¹Cruz-Rosas, H.I.,²Riquelme, F.,³Santiago, P.,³Rendón, L.,⁴Buhse, T.,⁵Ortega-Gutiérrez, F.,⁶Borja-Urby, R.,⁷Mendoza, D.,¹Gaona, C.,¹Miramontes, P.,³Cocho, G.
PLoS ONE 14, e0218750 Link to Article [DOI: 10.1371/journal.pone.0218750]
¹Facultad de Ciencias, Universidad Nacional Autónoma de México, Ciudad Universitaria, Cd. Mx., Mexico
²Laboratorio de Sistemática Molecular, Escuela de Estudios Superiores del Jicarero, Universidad Autónoma del Estado de Morelos, Jicarero, Morelos, Mexico
³Instituto de Física, Universidad Nacional Autónoma de México, Ciudad Universitaria, Cd. Mx., Mexico
⁴Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos, Mexico
⁵Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Cd. Mx., Mexico
⁶Centro de Nanociencias y Micro y Nanotecnologías, Instituto Politécnico Nacional, Zacatenco, Cd. Mx., Mexico
⁷Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Ciudad Universitaria, Cd. Mx., Mexico

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^1,2Naraoka, H.,¹Hashiguchi, M.,³Sato, Y.,^1,3Hamase, K.
Life 9, 62 Link to Article [DOI: 10.3390/life9030062]
¹Research Center for Planetary Trace Organic Compounds, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
²Department of Earth and Planetary Sciences, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
³Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka, 812-8582, Japan

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¹Francis M.McCubbin,^1,2Jessica J.Barnes
Earth and Planetary Science Letters 526, 115771 Link to Article [https://doi.org/10.1016/j.epsl.2019.115771]
¹Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX 77058, United States
²Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ 85721, United States
Copyright Elsevier

The presence of H2O within differentiated terrestrial bodies in the inner Solar System is well established; however, the source(s) of this H2O and the time of its arrival to the inner Solar System is an area of active study. At present, the prevailing model for the origin of inner Solar System H2O calls upon carbonaceous chondrites as the source. This is largely based on reported observations that H- and N-isotopic compositions of differentiated planetary bodies are largely the same and within a range of values that overlaps with carbonaceous chondrites as opposed to comets or the Sun. In this contribution, we evaluate the efficacy of this model and other models for the origin of inner Solar System H2O by considering geochronological constraints on early Solar System history, constraints on primary building blocks of differentiated bodies based on nucleosynthetic isotope anomalies, and constraints from dynamical models of planet formation. In addition to H- and N-isotopic data, these constraints indicate that an interstellar source of H2O was present in the inner Solar System within the first 4 Ma of CAI formation. Furthermore, the most H2O-rich carbonaceous chondrites are unlikely to be the source of H2O for the earliest-formed differentiated bodies based on their minimally overlapping primary accretion windows and the separation of their respective isotopic reservoirs by Jupiter in the timespan of about 1–4 Ma after CAI formation. The presence of deuterium-rich, non-nebular H2O sources in the inner Solar System prior to the formation of carbonaceous chondrites or comets implies early contributions of interstellar ices to both the inner and outer Solar System portions of the protoplanetary disk. Evidence for this interstellar ice component in the inner Solar System may be preserved in LL chondrites and in the mantle of Mars. In contrast to the earlier-formed bodies within the inner Solar System, Earth’s protracted accretion window may have facilitated incorporation of H2O in its interior from both the inner and outer Solar System, helping the Earth to become a habitable planet.

^1,2Asmaa Boujibar,^3,4,5Mya Habermann,¹Kevin Righter,^6,7D. Kent Ross,⁶Kellye Pando,⁸Minako Righter,⁹Bethany A. Chidester,⁶Lisa R. Danielson
American Mineralogist 104, 1221-1237 Link to Article [https://doi.org/10.2138/am-2019-7000]
¹NASA Johnson Space Center, 2101 E NASA Parkway, Houston, Texas 77058, U.S.A.
²Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
³Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, U.S.A.
⁴HX5, NASA Johnson Space Center, 2101 E NASA Parkway, Houston, Texas 77058, U.S.A.
⁵Department of Earth and Planetary Sciences, University of New Mexico, 221 Yale Boulevard NE, Albuquerque, New Mexico 87131, U.S.A.
⁶Jacobs, NASA Johnson Space Center, 2101 E NASA Parkway, Houston, Texas 77058, U.S.A.
⁷UTEP-CASSMAR, 500 W University Avenue, El Paso, Texas 79968, U.S.A.
⁸Department of Earth and Atmospheric Sciences, University of Houston, 3507 Cullen Boulevard, Houston, Texas 77004, U.S.A.
⁹Department of the Geophysical Sciences, University of Chicago, 5734 S Ellis Avenue, Chicago, Illinois 60637, U.S.A.
Copyright: The Mineralogical Society of America

The distribution of heat-producing elements (HPE) potassium (K), uranium (U), and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner solar system. To investigate the abundances of HPE in Mercury’s interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th, and K partitioning between metal, silicate, and sulfide (D^met/sil and D^sulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and that partitioning is enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts, respectively. Also, at low oxygen fugacity (log f_O2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. D^met/sil for U, Th, and K increases with decreasing oxygen fugacity, while Dmet/silUDUmet/sil and Dmet/silKDKmet/sil increase when the metal is enriched and depleted in O or Si, respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O, and C) in Fe metal and sulfide. We calculated chemical compositions of Mercury’s core after core segregation, for a range of f_O2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S, and negligible C. Also if core-mantle separation happened at a f_O2 lower than IW-4, the bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th, and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th, and K reservoirs into account. Without any sulfide layer, or if Mercury’s core segregated at a higher f_O2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance if core segregation occurred near IW-5.5 or more reduced conditions, and with a sulfide layer of at least 130 km thickness. At these conditions, the bulk Mercury K/Th ratio is close to Venus’s and Earth’s values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at an f_O2 close to, or lower than, IW-6 both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury’s surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar to or lower than the Earth’s and Venus’s, suggesting volatile depletion in the inner solar system. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.