208 research outputs found

    The Volatility Trend of Protosolar and Terrestrial Elemental Abundances

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    We present new estimates of protosolar elemental abundances based on an improved combination of solar photospheric abundances and CI chondritic abundances. These new estimates indicate CI chondrites and solar abundances are consistent for 60 elements. We compare our new protosolar abundances with our recent estimates of bulk Earth composition (normalized to aluminium), thereby quantifying the devolatilization in going from the solar nebula to the formation of the Earth. The quantification yields a linear trend log(f)=αlog(TC)+β\log(f) = \alpha\log(T_C) + \beta, where ff is the Earth-to-Sun abundance ratio and TCT_C is the 50%\% condensation temperature of elements. The best fit coefficients are: α=3.676±0.142\alpha = 3.676\pm 0.142 and β=11.556±0.436\beta = -11.556\pm 0.436. The quantification of these parameters constrains models of devolatilization processes. For example, the coefficients α\alpha and β\beta determine a critical devolatilization temperature for the Earth TD(E)=1391±15T_{\mathrm{D}}(\mathrm{E}) = 1391 \pm 15 K. The terrestrial abundances of elements with TC<TD(E)T_{C} < T_{\mathrm{D}}(\mathrm{E}) are depleted compared with solar abundances, whereas the terrestrial abundances of elements with TC>TD(E)T_{C} > T_{\mathrm{D}}(\mathrm{E}) are indistinguishable from solar abundances. The terrestrial abundance of Hg (TCT_C = 252 K) appears anomalously high under the assumption that solar and CI chondrite Hg abundances are identical. To resolve this anomaly, we propose that CI chondrites have been depleted in Hg relative to the Sun by a factor of 13±713\pm7. We use the best-fit volatility trend to derive the fractional distribution of carbon and oxygen between volatile and refractory components (fvolf_\mathrm{vol}, freff_\mathrm{ref}). We find (0.91±0.080.91\pm 0.08, 0.09±0.080.09 \pm 0.08) for carbon and (0.80±0.040.80 \pm 0.04, 0.20±0.040.20 \pm 0.04) for oxygen.Comment: Accepted for publication in Icarus. 28 pages, 12 figures, 5 tables. Compared to v1, the results and conclusion are the same, while discussion of results and implications is expanded considerabl

    The Elemental Abundances (with Uncertainties) of the Most Earth-like Planet

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    To first order, the Earth as well as other rocky planets in the Solar System and rocky exoplanets orbiting other stars, are refractory pieces of the stellar nebula out of which they formed. To estimate the chemical composition of rocky exoplanets based on their stellar hosts' elemental abundances, we need a better understanding of the devolatilization that produced the Earth. To quantify the chemical relationships between the Earth, the Sun and other bodies in the Solar System, the elemental abundances of the bulk Earth are required. The key to comparing Earth's composition with those of other objects is to have a determination of the bulk composition with an appropriate estimate of uncertainties. Here we present concordance estimates (with uncertainties) of the elemental abundances of the bulk Earth, which can be used in such studies. First we compile, combine and renormalize a large set of heterogeneous literature values of the primitive mantle (PM) and of the core. We then integrate standard radial density profiles of the Earth and renormalize them to the current best estimate for the mass of the Earth. Using estimates of the uncertainties in i) the density profiles, ii) the core-mantle boundary and iii) the inner core boundary, we employ standard error propagation to obtain a core mass fraction of 32.5±0.332.5 \pm 0.3 wt%. Our bulk Earth abundances are the weighted sum of our concordance core abundances and concordance PM abundances. Our concordance estimates for the abundances of Mg, Sn, Br, B, Cd and Be are significantly lower than previous estimates of the bulk Earth. Our concordance estimates for the abundances of Na, K, Cl, Zn, Sr, F, Ga, Rb, Nb, Gd, Ta, He, Ar, and Kr are significantly higher. The uncertainties on our elemental abundances usefully calibrate the unresolved discrepancies between standard Earth models under various geochemical and geophysical assumptions

