Total internal partition sums to support planetary remote sensing

Total internal partition sums are determined from 65 to 3010 K for ¹³C¹⁸O₂, ¹³C¹⁸O¹⁷O, ¹²CH₃D, ¹³CH₃D, H¹²C¹²CD, ¹³C¹²CH₆, ¹²CH₃⁷⁹Br, ¹²CH₃⁸¹Br, ¹²CF₄, H¹²C¹²C¹²C¹²CH, H¹²C¹²C¹²C¹⁴N, H¹²C¹²C¹³C¹⁴N, H¹²C¹³C¹²C¹⁴N, H¹³C¹²C¹²C¹⁴N, H¹²C¹²C¹²C¹⁵N, D¹²C¹²C¹²C¹⁴N, ¹⁴N¹²C¹²C¹⁴N, ¹⁵N¹²C¹²C¹⁵N, ¹²C³²S, ¹²C³³S, ¹²C³⁴S, ¹³C³²S, H₂, HD, ³²S¹⁶O, ³²S¹⁸O, ³⁴S¹⁶O, ¹²C₃H₄, ¹²CH₃, ¹²C³²S₂, ³²S¹²C³⁴S, ¹³C³²S₂, and ³²S¹²C³³S. These calculations complete the partition sum data needed for additional isotopologues in HITRAN2008 and also extend the partition sums to molecules of astrophysical interest. These data, at 25 K steps, are incorporated into a FORTRAN code (TIPS_2011.for) that can be used to rapidly generate the data at any temperature in the range 70-3000 K.     

Thermonuclear reaction rates from (p,n) reactions on targets withA = 92–122

From the experimentally measured (p,n) cross-sections for^92,94Zr,⁹³Nb,^95,96,98Mo,¹⁰³Rh,^107,109Ag,¹¹⁰Pd,¹¹⁵In,^117,122Sn nuclides, for proton energies below 7 MeV, thermonuclear reaction rates in the temperature range 1≤T ₉≤5(T₉=10⁹ K) have been extracted. These reaction rates have been fitted to a three parameter empirical expression proposed by Fowler.     

Cross sections from 5 to 35 MeV for the reactions natMg(3He,x)26Al, 27Al(3He,x)26Al,natCa(3He,x)41Ca,and natCa(3He,x)36Cl: Implications for early irradiation in the solar system

Abstract– Cross sections were measured for the nuclear reactions ^natMg(³He,x)²⁶Al, ²⁷Al(³He,x)²⁶Al, ^natCa(³He,x)⁴¹Ca, and ^natCa(³He,x)³⁶Cl in the energy region from approximately 5–35 MeV. The rates of these reactions are important for studies of early solar system irradiation scenarios. The ²⁶Al, ³⁶Cl, and ⁴¹Ca were separated chemically, and the numbers of atoms produced in each reaction channel were measured using accelerator mass spectrometry (AMS). From these results, 26 cross sections were determined and compared with predictions of the TALYS code. Agreement is within 40% for most cross sections. Our measurements were used to model the production of ⁷Be, ¹⁰Be, ²⁶Al, and ⁴¹Ca in the early solar system. For projectiles ¹H, ³He, and ⁴He, we assumed energy spectra of the general form E^?α. For a wide range of parameterizations, the modeled ratios of ⁷Be/Be and ¹⁰Be/Be on the one hand and of ²⁶Al/²⁷Al and ⁴¹Ca/Ca on the other are coupled because the excitation functions for the relevant nuclear reactions have similar shapes. Modeling of a closed system with the constraint that ¹⁰Be/⁹Be = 0.001 fails to reproduce simultaneously the range of ⁷Be/⁹Be, ²⁶Al/²⁷Al, and ⁴¹Ca/Ca ratios inferred for the early solar system from studies of meteorites.     

