The Case and Context for Atmospheric Methane as an Exoplanet Biosignature

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane's short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that, in contrast to an Earth-like biosphere, known abiotic processes cannot easily generate atmospheres rich in CH$_4$ and CO$_2$ with limited CO due to the strong redox disequilibrium between CH$_4$ and CO$_2$. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH$_4$ that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO$_2$ with comparatively little CO.Comment: 10 pages, 5 figures, 15 pages Supplementary Information, 3 Supplementary Figure

Developing the 60Fe-60Ni System for Early Solar System Chronology

Ph.D. University of Hawaii at Manoa 2015.Includes bibliographical references.This dissertation focuses on in situ Fe and Ni isotope analyses of chondrules from unequilibrated ordinary chondrites (UOCs) using the ion microprobe in order to constrain the initial 60Fe/56Fe ratio of UOC chondrules for early solar system chronology. Most of the chondrules analyzed for this dissertation do not have resolved excesses in 60Ni. A few chondrules have clear excesses in 60Ni (up to ~30‰) that can only be explained by the decay of 60Fe. However, the isochrons are clearly disturbed as shown by the weak correlation between the excesses in 60Ni and the Fe/Ni ratios. This, along with the discrepancies between the initial ratios inferred from bulk and in situ analyses, indicates that the Fe-Ni isotopic system in UOCs was disturbed. Synchrotron X-ray fluorescence maps of Fe and Ni and other trace elements in UOC chondrules confirm this. We found Fe and Ni enrichment along chondrule fractures, indicating extensive open system Fe-Ni redistribution occurred between chondrules and the surrounding matrix. These complications make the Fe-Ni isotope data difficult to interpret. Nevertheless, our data indicate that the initial 60Fe/56Fe ratio of UOC chondrules is between 5×10-8 and 2.6×10-7. Our ion microprobe measurements consist of counting Fe and Ni ions from a chondrule and calculating isotope ratios from those counts. However, ratios calculated this way are systematically higher than the true ratio in the sample. The bias increases proportionally with decreasing count rates of the normalizing isotope and can produce linear correlations similar to those of an isochron. This dissertation provides a detailed discussion of the influence of ratio bias on isochrons and it includes re-calculated ratios for several in situ studies, including most of the previously published in situ Fe-Ni data. Additionally, a study of the influence of ratio bias on in situ 26Al-26Mg (t1/2=0.7 Myr) systematics of plagioclase from H4 chondrites is included in this dissertation. We find that ratio bias is not significant for these analyses. We argue that the 26Al-26Mg ages for these chondrites date impact excavation and cooling at the surface of the H chondrite parent body, not cooling at depth as the onion shell model predicts

The case and context for atmospheric methane as an exoplanet biosignature.

Methane has been proposed as an exoplanet biosignature. Imminent observations with the James Webb Space Telescope may enable methane detections on potentially habitable exoplanets, so it is essential to assess in what planetary contexts methane is a compelling biosignature. Methane’s short photochemical lifetime in terrestrial planet atmospheres implies that abundant methane requires large replenishment fluxes. While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that—in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2. Methane is thus more likely to be biogenic for planets with 1) a terrestrial bulk density, high mean-molecular-weight and anoxic atmosphere, and an old host star; 2) an abundance of CH4 that implies surface fluxes exceeding what could be supplied by abiotic processes; and 3) atmospheric CO2 with comparatively little CO

Composition of terrestrial exoplanet atmospheres from meteorite outgassing experiments

Terrestrial exoplanets likely form initial atmospheres through outgassing during and after accretion, although there is currently no first-principles understanding of how to connect a planet's bulk composition to its early atmospheric properties. Important insights into this connection can be gained by assaying meteorites, representative samples of planetary building blocks. We perform laboratory outgassing experiments that use a mass spectrometer to measure the abundances of volatiles released when meteorite samples are heated to 1200 $^{\circ}$C. We find that outgassing from three carbonaceous chondrite samples consistently produce H$_2$O-rich (averaged ~66 %) atmospheres but with significant amounts of CO (~18 %) and CO$_2$ (~15 %) as well as smaller quantities of H$_2$ and H$_2$S (up to 1 %). These results provide experimental constraints on the initial chemical composition in theoretical models of terrestrial planet atmospheres, supplying abundances for principal gas species as a function of temperature

Composition of terrestrial exoplanet atmospheres from meteorite outgassing experiments

Terrestrial exoplanets likely form initial atmospheres through outgassing during and after accretion, although there is currently no first-principles understanding of how to connect a planet's bulk composition to its early atmospheric properties. Important insights into this connection can be gained by assaying meteorites, representative samples of planetary building blocks. We perform laboratory outgassing experiments that use a mass spectrometer to measure the abundances of volatiles released when meteorite samples are heated to 1200 $^{\circ}$C. We find that outgassing from three carbonaceous chondrite samples consistently produce H$_2$O-rich (averaged ~66 %) atmospheres but with significant amounts of CO (~18 %) and CO$_2$ (~15 %) as well as smaller quantities of H$_2$ and H$_2$S (up to 1 %). These results provide experimental constraints on the initial chemical composition in theoretical models of terrestrial planet atmospheres, supplying abundances for principal gas species as a function of temperature

