¹K. Righter,²K. Pando,²D. K. Ross,³M. Righter,³T. J. Lapen
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13285]
¹NASA JSC, Mailcode XI2, 2101 NASA Pkwy, Houston, Texas, 77058 USA
²UTC–Jacobs JETS Contract, NASA JSC, Houston, Texas, 77058 USA
³Department of Earth and Atmospheric Sciences, University of Houston, Houston, Texas, 77204 USA
Published by arrangement with John Wiley & Sons

The depletion of volatile siderophile elements (VSE) Sn, Ag, Bi, Cd, and P in mantles of differentiated planetary bodies can be attributed to volatile‐depleted precursor materials (building blocks), fractionation during core formation, fractionation into and retention in sulfide minerals, and/or volatile loss associated with magmatism. Quantitative models to constrain the fractionation due to core formation have not been possible due to the lack of activity and partitioning data. Interaction parameters in Fe‐Si liquids have been measured at 1 GPa, 1600 °C and increase in the order Cd (~6), Ag (~10), Sn (~28), Bi (~46), and P (~58). These large and positive values contrast with smaller and negative values in Fe‐S liquids indicating that any chalcophile behavior exhibited by these elements will be erased by dissolution of a small amount of Si in the metallic liquid. A newly updated activity model is applied to Earth, Mars, and Vesta. Five elements (P, Zn, Sn, Cd, and In) in Earth’s primitive upper mantle can largely be explained by metal‐silicate equilibrium at high PT conditions where the core‐forming metal is a Fe‐Ni‐S‐Si‐C metallic liquid, but two other—Ag and Bi—become overabundant during core formation and require a removal mechanism such as late sulfide segregation. All of the VSE in the mantle of Mars are consistent with core formation in a volatile element depleted body, and do not require any additional processes. Only P and Ag in Vesta’s mantle are consistent with combined core formation and volatile‐depleted precursors, whereas the rest require accretion of chondritic or volatile‐bearing material after core formation. The concentrations of Zn, Ag, and Cd modeled for Vesta’s core are similar to the concentration range measured in magmatic iron meteorites indicating that these volatile elements were already depleted in Vesta’s precursor materials.

¹Mehmet Yesiltas,²Timothy D. Glotch,²Steven Jaret,³Alexander B. Verchovsky,³Richard C. Greenwood
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13287]
¹Faculty of Aeronautics and Space Sciences, Kirklareli University, Kirklareli, 39100 Turkey
²Department of Geosciences, Stony Brook University, Stony Brook, New York, 11794 USA
³School of Physical Sciences, The Open University, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

As of today, the Sariçiçek (SC) meteorite is the newest howardite and the only confirmed fall among the 17 known howardites. In this study, we present isotopic, infrared, and Raman data on three distinct pieces of the SC meteorite. Our oxygen isotopic measurements show that Δ¹⁷O values of the pieces are close to each other, and are in good agreement with other howardites, eucrites, and diogenites. The carbon isotopic measurements, which were conducted by combusting terrestrial contamination selectively at temperatures lower than 500–600 °C, show the presence of indigenous carbon in the SC specimens. The matrix of these specimens, investigated via infrared microspectroscopy, appears to be dominated by clinopyroxene/orthopyroxene, forsterite, and fayalite, with minor contributions from ilmenite, plagioclase, and enstatite. Carbon‐rich regions were mapped and studied via Raman imaging microspectroscopy, which reveals that both amorphous and graphitic carbon exist in these samples. Synchrotron‐based infrared microspectroscopy data show the presence of very little aliphatic and aromatic hydrocarbons. The SC meteorite is suggested to be originating from the Antonia impact crater in the Rheasilvia impact basin on 4 Vesta (Unsalan et al. 2019). If this is in fact the case, then the carbon phases present in the SC samples might provide clues regarding the impactor material (e.g., carbonaceous chondrites).

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.

Colette Salyk¹, John Lacy², Matt Richter³, Ke Zhang⁴, Klaus Pontoppidan⁵, John S. Carr⁶, Joan R. Najita⁷, and Geoffrey A. Blake⁸
Astrophysical Journal 874, 24 Link to Article [DOI: 10.3847/1538-4357/ab05c3 ]
¹Department of Physics and Astronomy, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12604, USA
²Department of Astronomy, University of Texas at Austin, Austin, TX 78712, USA
³Physics Department, University of California at Davis, Davis, CA 95616, USA
⁴Department of Astronomy, University of Michigan, 311 West Hall, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁵Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶Naval Research Laboratory, Code 7213, Washington, DC 20375, USA
⁷National Optical Astronomy Observatory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA
⁸Division of Geological and Planetary Sciences, California Institute of Technology, MC 150-21, 1200 E California Boulevard, Pasadena, CA 91125, USA