Calibration of cosmogenic noble gas production based on 36Cl‐36Ar ages. Part 2. The 81Kr‐Kr dating technique

We calibrated the ⁸¹Kr‐Kr dating system for ordinary chondrites of different sizes using independent shielding‐corrected ³⁶Cl‐³⁶Ar ages. Krypton concentrations and isotopic compositions were measured in bulk samples from 14 ordinary chondrites of high petrologic type and the cosmogenic Kr component was obtained by subtracting trapped Kr from phase Q. The thus‐determined average cosmogenic ⁷⁸Kr/⁸³Kr, ⁸⁰Kr/⁸³Kr, ⁸²Kr/⁸³Kr, and ⁸⁴Kr/⁸³Kr ratiC(Lavielle and Marti 1988; Wieler 2002). The cosmogenic ⁷⁸Kr/⁸³Kr ratio is correlated with the cosmogenic ²²Ne/²¹Ne ratio, confirming that ⁷⁸Kr/⁸³Kr is a reliable shielding indicator. Previously, ⁸¹Kr‐Kr ages have been determined by assuming the cosmogenic production rate of ⁸¹Kr, P(⁸¹Kr)_c, to be 0.95 times the average of the cosmogenic production rates of ⁸⁰Kr and ⁸²Kr; the factor Y = 0.95 therefore accounts for the unequal production of the various Kr isotopes (Marti 1967a). However, Y should be regarded as an empirical adjustment. For samples whose ⁸⁰Kr and ⁸²Kr concentrations may be affected by neutron‐capture reactions, the shielding‐dependent cosmogenic (⁷⁸Kr/⁸³Kr)_c ratio has been used instead to calculate P(⁸¹Kr)/P(⁸³Kr), as for some lunar samples, this ratio has been shown to linearly increase with (⁷⁸Kr/⁸³Kr)_c (Marti and Lugmair 1971). However, the ⁸¹Kr‐Kr ages of our samples calculated with these methods are on average ~30% higher than their ³⁶Cl‐³⁶Ar ages, indicating that most if not all the ⁸¹Kr‐Kr ages determined so far are significantly too high. We therefore re‐evaluated both methods to determine P(⁸¹Kr)_c/P(⁸³Kr)_c. Our new Y value of 0.70 ± 0.04 is more than 25% lower than the value of 0.95 used so far. Furthermore, together with literature data, our data indicate that for chondrites, P(⁸¹Kr)_c/P(⁸³Kr)_c is rather constant at 0.43 ± 0.02, at least for the shielding range covered by our samples ([⁷⁸Kr/⁸³Kr]_c = 0.119–0.185; [²²Ne/²¹Ne]_c = 1.083–1.144), in contrast to the observations on lunar samples. As expected considering the method used, ⁸¹Kr‐Kr ages calculated either directly with this new P(⁸¹Kr)_c/P(⁸³Kr)_c value or with our new Y value both agree with the corresponding ³⁶Cl‐³⁶Ar ages. However, the average deviation of 2% indicates the accuracy of both new ⁸¹Kr‐Kr dating methods and the precision of the new dating systems of ~10% is demonstrated by the low scatter in the data. Consequently, this study indicates that the ⁸¹Kr‐Kr ages published so far are up to 30% too high.     

Franck-Condon factors and <Emphasis Type="Italic">r</Emphasis>-centroids for certain band systems of astrophysical molecules SrF and ScF

The Franck-Condon factors and r-centroids, which are very closely related to vibrational transition probabilities, have been evaluated by the more reliable numerical integration procedure for the bands of B ²∑⁺ − X ²∑⁺, F ²∑⁺ − X ²∑⁺ systems of SrF and C ¹∑⁺ − X ¹∑⁺, G ¹Π − X ¹∑⁺ systems of ScF molecules of astrophysical interest, using a suitable potential.     