    Europium as a lodestar: diagnosis of radiogenic heat production in terrestrial exoplanets

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    Long-lived radioactive nuclides, such as 40^{40}K, 232^{232}Th, 235^{235}U and 238^{238}U, contribute to persistent heat production in the mantle of terrestrial-type planets. As refractory elements, the concentrations of Th and U in a terrestrial exoplanet are implicitly reflected in the photospheric abundances in the stellar host. However, a robust determination of these stellar abundances is difficult in practice owing to the general paucity and weakness of the relevant spectral features. We draw attention to the refractory, rr-process element europium, which may be used as a convenient and practical proxy for the population analysis of radiogenic heating in exoplanetary systems. As a case study, we present a determination of Eu abundances in the photospheres of α\alpha Cen A and B. We find that europium is depleted with respect to iron by \sim 0.1 dex and to silicon by \sim 0.15 dex compared to solar in both binary components. To first order, the measured Eu abundances can be converted to the abundances of 232^{232}Th, 235^{235}U and 238^{238}U with observational constraints while the abundance of 40^{40}K is approximated independently with a Galactic chemical evolution model. We find that the radiogenic heat budget in an α\alpha-Cen-Earth is 73.46.9+8.373.4^{+8.3}_{-6.9} TW upon its formation and 8.81.3+1.78.8^{+1.7}_{-1.3} TW at the present day, respectively 23±523\pm5 % and 54±554\pm5 % lower than that in the Hadean and modern Earth. As a consequence, mantle convection in an α\alpha-Cen-Earth is expected to be overall weaker than that of the Earth (assuming other conditions are the same) and thus such a planet would be less geologically active, suppressing its long-term potential to recycle its crust and volatiles. With Eu abundances being available for a large sample of Sun-like stars, the proposed approach can extend our ability to make predictions about the nature of other rocky worlds.Comment: Accepted for publication in Astronomy & Astrophysics. 11 pages, 4 figures, and 4 table

    Downgrading MELD Improves the Outcomes after Liver Transplantation in Patients with Acute-on-Chronic Hepatitis B Liver Failure

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    Background: High score of model for end-stage liver diseases (MELD) before liver transplantation (LT) indicates poor prognosis. Artificial liver support system (ALSS) has been proved to effectively improve liver and kidney functions, and thus reduce the MELD score. We aim to evaluate whether downgrading MELD score could improve patient survival after LT. Methodology/Principal Findings: One hundred and twenty-six LT candidates with acute-on-chronic hepatitis B liver failure and MELD score 30wereincludedinthisprospectivestudy.Ofthe126patients,42receivedemergencyLTwithin72h(ELTgroup)andtheother84weregivenALSSassalvagetreatment.Ofthe84patients,33werefoundtohavereducedMELDscore(,30)onthedayofLT(DGMgroup),51underwentLTwithpersistenthighMELDscore(NDGMgroup).Themedianwaitingtimeforadonorwas10forDGMgroupand9.5daysforNDGMgroup.InNDGMgroupthereisasignificantlyhigheroverallmortality(43.130 were included in this prospective study. Of the 126 patients, 42 received emergency LT within 72 h (ELT group) and the other 84 were given ALSS as salvage treatment. Of the 84 patients, 33 were found to have reduced MELD score (,30) on the day of LT (DGM group), 51 underwent LT with persistent high MELD score (N-DGM group). The median waiting time for a donor was 10 for DGM group and 9.5 days for N-DGM group. In N-DGM group there is a significantly higher overall mortality (43.1%) than that in ELT group (16.7%) and DGM group (15.2%). N-DGM (vs. ECT and DGM) was the only independent risk factor of overall mortality (P = 0.003). Age.40 years and the interval from last ALSS to LT.48 h were independent negative influence factors of downgrading MELD. Conclusions/Significance: Downgrading MELD for liver transplant candidates with MELD score 30 was effective i