Prabal Saxena^1,2, Rosemary M. Killen¹, Vladimir Airapetian^1,3, Noah E. Petro¹, Natalie M. Curran^1,4, and Avi M. Mandell¹
Astrophysical Journal Letters 876, L16 Link to Article [DOI: 10.3847/2041-8213/ab18fb]
¹NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
²CREST II/University of Maryland, College Park, MD 20742, USA
³American University, Washington, DC 20016, USA
⁴USRA, Columbia, MD, 21046, USA

While the Earth and Moon are generally similar in composition, a notable difference between the two is the apparent depletion in moderately volatile elements in lunar samples. This is often attributed to the formation process of the Moon, and it demonstrates the importance of these elements as evolutionary tracers. Here we show that paleo space weather may have driven the loss of a significant portion of moderate volatiles, such as sodium and potassium, from the surface of the Moon. The remaining sodium and potassium in the regolith is dependent on the primordial rotation state of the Sun. Notably, given the joint constraints shown in the observed degree of depletion of sodium and potassium in lunar samples and the evolution of activity of solar analogs over time, the Sun is highly likely to have been a slow rotator. Because the young Sun’s activity was important in affecting the evolution of planetary surfaces, atmospheres, and habitability in the early Solar System, this is an important constraint on the solar activity environment at that time. Finally, as solar activity was strongest in the first billion years of the Solar System, when the Moon was most heavily bombarded by impactors, evolution of the Sun’s activity may also be recorded in lunar crust and would be an important well-preserved and relatively accessible record of past Solar System processes.

¹Haijun Cao,¹Jian Chen,¹Zongcheng Ling
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.05.039]
¹Shandong Provincial Key Laboratory of Optical Astronomy & Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai 264209, China
Copyright Elsevier 

Orbital remote sensing has recently identified alunite as one of the types of Al-sulfate on Mars. As other types of hydrated Al-sulfates may also exist abundantly in the soil and rocks on Mars, it is important to perform systematic experimental investigations of the spectral characterization of hydrous Al-sulfates to identify and determine their potential distribution on Mars. We successfully used the humidity buffer technique to synthesize five alunogen series of Al-sulfates, Al₂(SO₄)₃·xH₂O (x = 0, 4, 8, 12, 14) and AlH(SO₄)₂·H₂O, with different degrees of hydration. X-ray diffraction (XRD) was used to identify them from the PDF 2004 database, except for the species with 12 structural water molecules; the quantity of structural water in the latter was confirmed by thermogravimetry and differential scanning calorimetry measurements after heating to >500 °C. Raman, mid-infrared (MIR), and visible and near-infrared (VNIR) spectra were acquired to evaluate vibrational spectroscopic properties related to crystal structure. The prominent ν₁ modes of SO₄ tetrahedra of six Al-sulfates from Raman and MIR spectra have obvious shifts to higher wavenumbers (from 993.4 to 1133.6 cm⁻¹) with decrease in hydration state. With no absorption bands from 250 to 1000 nm, all absorption features of the VNIR spectra of alunogen series of Al-sulfates are derived from overtones and combinations of fundamental vibrational modes from OH/H₂O and SO₄ groups, generally showing a red shift toward longer wavelengths with increasing hydration state. The XRD, Raman, MIR, and VNIR spectroscopic data of these Al-sulfates can provide crucial data supporting their identification for future remote sensing and in situ detection on Mars.

¹Driss Takir,²Karen R.Stockstill-Cahill,²Charles A.Hibbitts,³Yusuke Nakauchi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.05.012]
¹Jacobs/ARES, NASA Johnson Space Center, Houston, TX 77058-3696, USA
²Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20273, USA
³JAXA Institute of Space and Astronautical Science, Sagamihara, Japan
Copyright Elsevier