Extinct isotope heterogeneities in the mantles of Earth and Mars: Implications for mantle stirring rates

Heterogeneities in terrestrial samples for ¹⁸²W/¹⁸³W and ¹⁴²Nd/¹⁴⁴Nd are only preserved in Hadean and Archean rocks while heterogeneities in ¹²⁹Xe/¹³⁰Xe and ¹³⁶Xe/¹³⁰Xe persist to very young mantle‐derived rocks. In contrast, meteorites from Mars show that the Martian mantle preserves heterogeneities in ¹⁸²W/¹⁸³W and ¹⁴²Nd/¹⁴⁴Nd up to the present. As a consequence of the probable “deep magma ocean” core formation process, we assume that the Earth and Mars both had a very early two‐mantle‐reservoir structure with different initial extinct nuclide isotopic compositions (different ¹⁸²W/¹⁸³W, ¹⁴²Nd/¹⁴⁴Nd, ¹²⁹Xe/¹³⁰Xe, ¹³⁶Xe/¹³⁰Xe ratios). Based on this assumption, we developed a simple stochastic model to trace the evolution of a mantle with two initially distinct layers for the extinct isotopes and its development into a heterogeneous mantle by convective mixing and stretching of these two layers. Using the extinct isotope system ¹⁸²Hf‐¹⁸²W, we find that the mantles of Earth and Mars exhibit substantially different mixing or stirring rates. This is consistent with Mars having cooled faster than the Earth due to its smaller size, resulting in less efficient mantle mixing for Mars. Moreover, the mantle stirring rate obtained for Earth using ¹⁸²Hf‐¹⁸²W is consistent with the mantle stirring rate of ~500 Myr constrained by the long‐lived isotope system, ⁸⁷Rb‐⁸⁷Sr and ¹⁴⁷Sm‐¹⁴³Nd. The apparent absence of ¹⁸²W/¹⁸³W isotopic heterogeneity in modern terrestrial rocks is attributed to very active mantle stirring which reduced the ¹⁸²W/¹⁸³W isotopic heterogeneity to a relatively small scale (~83 m for a mantle stirring rate of 500 Myr) compared to the common sampling scale of terrestrial basalts (~30 or 100 km). Our results also support the “deep magma ocean” core formation model as being applicable to both Mars and Earth.     

Anomalous Cosmic-Ray Helium,Nitrogen and Oxygen in 1996 – Measurements of the ERNE Instrument on-board SOHO

The energy spectra of the anomalous components of helium, nitrogen and oxygen have been measured by the ERNE experiment on board the SOHO spacecraft. During February 28–April 30, 1996, the maximum intensity of anomalous helium was found to be 3.8 × 10^-5 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 in the energy range 10–15 MeV nucl^-1. During the period January 26–April 30, 1996, the maximum oxygen intensity was 1.2 × 10^-5 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 at 4–7 MeV nucl^-1, and the maximum nitrogen intensity 1.7 × 10^-6 cm^-2 sr^-1 s^-1 (MeV nucl^-1)^-1 at 4–9 MeV nucl^-1. These peak intensities are at the same level as two solar cycles ago in 1977, but significantly higher than in 1986. This gives observational evidence for a 22-year solar modulation cycle. A noteworthy point is that the spectra of anomalous nitrogen and oxygen appear to be somewhat broader than in 1977.     

X-ray observation of the Jovian impacts of comet Shoemaker - Levy 9

An ASCA observation of the Jovian impact of the comet Shoemaker-Levy 9 is reported. Four impacts of H, L, Q1 and R were observed and four impacts of B, C, G, and Q2 were observed within 60 minutes after their impacts. No significant flaring of X-ray emission was observed. Upper limit X-ray fluxes of 90 % confidence level, averaged 5 minutes just after the impacts, were 2.4 × 10^–13 erg sec^–1 cm^–2, 3.5 × 10^–13 erg sec^–1 cm^–2, 1.6 × 10^–13 erg sec^–1 cm^–2 and 2.9 × 10^–13 erg sec^–1 cm^–2 for the impacts of H, L, Q1 and R, respectively, in the 0.5(0.7 for H and Q1)–10 keV energy range. However, a hint of X-ray enhancement around Jupiter from July 17 to July 19 was detected with about 2 6 × 10^–14 erg sec^–1 cm^–2 in the 0.5–10 keV energy range.     