Iron, zinc, magnesium and uranium isotopic fractionation during continental crust differentiation: The tale from migmatites, granitoids, and pegmatites

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Iron, zinc, magnesium and uranium isotopic fractionation during continental crust differentiation: The tale from migmatites, granitoids, and pegmatites

The causes of some stable isotopic variations in felsic rocks are not well understood. In particular, the origin of the heavy Fe isotopic compositions (i.e., high δ^(56)Fe values, deviation in ‰ of the ^(56)Fe/^(54)Fe ratio relative to IRMM-014) of granites with SiO_2 > 70 wt.% compared with less silicic rocks is still debated. It has been interpreted to reflect isotopic fractionation during late stage aqueous fluid exsolution, magma differentiation, partial melting, or Soret (thermal) diffusion. The present study addresses this issue by comparing the Fe isotopic compositions of a large range of differentiated crustal rocks (whole rocks of migmatites, granitoids, and pegmatites; mineral separates) with the isotopic compositions of Zn, Mg and U. The samples include granites, migmatites and pegmatites from the Black Hills, South Dakota (USA), as well as I-, S-, and A-type granitoids from Lachlan Fold Belt (Australia). The nature of the protolith (i.e., I- or S-type) does not influence the Fe isotopic composition of granitoids. Leucosomes (partial melts in migmatites) tend to have higher δ^(56)Fe values than melanosomes (melt residues) indicating that partial melting of continental crust material can possibly fractionate Fe isotopes. No clear positive correlation is found between the isotopic compositions of Mg, U and Fe, which rules out the process of Soret diffusion in the systems studied here. Zinc isotopes were measured to trace fluid exsolution because Zn can easily be mobilized by aqueous fluids as chloride complexes. Pegmatites and some granitic rocks with high δ^(56)Fe values also have high δ^(66)Zn values. In addition, high-SiO_2 granites show a large dispersion in the Zn/Fe ratio that cannot easily be explained by magma differentiation alone. These results suggest that fluid exsolution is responsible for some of the Fe isotopic fractionation documented in felsic rocks and in particular in pegmatites. However, some granites with high δ^(56)Fe values have unfractionated δ^(66)Zn values and were presumably poor in fluids (e.g., A-type). For these samples, iron isotopic fractionation during magma differentiation is a viable interpretation. Equilibrium Fe isotopic fractionation factors between silicic melts and minerals remain to be characterized to quantitatively assess the role of fractional crystallization on iron isotopes in granitoids

Iron, zinc, magnesium and uranium isotopic fractionation during continental crust differentiation: The tale from migmatites, granitoids, and pegmatites

The causes of some stable isotopic variations in felsic rocks are not well understood. In particular, the origin of the heavy Fe isotopic compositions (i.e., high δ^(56)Fe values, deviation in ‰ of the ^(56)Fe/^(54)Fe ratio relative to IRMM-014) of granites with SiO_2 > 70 wt.% compared with less silicic rocks is still debated. It has been interpreted to reflect isotopic fractionation during late stage aqueous fluid exsolution, magma differentiation, partial melting, or Soret (thermal) diffusion. The present study addresses this issue by comparing the Fe isotopic compositions of a large range of differentiated crustal rocks (whole rocks of migmatites, granitoids, and pegmatites; mineral separates) with the isotopic compositions of Zn, Mg and U. The samples include granites, migmatites and pegmatites from the Black Hills, South Dakota (USA), as well as I-, S-, and A-type granitoids from Lachlan Fold Belt (Australia). The nature of the protolith (i.e., I- or S-type) does not influence the Fe isotopic composition of granitoids. Leucosomes (partial melts in migmatites) tend to have higher δ^(56)Fe values than melanosomes (melt residues) indicating that partial melting of continental crust material can possibly fractionate Fe isotopes. No clear positive correlation is found between the isotopic compositions of Mg, U and Fe, which rules out the process of Soret diffusion in the systems studied here. Zinc isotopes were measured to trace fluid exsolution because Zn can easily be mobilized by aqueous fluids as chloride complexes. Pegmatites and some granitic rocks with high δ^(56)Fe values also have high δ^(66)Zn values. In addition, high-SiO_2 granites show a large dispersion in the Zn/Fe ratio that cannot easily be explained by magma differentiation alone. These results suggest that fluid exsolution is responsible for some of the Fe isotopic fractionation documented in felsic rocks and in particular in pegmatites. However, some granites with high δ^(56)Fe values have unfractionated δ^(66)Zn values and were presumably poor in fluids (e.g., A-type). For these samples, iron isotopic fractionation during magma differentiation is a viable interpretation. Equilibrium Fe isotopic fractionation factors between silicic melts and minerals remain to be characterized to quantitatively assess the role of fractional crystallization on iron isotopes in granitoids