¹De‐Liang Chen,^1,2Ai‐Cheng Zhang,¹Run‐Lian Pang,¹Jia‐Ni Chen,³Yang Li
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13286]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210023 China
²CAS Center for Excellence in Comparative Planetology, Hefei, 230026 China
³Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081 China
Published by arrangement with John Wiley & Sons

Anorthite is an important constituent mineral in basaltic achondrites from small celestial bodies. Its high‐pressure phase transformation in shocked meteorites has not been systematically studied. In this study, we report the diverse phase transformation behaviors of anorthite in a shocked eucrite Northwest Africa (NWA) 2650, which also contains coesite, stishovite, vacancy‐rich clinopyroxene, super‐silicic garnet, and reidite. Anorthite in NWA 2650 has transformed into anorthite glass (anorthite glassy vein, maskelynite, and glass with a schlieren texture and vesicles), tissintite and dissociated into three‐phase assemblage grossular + kyanite + silica glass. Different occurrences of anorthite glass might have formed via the mechanism involving shear melting, solid‐state transformation, and postshock thermally melting, respectively. Tissintite could have crystallized from a high‐pressure plagioclase melt. The nucleation of tissintite might be facilitated by relict pyroxene fragments and the early formed vacancy‐rich clinopyroxene. The three‐phase assemblage grossular, kyanite, and silica glass should have formed from anorthitic melt at high‐pressure and high‐temperature conditions. The presence of maskelynite and reidite probably suggests a minimum peak shock pressure up to 20 GPa, while the other high‐pressure phases indicate that the shock pressure during the crystallization of shock melt veins might vary from >8 GPa to >2 GPa with a heterogeneous temperature distribution.

Edward W. Schwieterman^1,2,3,4, Christopher T. Reinhard^3,5, Stephanie L. Olson^3,6, Kazumi Ozaki^3,5,7, Chester E. Harman^3,8,9, Peng K. Hong¹⁰, and Timothy W. Lyons^1,3
Astrophysical Journal 874, 9 Link to Article [DOI: 10.3847/1538-4357/ab05e1 ]
¹Department of Earth Sciences, University of California, Riverside, CA, USA
²NASA Postdoctoral Program, Universities Space Research Association, Columbia, MD, USA
³NASA Astrobiology Institute, Alternative Earths & Virtual Planetary Laboratory Teams, USA
⁴Blue Marble Space Institute of Science, Seattle, WA, USA
⁵School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
⁶Department of Geophysical Sciences, University of Chicago, Chicago, IL, USA
⁷Department of Environmental Science, Toho University, Tokyo, Japan
⁸NASA Goddard Institute for Space Studies, New York, NY, USA
⁹Department of Applied Mathematics and Applied Physics, Columbia University, New York, NY, USA
¹⁰Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan

Some atmospheric gases have been proposed as counter indicators to the presence of life on an exoplanet if remotely detectable at sufficient abundance (i.e., antibiosignatures), informing the search for biosignatures and potentially fingerprinting uninhabited habitats. However, the quantitative extent to which putative antibiosignatures could exist in the atmospheres of inhabited planets is not well understood. The most commonly referenced potential antibiosignature is CO, because it represents a source of free energy and reduced carbon that is readily exploited by life on Earth and is thus often assumed to accumulate only in the absence of life. Yet, biospheres actively produce CO through biomass burning, photooxidation processes, and release of gases that are photochemically converted into CO in the atmosphere. We demonstrate with a 1D ecosphere-atmosphere model that reducing biospheres can maintain CO levels of ~100 ppmv even at low H₂ fluxes due to the impact of hybrid photosynthetic ecosystems. Additionally, we show that photochemistry around M dwarf stars is particularly favorable for the buildup of CO, with plausible concentrations for inhabited, oxygen-rich planets extending from hundreds of ppm to several percent. Since CH₄ buildup is also favored on these worlds, and because O₂ and O₃ are likely not detectable with the James Webb Space Telescope, the presence of high CO (>100 ppmv) may discriminate between oxygen-rich and reducing biospheres with near-future transmission observations. These results suggest that spectroscopic detection of CO can be compatible with the presence of life and that a comprehensive contextual assessment is required to validate the significance of potential antibiosignatures.

^1,2Yuan Yin,^1,2Zeming Li,¹Shuangmeng Zhai
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.03.037]
¹Key Laboratory of High-temperature and High-pressure Study of the Earth’s Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
²University of Chinese Academy of Sciences, Beijing 100049, China
Copyright Elsevier

Phosphorus is a potential candidate in the metallic core of the Moon. The phase diagram of the Fe-P binary system was investigated at the pressure of 3 GPa and temperatures of up to 1600 °C. Up to 3.0 wt% and 10.4 wt% phosphorus can dissolve in the solid iron and liquid Fe-P phases at 1100 °C and 3 GPa, respectively. The eutectic temperature on the iron-rich side was determined as 1085 °C at 3 GPa. The solubility of phosphorus in the iron decreases from ∼1.4 wt% at 1100 °C to ∼0.7 wt% at 1500 °C and 3 GPa. Structure of the solid iron in the quenched sample is the body-center cubic, corresponding to α-Fe phase. Extending the phosphorus solubility in the solid iron to the present lunar core conditions yields a maximum phosphorus concentration in a fully crystallized iron core of 0.85 ± 0.15 wt%. If there are Ni and C in the core, the value would be depressed to 0.4 ± 0.1 wt%. In addition, based on a simple siderophile mass balance between the bulk Moon (BM) and bulk silicate Moon (BSM) and a modeled phosphorus partition coefficient, Dcore/mantle P-Moon(40 – 200) for the lunar magma ocean, a bulk silicate Earth-like P content (82 ± 8 ppm) in the initial Moon yields a lunar core with < 0.3 wt% P. Some other potential light elements such as S and C could reduce the P content in the lunar core. Furthermore, the partition coefficient of phosphorus in the iron and liquid melt (DSM/LM P) was found to be 0.10 ± 0.03 at 3 GPa. Taking the sulfur into account, the DSM/LM P increase to 0.18 ± 0.02 at 5 GPa in the S-rich liquid metal (∼8 wt%). In the case of a solid lunar inner core and S-bearing liquid outer core, their P contents were assessed to be less than 0.09 ± 0.01 wt% and 0.51 ± 0.01 wt%, respectively, when the lunar core’s storage of P is <0.3 wt%. The moderate phosphorus solubility in the solid iron, combined with the assumption of abundant phosphorus in the bulk Moon, indicates that the phosphorus concentration in the lunar core could higher than previously thought.