Search for extinct aluminum‐26 and titanium‐44 in nanodiamonds from the Allende CV3 and Murchison CM2 meteorites

Abstract– We report Mg‐Al and Ca‐Ti isotopic data for meteoritic nanodiamonds separated from the Allende CV3 and Murchison CM2 meteorites. The goal of this study was to search for excesses in ²⁶Mg and ⁴⁴Ca, which can be attributed to the in situ decay of radioactive and now extinct ²⁶Al and ⁴⁴Ti, respectively. Previous work on presolar SiC and graphite had shown that ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios in presolar grains can be used to discriminate between different types of stellar sources. Aluminum and Ti concentrations are low in the meteoritic nanodiamonds of this study. Murchison nanodiamonds have higher Al and Ti concentrations than the Allende nanodiamonds. This can be attributed to contamination and the presence of presolar SiC in the Murchison nanodiamond samples. ²⁶Mg/²⁴Mg and ⁴⁴Ca/⁴⁰Ca ratios are close to normal in Allende nanodiamonds with upper limits on the initial ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios of approximately 1 × 10^?3. These ratios are factors of 10–1000 and, respectively, 1–1000 lower than those of presolar SiC and graphite grains from supernovae. The ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti data for nanodiamonds are compatible with an asymptotic giant branch star or solar system origin, but not with a supernova origin of a major fraction of meteoritic nanodiamonds. The latter possibility cannot be excluded, though, as the diamond separates may contain significant amounts of contaminating Al and Ti, which would lower the inferred ²⁶Al/²⁷Al and ⁴⁴Ti/⁴⁸Ti ratios considerably.     

Mars: Chemical weathering as a massive volatile sink

Photostimulated oxidation weathering irreversibly removes both oxygen and hydrogen from the atmosphere at a rate of 10⁸ to 10¹¹ cm^?2sec^?1. This corresponds to a net loss of 10²⁵ to 10²⁸ molecules cm^?2 (10² to 10⁵ g cm^?2 of H₂O, assuming a uniform rate over geologic time. Additional H₂O is removed through hydration of Fe₂O₃ and clay minerals, but the loss is reversible and the extent of regolith storage is uncertain. CO₂ is irreversibly removed from the atmosphere through the formation of CaCO₃ at a rate of 10⁷?10¹⁰cm^?2sec^?1. Over geologic time this corresponds to a net loss of 10²⁴?10²⁷ molecules cm^?2 (10¹?10⁴g cm^?2) of CO₂. Previously, it was proposed that exospheric escape was the principal irreversible volatile sink, amounting to only 10²g cm^?2 of H₂O and 10⁰g cm^?2 of CO₂ over geologic time. A recent tentative identification of abundant argon on Mars suggests that the planet may have degassed up to 10⁵g cm^?2 of H₂O and 10⁴g cm^?2 of CO₂. If the amounts of H₂O and CO₂ removed by photostimulated oxidation are close to the upper limits proposed here, it is possible that chemical weathering may have had a major effect on limiting the supply of H₂O and CO₂ trapped in the regolith and polar caps.     

Neodymium,strontium and chromium isotopic studies of the LEW86010 and Angra dos Reis meteorites and the chronology of the angrite parent body