We present the largest survey of spectrally resolved mid-infrared water emission to date, with spectra for 11 disks obtained with the Michelle and TEXES spectrographs on Gemini North. Water emission is detected in six of eight disks around classical T Tauri stars. Water emission is not detected in the transitional disks SR 24 N and SR 24 S, in spite of SR 24 S having pretransitional disk properties like DoAr 44, which does show water emission. With R ~ 100,000, the TEXES water spectra have the highest spectral resolution possible at this time, and allow for detailed line shape analysis. We find that the mid-IR water emission lines are similar to the “narrow component” in CO rovibrational emission, consistent with disk radii of a few astronomical units. The emission lines are either single peaked, or consistent with a double peak. Single-peaked emission lines cannot be produced with a Keplerian disk model, and may suggest that water participates in the disk winds proposed to explain single-peaked CO emission lines. Double-peaked emission lines can be used to determine the radius at which the line emission luminosity drops off. For HL Tau, the lower limit on this measured dropoff radius is consistent with the 13 au dark ring. We also report variable line/continuum ratios from the disks around DR Tau and RW Aur, which we attribute to continuum changes and line flux changes, respectively. The reduction in RW Aur line flux corresponds with an observed dimming at visible wavelengths.

Debanjan Sengupta^1,2, Sarah E. Dodson-Robinson^1,3, Yasuhiro Hasegawa², and Neal J. Turner²
Astrophysical Journal 874, 26 Link to Article [DOI: 10.3847/1538-4357/aafc36 ]
¹Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA
²Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
³Bartol Research Institute, Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA

Despite making a small contribution to total protoplanetary disk mass, dust affects the disk temperature by controlling the absorption of starlight. As grains grow from their initial interstellar-medium-like size distribution, settling depletes the disk’s upper layers of dust and decreases the optical depth, cooling the interior. Here we investigate the effect of collisional growth of dust grains and their dynamics on the thermal and optical profile of the disk, and explore the possibility that cooling induced by grain growth and settling could lead to gravitational instability. We develop a Monte Carlo dust collision model with a weighting technique and allow particles to collisionally evolve through sticking and fragmentation, along with vertical settling and turbulent mixing. We explore three disk models and perform simulations for both constant and spatially variable turbulence profile. We then calculate mean wavelength-dependent opacities for the evolving disks and perform radiative transfer to calculate the temperature profile. Finally, we calculate the Toomre Q parameter, a measure of the disk’s stability against self-gravity, after it reaches a steady-state dust-size distribution. We find that even weak turbulence can keep submicrometer-sized particles stirred in the disk’s upper layer, affecting its optical and thermal profiles, and the growth of large particles in the midplane can make a massive disk optically thick at millimeter wavelengths, making it difficult to calculate the surface density of dust available for planet formation in the inner disk. Also, for all our initially marginally stable annuli, we find a small but noticeable reduction in Q.

Yanqin Wu
Astrophysical Journal 874, 91 Link to Article [DOI: 10.3847/1538-4357/ab06f8 ]
Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada

The majority of the transiting planets discovered by the Kepler mission (called super-Earths here, includes the so-called “sub-Neptunes”) orbit close to their stars. As such, photoevaporation of their hydrogen envelopes etches sharp features in an otherwise bland space spanned by planet radius and orbital period. This, in turn, can be exploited to reveal the mass of these planets, in addition to techniques such as radial velocity and transit-timing-variation. Here, using updated radii for Keplerplanet hosts from Gaia DR2, I show that the photoevaporation features shift systematically to larger radii for planets around more massive stars (ranging from M-dwarfs to F-dwarfs), corresponding to a nearly linear scaling between planet mass and its host mass. By modeling planet evolution under photoevaporation, one further deduces that the masses of super-Earths peak narrowly around 8 M_⊕(M _*/M _⊙). When such a stellar mass dependence is scaled out, Kepler planets appear to be a homogeneous population surprisingly uniform in mass, in core composition (likely terrestrial), and in initial mass fraction of their H/He envelope (a couple percent). The masses of these planets do not appear to depend on the metallicity values of their host stars, while they may weakly depend on the orbital separation. Taken together, the simplest interpretation of our results is that super-Earths are at the so-called “thermal mass”, where the planet’s Hill radius is equal to the vertical scale height of the gas disk.

¹C.J.Manser et al. (>10)
Science 364, 66-69 Link to Article [DOI: 10.1126/science.aat5330]
¹Department of Physics, University of Warwick, Coventry CV4 7AL, UK. Reprinted with permission from AAAS

Many white dwarf stars show signs of having accreted smaller bodies, implying that they may host planetary systems. A small number of these systems contain gaseous debris discs, visible through emission lines. We report a stable 123.4-minute periodic variation in the strength and shape of the Ca ii emission line profiles originating from the debris disc around the white dwarf SDSS J122859.93+104032.9. We interpret this short-period signal as the signature of a solid-body planetesimal held together by its internal strength.

¹D.S.Lauretta et al. (>10)
Nature 568, 55-60 Link to Article [https://doi.org/10.1038/s41586-019-1033-6]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

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