraction of its interior being outgassed, and to an early reduction in its outgassing rate. Venus has lost virtually all of its primordial He and some of its radiogenic He. The escape flux of He may have been quite substantial in Venus' early history, but much diminished at later times, with this time variation being perhaps strongly influenced by massive losses of H2 resulting from efficient H2O loss processes.Key trends in the primordial noble gas content of terrestial planetary atmospheres include (1) a several orders of magnitude decrease in 20Ne and 36Ar from Venus to Earth to Mars; (2) a nearly constant 20Ne/36Ar ratio which is comparable to that found in the more primitive carbonaceous chondrites and which is two orders of magnitude smaller than the solar ratio; (3) a sizable fractionation of Ar, Kr, and Xe from their solar ratios, although the degree of fractionation, especially for 36Ar/132Xe, seems to decrease systematically from carbonaceous chondrites to Mars to Earth to Venus; and (4) large differences in Ne and Xe isotopic ratios among Earth, meteorites, and the Sun. Explaining trends (2), (2) and (4), and (1) pose the biggest problems for the solar-wind implantation, primitive atmosphere, and late veneer hypotheses, respectively. It is suggested that the grain-accretion hypothesis can explain all four trends, although the assumptions needed to achieve this agreement are far from proven. In particular, trends (1), (2), (3), and (4) are attributed to large pressure but small temperature differences in various regions of the inner solar system at the times of noble gas incorporation by host phases; similar proportions of the host phases that incorporated most of the He and Ne on the one hand (X) and Ar, Kr, and Xe on the other hand (Q); a decrease in the degree of fractionation with increasing noble-gas partial pressure; and the presence of interstellar carriers containing isotopically anomalous noble gases.Our analysis also suggests that primordial noble gases were incorporated throughout the interior of the outer terrestial planets, i.e., homogeneous accretion is favored over inhomogeneous accretion. In accord with meteorite data, we propose that carbonaceous materials were key hosts for the primordial noble gases incorporated into planets and that they provided a major source of the planets' CO2 and N2.  相似文献   

13.
Abstract– We review current observational and theoretical constraints on the galactic chemical evolution (GCE) of oxygen isotopes to explore whether GCE plays a role in explaining the lower 17O/18O ratio of the Sun, relative to the present‐day interstellar medium, or the existence of distinct 16O‐rich and 16O‐poor reservoirs in the solar system. Although the production of both 17O and 18O are related to the metallicity of progenitor stars, 17O is most likely produced in stars that evolve on longer timescales than those that produce 18O. Therefore, the 17O/18O ratio need not have remained constant over time, contrary to preconceptions and the simplest models of GCE. An apparent linear, slope‐one correlation between δ17O and δ18O in the ISM need not necessarily reflect an O isotopic gradient, and any slope‐one galactocentric gradient need not correspond to evolution in time. Instead, increasing 17O/18O is consistent both with observational data from molecular clouds and with modeling of the compositions of presolar grains. Models in which the rate of star formation has decelerated over the past few Gyr or in which an enhanced period of star formation occurred shortly before solar birth (“starburst”) can explain the solar‐ISM O‐isotopic difference without requiring a local input of supernova ejecta into the protosolar cloud. “Cosmic chemical memory” models in which interstellar dust is on average older than interstellar gas predict that primordial solar system solids should be 16O‐rich, relative to the Sun, in conflict with observations. However, scenarios can be constructed in which the 16O‐rich contribution of very massive stars could lead to 16O‐poor solids and a 16O‐rich bulk Sun, if the solar system formed shortly after a starburst, independent of the popular scenario of photochemical self‐shielding of CO.  相似文献   