Abstract— Neodymium, strontium, and chromium isotopic studies of the LEW86010 angrite established its absolute age and the formation interval between its crystallization and condensation of Allende CAIs from the solar nebula. Pyroxene and phosphate were found to contain ～98% of its Sm and Nd inventory. A conventional ¹⁴⁷Sm-¹⁴³Nd isochron yielded an age of 4.53 ± 0.04 Ga (2 σ) and ?¹⁴³ _Nd = 0.45 ± 1.1. An ¹⁴⁶Sm-¹⁴²Nd isochron gives initial ¹⁴⁶Sm/¹⁴⁴Sm = 0.0076 ± 0.0009 and ?¹⁴³ _Nd = ?2.5 ± 0.4. The Rb-Sr analyses give initial ⁸⁷Sr/⁸⁶Sr (I⁸⁷_Sr) = 0.698972 ± 8 and 0.698970 ± 18 for LEW and ADOR, respectively, relative to ⁸⁷Sr/⁸⁶Sr = 0.71025 for NBS987. The difference, ΔI⁸⁷_Sr, between I⁸⁷_Sr for the angrites and literature values for Allende CAIs, corresponds to ～9 Ma of growth in a solar nebula with a CI chondrite value of ⁸⁷Rb/⁸⁶Sr = 0.91, or ～5 Ma in a nebula with solar photospheric ⁸⁷Rb/⁸⁶Sr = 1.51. Excess ⁵³Cr from extinct ⁵³Mn (t1/2 = 3.7 Ma) in LEW86010 corresponds to initial ⁵³Mn/⁵⁵Mn = 1.44 ± 0.07 × 10^?6 and closure to Cr isotopic homogenization 18.2 ± 1.7 Ma after formation of Allende inclusions, assuming initial ⁵³Mn/⁵⁵Mn = 4.4 ± 1.0 × 10^?5 for the inclusions as previously reported by the Paris group (Birck and Allegre, 1988). The ¹⁴⁶Sm/¹⁴⁴Sm value found for LEW86010 corresponds to solar system initial (¹⁴⁶Sm/¹⁴⁴Sm)_o = 0.0080 ± 0.0009 for crystallization 8 Ma after Allende, the difference between Pb-Pb ages of angrites and Allende, or 0.0086 ± 0.0009 for crystallization 18 Ma after Allende, using the Mn-Cr formation interval. The isotopic data are discussed in the context of a model in which an undifferentiated “chondritic” parent body formed from the solar nebula ～2 Ma after Allende CAIs and subsequently underwent differentiation accompanied by loss of volatiles. Parent bodies with Rb/Sr similar to that of CI, CM, or CO chondrites could satisfy the Cr and Sr isotopic systematics. If the angrite parent body had Rb/Sr similar to that of CV meteorites, it would have to form slightly later, ～2.6 Ma after the CAIs, to satisfy the Sr and Cr isotopic systematics.     

Search for spatial variation in the jovian N/N ratio from Cassini/CIRS observations

We have used the spectra obtained by the Composite Infrared Spectrometer (CIRS) onboard the Cassini spacecraft to search for latitudinal variation in the ¹⁵N/¹⁴N ratio on Jupiter. We found no variations statistically significant given the observational and model uncertainties. The absence of latitudinal variations demonstrates that ¹⁵NH₃ is not fractionated in Jupiter's atmosphere, and that the measured ¹⁵N/¹⁴N represents Jupiter's global value. Our mean value for the global jovian ¹⁵N/¹⁴N ratio of (2.22±0.52)×10⁻³ agrees with previous measurements made by Fouchet et al. (2000, Icarus 143, 223-243) and Owen et al. (2001, Astrophys. J. 553, L77-L79). We argue that the jovian isotopic ¹⁵N/¹⁴N ratio must represent the solar nitrogen isotopic composition. The solar ¹⁵N/¹⁴N ratio hence significantly differs from the terrestrial value: (¹⁵N/¹⁴N)_⊕=3.68×10⁻³. This supports the proposition that terrestrial nitrogen originates from a nitrogen reservoir isolated from the main nitrogen reservoir in the proto-solar nebula. The origin and carrier of this isolated reservoir are still unknown.     

A Study on the CO Isotopic Lines of the Star Forming Region AFGL 5157

By the mapping observations simultaneously at the ¹²CO (J=1-0), ¹³CO (J=1-0), and C¹⁸O (J=1-0) lines on the area of 24’×24’ (12 pc×12 pc) of the star forming region AFGL 5157, we have obtained the distribution and averaged physical parameters for the respective ¹³CO and C¹⁸O cores of this molecu- lar cloud. At the edge of the molecular cloud, the isotopic abundance ratio is X [(¹³CO)/(C¹⁸O)] ≈ 10, close to the ratio of a giant molecular cloud. The viral masses of the ¹³CO and C¹⁸O cores are less than the masses of the molecu-lar cloud cores, so the molecular cloud cores are gravitationally unstable, and the C¹⁸O molecular cloud core is more easy to collapse. The column density distributions of the C¹⁸O molecular cloud core in the northeast and southwest directions are, respectively, 1.1 × 10²³× z^−0.43 and 4.6 × 10²⁵× z^−0.58, where z is the distance from the center of the molecular cloud core. The high velocity molecular out?ow has been con?rmed from our ¹²CO spectra, the mass loss rate of the out?ow has been estimated, and the mass-velocity relation of the out?ow is ?tted by a power-law function of m ∝ v^−1.8. The star formation rate of the ¹³CO molecular cloud core is as high as 23%, probably, under the in?uence of     

Mg isotopic heterogeneity,Al‐Mg isochrons,and canonical 26Al/27Al in the early solar system

There is variability in the Mg isotopic composition that is a reflection of the widespread heterogeneity in the isotopic composition of the elements in the solar system at approximately 100 ppm. Measurements on a single calcium‐aluminum‐rich inclusion (CAI) gave a good correlation of ²⁶Mg/²⁴Mg with ²⁷Al/²⁴Mg, yielding an isochron corresponding to an initial (²⁶Al/²⁷Al)_o = (5.27 ± 0.18) × 10^?5 and an initial (²⁶Mg/²⁴Mg)_o = ?0.127 ± 0.032‰ relative to the standard. This isochron is parallel to that obtained by Jacobsen et al. (2008) , but is distinctively offset. This demonstrates that there are different initial Mg isotopic compositions in different samples with the same ²⁶Al/²⁷Al. No inference about uniformity/heterogeneity of ²⁶Al/²⁷Al on a macro scale can be based on the initial (²⁶Mg/²⁴Mg)_o values. Different values of ²⁶Al/²⁷Al for samples representing the same point in time would prove heterogeneity of ²⁶Al/²⁷Al. The important issue is whether the bulk solar inventory of ²⁶Al/²⁷Al was approximately 5 × 10^?5 at some point in the early solar system. We discuss ultra refractory phases of solar type oxygen isotope composition with ²⁶Al/²⁷Al from approximately 5 × 10^?5 to below 0.2 × 10^?5. We argue that the real issues are: intrinsic heterogeneity in the parent cloud; mechanism and timing for the later production of ¹⁶O‐poor material; and the relationship to earlier formed ¹⁶O‐rich material in the disk. ²⁶Al‐free refractories can be produced at a later time by late infall, if there is an adequate heat source, or from original heterogeneities in the placental molecular cloud from which the solar system formed.     

Theoretical Ne v emission line ratios compared to solar observations

The recent level population calculations for Ne v by Aggarwal are used to determine the theoretical emission line ratios R ₁ = I(2s2p ^{3 1}D^o - 2s²2p^{2 1}D^e)/I(2s2p^{3 3}D ₂ ⁰ - 2s²2p^{2 3}P ₁ ^e ) and R ₂ = I(2s2p ^{3 1}D^o-2s²2p^{2 1}D^e)/I(2s2p ^{3 3}D ₃ ⁰ -2s²2p^{2 3}P ₂ ^e ). A comparison of these with observational data for a solar flare and erupting prominence obtained with the NRL XUV spectrograph on board Skylab reveals that R ₁ and R ₂ are in their predicted high density limits. Although the ratios cannot be used as density diagnostics for values of n _e typical of the solar transition region, it is shown that they are temperature sensitive and hence may be employed to determine the electron temperatures of Ne v line emitting regions.     