14.
Abstract— The noble gases He, Ne, Ar, Kr, and Xe were measured in 27 individual Antarctic micrometeorites (AMMs) in the size range 60 to 250 μm that were collected at the Dome Fuji Station. Eleven of the AMMs were collected in 1996 (F96 series) and 16 were collected in 1997 (F97 series). One of the F97 AMMs is a totally melted spherule, whereas all other particles are irregular in shape. Noble gases were extracted using a Nd‐YAG continuous wave laser with an output power of 2.5‐3.5 W for ?5 min. Most particles released measurable amounts of noble gases. 3He/4He ratios are determined for 26 AMMs ((0.85‐9.65) × 10?4). Solar energetic particles (SEP) are the dominant source of helium in most AMMs rather than solar wind (SW) and cosmogenic He. Three samples had higher 3He/4He ratios compared to that of SW, showing the presence of spallogenic 3He. The Ne isotopic composition of most AMMs resembled that of SEP as in the case of helium. Spallogenic 21Ne was detected in three samples, two of which had extremely long cosmic‐ray exposure ages (> 100 Ma), calculated by assuming solar cosmic‐ray (SCR) + galactic cosmic‐ray (GCR) production. These two particles may have come to Earth directly from the Kuiper Belt. Most AMMs had negligible amounts of cosmogenic 21 Ne and exposure ages of <1 Ma. 40Ar/36Ar ratios for all particles (3.9–289) were lower than that of the terrestrial atmosphere (296), indicating an extraterrestrial origin of part of the Ar with a very low 40Ar/36Ar ratio plus some atmospheric contamination. Indeed, 40Ar/36Ar ratios for the AMMs are higher than SW, SEP, and Q‐Ar values, which is explained by the presence of atmospheric 40Ar. The average 38Ar/36Ar ratio of 24 AMMs (0.194) is slightly higher than the value of atmospheric or Q‐Ar, suggesting the presence of SEP‐Ar which has a relatively high 38Ar/36Ar ratio. According to the elemental compositions of the heavy noble gases, Dome Fuji AMMs can be classified into three groups: chondritic (eight particles), air‐affected (nine particles), and solar‐affected (eight particles). The eight AMMs classified as chondritic preserve the heavy noble gas composition of primordial trapped component due to lack of atmospheric adsorption and solar implantation. The average of 129Xe/132Xe ratio for the 16 AMMs not affected by atmospheric contamination (1.05) corresponds to the values in matrices of carbonaceous chondrites (?1.04). One AMM, F96DK038, has high 129Xe/132Xe in excess of this ratio. Our results imply that most Dome Fuji AMMs originally had chondritic heavy noble gas compositions, and carbonaceous chondrite‐like objects are appropriate candidate sources for most AMMs.  相似文献   

15.
Abstract— Abundances and isotopic compositions of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite were measured. The abundance ratios of spallogenic components in metal reveal a 3He deficiency which is due to the diffusive loss of parent isotopes, that is, tritium (Tilles, 1963; Schultz, 1967). The diffusive loss likely has been induced by thermal heating by the Sun during cosmic‐ray exposure (～160 Ma; Lavielle et al, 1999). Thermal process such as impact‐induced partial loss may have affected the isotopic composition of spallogenic Ne. The 129Xe/131Xe ratio of cosmogenic components in the metal indicates an enhanced production of epi‐thermal neutrons. The abundance ratios of spallogenic components in the graphite reveal that it contained small amounts of metal and silicates. The isotopic composition of heavy noble gases in graphite itself was obtained from graphite treated with HF/HCl. The isotopic composition of the etched graphite shows that it contains two types of primordial Xe (i.e., Q‐Xe and El Taco Xe). The isotopic heterogeneity preserved in the Bohumilitz graphite indicates that the Bohumilitz graphite did not experience any high‐temperature event and, consequently, must have been emplaced into the metal at subsolidus temperatures. This situation is incompatible with an igneous model as well as the impact melting models for the IAB‐IIICD iron meteorites as proposed by Choi et al. (1995) and Wasson et al (1980).  相似文献   

16.
We report new data from Pesyanoe‐90,1 (dark lithology) on the isotopic signature of solar wind (SW) Xe as recorded in this enstatite achondrite which represents a soil‐breccia of an asteroidal regolith. The low temperature (≤800°C) steps define the Pesyanoe‐S xenon component, which is isotopically consistent with SW Xe reported for the lunar regolith. This implies that the SW Xe isotopic signature was the same at two distinct solar system locations and, importantly, also at different times of solar irradiation. Further, we compare the calculated average solar wind “SW‐Xe” signature to Chass‐S Xe, the indigenous Xe observed in SNC (Mars) meteorites. Again, a close agreement between these compositions is observed, which implies that a mass‐dependent differential fractionation of Xe between SW‐Xe and Chass‐S Xe is >1.5%o per amu. We also observe fractionated (Pesyanoe‐F) Xe and Ar components in higher temperature steps and we document a fission component due to extinct 244Pu. Interestingly, the Pesyanoe‐F Xe component is revealed only at the highest temperatures (>1200°C). The Pesyanoe‐F gas reveals Xe isotopic signatures that are consistent with lunar solar energetic particles (SEP) data and may indicate a distinct solar energetic particle radiation as was inferred for the moon. However, we cannot rule out fractionation processes due to parent body processes. We note that ratios 36Ar/38Ar≤5 are also consistent with SEP data. Calculated abundances of the fission component correlate well with radiogenic 40Ar concentrations, revealing rather constant 244Pu/K ratios in Pesyanoe, and separates thereof, and indicate that both components were retained. We identify a nitrogen component (δ15N = 44%o) of non‐solar origin with an isotopic signature distinct from indigenous N (δ15N = ?33%o). While large excesses at 128Xe and 129Xe are observed in the lunar regolith samples, these excesses in Pesyanoe are small. On the other hand, significant 126Xe isotopic excesses, comparable to relative excesses observed in lunar soils and breccias, are prominent in the intermediate temperature steps of Pesyanoe‐90,1.  相似文献   