Calibration of cosmogenic noble gas production in ordinary chondrites based on 36Cl‐36Ar ages. Part 1: Refined produced rates for cosmogenic 21Ne and 38Ar

We measured the concentrations and isotopic compositions of He, Ne, and Ar in bulk samples and metal separates of 14 ordinary chondrite falls with long exposure ages and high metamorphic grades. In addition, we measured concentrations of the cosmogenic radionuclides ¹⁰Be, ²⁶Al, and ³⁶Cl in metal separates and in the nonmagnetic fractions of the selected meteorites. Using cosmogenic ³⁶Cl and ³⁶Ar measured in the metal separates, we determined ³⁶Cl‐³⁶Ar cosmic‐ray exposure (CRE) ages, which are shielding‐independent and therefore particularly reliable. Using the cosmogenic noble gases and radionuclides, we are able to decipher the CRE history for the studied objects. Based on the correlation ³He/²¹Ne versus ²²Ne/²¹Ne, we demonstrate that, among the meteorites studied, only one suffered significant diffusive losses (about 35%). The data confirm that the linear correlation ³He/²¹Ne versus ²²Ne/²¹Ne breaks down at high shielding. Using ³⁶Cl‐³⁶Ar exposure ages and measured noble gas concentrations, we determine ²¹Ne and ³⁸Ar production rates as a function of ²²Ne/²¹Ne. The new data agree with recent model calculations for the relationship between ²¹Ne and ³⁸Ar production rates and the ²²Ne/²¹Ne ratio, which does not always provide unique shielding information. Based on the model calculations, we determine a new correlation line for ²¹Ne and ³⁸Ar production rates as a function of the shielding indicator ²²Ne/²¹Ne for H, L, and LL chondrites with preatmospheric radii less than about 65 cm. We also calculated the ¹⁰Be/²¹Ne and ²⁶Al/²¹Ne production rate ratios for the investigated samples, which show good agreement with recent model calculations.     

Under the shadow of the Magellanic Bridge: A measurement of the extragalactic background at ∼ 0.7 keV

We measured the extragalactic 0.7 keV X-ray background by observing the X-ray shadow of a neutral gas cloud in the Magellanic Bridge region. Two ROSAT PSPC observations of total 104 ks were complemented by a detailed H I mapping of the cloud with both the Parkes 64 m telescope and the Australia Telescope Compact Array. From the detected anti-correlation between the observed background intensity and the H I column density of the cloud, we derived the unabsorbed extragalactic background intensity as ∼ 28 keV s⁻¹ cm⁻² keV⁻¹ sr⁻¹ at ∼ 0.7 keV. The 95% confidence lower limit 18 keV s⁻¹ cm⁻² keV⁻¹ sr⁻¹ is greater than the expected point-like source contribution ? 14 keV s⁻¹ cm⁻² keV⁻¹ sr⁻¹, constrained by the mean source spectrum together with the total background intensity in the 1-2 keV band. A significant fraction of the 0.7 keV background likely arises in a diffuse hot intergalactic medium of a few million degrees, as has been predicted in hydrodynamic simulations of cosmological structure formation.Richard McCray     

On the Bur Gheluai H5 chondrite and other meteorites with complex exposure histories

Abstract— We present the ¹⁴C, ²⁶Al, ¹⁰Be, ³He, ⁴He, ²⁰Ne, ²¹Ne, ²²Ne, ³⁶Ar, ³⁸Ar, and ⁴⁰Ar concentrations and the track densities measured in up to 13 samples of the Bur Gheluai (H5) meteorite fall. Only a multi-stage exposure history can explain the data in a self-consistent way. Parameters for a model two-stage history obtained by simultaneous, least-squares fitting of the concentrations of ¹⁴C, ²⁶Al, ¹⁰Be, and ²¹Ne were: first stage duration ～10 Ma and radius >2 m; second stage duration ～0.6 Ma and radius 40–100 cm. Nominal one-stage ²¹Ne production rates (P₂₁) inferred from ²⁶Al in Bur Gheluai samples exceed those inferred from ¹⁰Be as expected for a meteorite with a complex history. Nonetheless, data for other meteorites indicate that multi-stage irradiations alone do not account for all the high reported values of P₂₁ based on ²⁶A***l: The equations describing the production of cosmogenic nuclides show that uncorrected shielding effects may also play a role. A compilation of ordinary, solar-gas-poor chondrites for which two-stage histories have been proposed includes many with short second stages but none with unambiguously long first stages (>0.2 Ga).     

Noble gases and strontium isotopes in the unique meteorite Divnoe

Abstract— We present an isotope study of noble gases in Divnoe, an anomalous meteorite, and also Rb-Sr and K-Ar dating of this meteorite. The relatively young Rb-Sr age obtained (3.39 Ga) seems doubtful and, most probably, results from weathering or contamination. The ancient K-Ar age (4.67^+0.20_–0.40), together with clear excess of ¹²⁹Xe, allows the suggestion of very early formation of the Divnoe meteorite. Concentrations and isotope ratios of noble gases in Divnoe are: 17.9 ≤ ³He ≤ 29.0 × 10^?8; ²⁰Ne = 6.22 × 10^?8; 2.44 ≤ ³⁶Ar ≤ 5.10 × 10^?8; ¹³⁰Xe = 41.3 × 10^?12 cm³/g; 0.079 ≤ ³He/⁴He ≤ 0.193; ²⁰Ne/²²Ne = 0.860; ²¹Ne/²²Ne = 0.927; 3.47 ≤ ⁴⁰Ar/³⁶Ar ≤ 9.47; 2.22 ≤ 36Ar/³⁸Ar ≤ 3.27; ¹²⁹Xe/¹³²Xe = 1.09. The exposure age calculated from cosmogenic ³He, ²¹Ne, and ³⁸Ar is 17.9 ± 0.9 Ma. On the basis of the isotope data for the noble gases and O, and abundances of K, Rb, and Sr, an attempt was made to estimate the relationship of Divnoe to other meteorite types. The O-isotope characteristics of Divnoe are clearly distinct from those of ordinary chondrites, acapulcoites/lodranites, and SNC meteorites (Petaev et al., 1994, Clayton, 1993). In plots of ¹³⁶Xe vs. ¹²⁹Xe/¹³⁰Xe, the Divnoe data fall outside of the data fields for carbonaceous and enstatite chondrites. The light noble gas data, especially the ⁴⁰Ar/³⁸Ar ratio, and the ⁴⁰Ar, ³⁸Ar, ³He, and ⁴He contents of Divnoe differ significantly from those of all meteorite types except diogenites. The K, Rb, and Sr abundances in Divnoe are substantially lower than in most other meteorites. In the concentrations of these elements, as well as in the REE pattern, the Divnoe meteorite is similar only to diogenites. Divnoe probably should be treated as a restite remaining after partial melting of the chondritic mantle of a parent asteroid body.     

Cosmic‐ray exposure history of the Norton County enstatite achondrite

Abstract– We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of ³⁶Cl, ⁴¹Ca, ²⁶Al, and ¹⁰Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N‐Particle codes. Assuming a one‐stage irradiation of a meteoroid with a pre‐atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of ¹⁰Be, ²⁶Al, and ⁵³Mn (<6%) but overestimates the ⁴¹Ca concentrations by about 20%. ³He, ²¹Ne, and ²⁶Al data give a one‐stage cosmic‐ray exposure (CRE) age of 115 Ma. Argon‐36 released at intermediate temperatures, ³⁶Ar_n, is attributed to production by thermal neutrons. From the values of ³⁶Ar_n, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 10¹⁶ neutrons cm^?2. We infer comparable values from ε¹⁴⁹Sm and ε¹⁵⁰Sm. Values calculated from ⁴¹Ca and a CRE age of 115 Ma, 0.2–1.4 × 10¹⁶ neutrons cm^?2, are lower by a factor of approximately 2.5, indicating that nearly half of the ¹⁴⁹Sm captures occurred earlier. One possible irradiation history places the center of proto‐Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.