17.
Abstract— The outer layers of the Sun are thought to preserve the average isotopic and chemical composition of the solar system. The solar O-isotopic composition is essentially unmeasured, though models based on variations in meteoritic materials yield several predictions. These predictions are reviewed and possible variations on these predictions are explored. In particular, the two-component mixing model of Clayton and Mayeda (1984) (slightly revised here) predicts solar compositions to lie along an extension of the calcium-aluminum-rich inclusion (CAI) 16O line between (δ18O, δ17O) = (16.4, 11.4)%0 and (12.3, 7.5)%0. Consideration of data from ordinary chondrites suggests that the range of predicted solar composition should extend to slightly lower δ18O values. The predicted solar composition is critically sensitive to the solid/gas ratio in the meteorite-forming region, which is often considered to be significantly enriched over solar composition. A factor of two solid/gas enrichment raises the predicted solar (δ18O, δ17O) values along an extension of the CAI 16O line to (33, 28)%0. The model is also sensitive to the nebular O gas phase. If conversion of most of the gaseous O from CO to H2O occurred at relatively low temperatures and was incomplete at the time of CM aqueous alteration, the predicted nebular gas composition (and hence the solar composition) would be isotopically heavier along a slope 1/2 line. The likelihood of having a single solid nebular O component is discussed. A distribution of initial solid compositions along the CAI 16O line (rather than simply as an end-member) would not significantly change the predictions above in at least one scenario. Even considering these variations within the mixing model, the predicted range of solar compositions is distinct from that expected if the meteoritic variations are due to non-mass-dependent fractionation. Thus, a measurement of the solar O composition to a precision of several permil would clearly distinguish between these theories and should clarify a number of other important issues regarding solar system formation.  相似文献   

18.
Abstract— We present results from a model of oxygen isotopic anomaly production through selective photodissociation of CO within the collapsing proto‐solar cloud. Our model produces a proto‐Sun with a wide range of Δ17O values depending on the intensity of the ultraviolet radiation field. Dramatically different results from two recent solar wind oxygen isotope measurements indicate that a variety of compositions remain possible for the solar oxygen isotope composition. However, constrained by other measurements from comets and meteorites, our models imply the birth of the Sun in a stellar cluster with an enhanced radiation field and are therefore consistent with a supernova source for 60Fe in meteorites.  相似文献   

19.
Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., 26Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to 60Fe predicted from stars with initial mass in the range approximately 7–11 M⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 108 K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce 26Al/27Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the 26Al/27Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 102 to several 103 of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ17O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides 41Ca and 60Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as 107Pd.  相似文献   

20.
Abstract– The interior texture and chemical and noble gas composition of 99 cosmic spherules collected from the meteorite ice field around the Yamato Mountains in Antarctica were investigated. Their textures were used to classify the spherules into six different types reflecting the degree of heating: 13 were cryptocrystalline, 40 were barred olivine, 3 were porphyritic A, 24 were porphyritic B, 9 were porphyritic C, and 10 were partially melted spherules. While a correlation exists between the type of spherule and its noble gas content, there is no significant correlation between its chemical composition and noble gas content. Fifteen of the spherules still had detectable amounts of extraterrestrial He, and the majority of them had 3He/4He ratios that were close to that of solar wind (SW). The Ne isotopic composition of 28 of the spherules clustered between implantation‐fractionated SW and air. Extraterrestrial Ar, confirmed to be present because it had a 40Ar/36Ar ratio lower than that of terrestrial atmosphere, was found in 35 of the spherules. An enigmatic spherule, labeled M240410, had an extremely high concentration of cosmogenic nuclides. Assuming 4π exposure to galactic and solar cosmic rays as a micrometeoroid and no exposure on the parent body, the cosmic‐ray exposure (CRE) age of 393 Myr could be computed using cosmogenic 21Ne. Under these model assumptions, the inferred age suggests that the particle might have been an Edgeworth‐Kuiper Belt object. Alternatively, if exposure near the surface of its parent body was dominant, the CRE age of 382 Myr can be estimated from the cosmogenic 38Ar using the production rate of the 2π exposure geometry, and implies that the particle may have originated in the mature regolith of an asteroid.  相似文献   

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