We measured 3-μm reflectance spectra of 21 meteorites that represent all carbonaceous chondrite types available in terrestrial meteorite collections. The measurements were conducted at the Laboratory for Spectroscopy under Planetary Environmental Conditions (LabSPEC) at the Johns Hopkins University Applied Physics Laboratory (JHU APL) under vacuum and thermally-desiccated conditions (asteroid-like conditions). This is the most comprehensive 3-μm dataset of carbonaceous chondrites ever acquired in environments similar to the ones experienced by asteroids. The 3-μm reflectance spectra are extremely important for direct comparisons with and appropriate interpretations of reflectance data from ground-based telescopic and spacecraft observations of asteroids. We found good agreement between 3-μm spectral characteristics of carbonaceous chondrites and carbonaceous chondrite classifications. The 3-μm band is diverse, indicative of varying composition, thus suggesting that these carbonaceous chondrites experienced distinct parent body aqueous alteration and metamorphism environments. The spectra of CI chondrites, from which significant amount of water adsorbed under ambient conditions was removed, are consistent with Mg-serpentine and clay minerals. The high abundances of organics in CI chondrites is also associated with the mineralogy of these chondrites, oxyhydroxides- and complex clay minerals-rich. CM chondrites, which are cronstedtite-rich, have shallower 3-μm band than CI chondrites, suggesting they experienced less aqueous alteration. CR chondrites showed moderate aqueous alteration relative to CI and CM chondrites. CV chondrites, except for Efremovka, have a very shallow 3-μm band, consistent with their lower phyllosilicate proportions. CO chondrites, like most CVs, have a very shallow 3-μm band that suggest they experienced minor aqueous alteration. The 3-μm band in CH/CBb is deep and broad centered ~3.11 μm, possibly due to the high abundance of FeNi metal and presence of heavily hydrated clasts in these chondrites. The 3-μm spectra of Essebi (C2-ung) and EET 83226 are more consistent with CM chondrites’ spectra. The 3-μm spectra of Tagish lake (C2-ung), on the other hand, are consistent with CI chondrites. None of these spectral details could have been resolved without removing the adsorbed water before acquiring spectra.

¹Youyue Zhang,¹Takashi Yoshino,¹Akira Yoneda,²Masahiro Osako
Earth and Planetary Science Letters 519, 109-119 Link to Article [https://doi.org/10.1016/j.epsl.2019.04.048]
¹Institute for Planetary Materials, Okayama University, Misasa, Tottori 682-0193, Japan
²National Museum of Nature and Science, Tsukuba, Ibaraki 305-0005, Japan
Copyright Elsevier

The influence of Fe concentration on heat transport properties of olivine was investigated to understand the cooling history of rocky planets such as Mercury, Mars and asteroids. Thermal conductivity (λ) and thermal diffusivity (κ) were measured simultaneously for olivine polycrystal with different Fe contents (Fo, Fo₉₀, Fo₇₀, Fo₅₀, Fo₃₁ and Fo₀) up to 10 GPa and 1100 K by a pulse heating method. With increasing Fe in olivine, thermal conductivity of olivine first decreases and then slightly increases. The minimum λ was found to be at composition near Fo₃₁; the absolute λ value of Fo₃₁ is about 65% lower than that of Fo. Small amounts of Fe in olivine can strongly reduce the thermal conductivity at low temperature; λ value of Fo₉₀ is about 50% of Fo at room temperature. Thermal conductivities of polycrystalline olivine have smaller absolute values and weaker pressure and temperature dependences, compared with those of natural single crystal olivine determined by previous studies. Heat capacity of Fo₇₀ and Fo₅₀ calculated from λ and κ is independent of pressure and is controlled by nearly constant thermal expansion coefficient with increasing temperature. Smaller λ of olivine aggregate with high Fe content would produce a warmer mantle and, in turn, possibly a thicker crust in the Fe-rich Mars, while heat in the Fe-poor Mercury can escape faster than the other terrestrial planets. Olivine-dominant asteroids with high Fe concentration could have longer cooling history and lower thermal inertia on the surface.

¹Keith D. Putirka,¹John C. Rarick
American Mineralogist 104, 817-829 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2019/Abstracts/AM104P0817.pdf]
¹Department of Earth and Environmental Sciences, Fresno State, 2345 E. San Ramon Avenue, MS/MH24, Fresno, California 93720, U.S.A.
Copyright: The Mineralogical Society of America

Combining occurrence rates of rocky exoplanets about sun-like stars, with the number of such stars that occupy possibly hospitable regions of the Milky Way, we estimate that at least 1.4 × 108 near-Earthsized planets occupy habitable orbits about habitable stars. This number is highly imprecise to be sure, and it is likely much higher, but it illustrates that such planets are common, not rare. To test whether such rocky exoplanets might be geologically similar to Earth, we survey >4000 star compositions from the Hypatia Catalog—the most compositionally broad of such collections. We find that rocky exoplanets will have silicate mantles dominated by olivine and/or orthopyroxene, depending upon Fe partitioning during core formation. Some exoplanets may be magnesiowüstite- or quartz-saturated, and we present a new classification scheme based on the weight percent ratio (FeO+MgO)/SiO2, to differentiate rock types. But wholly exotic mantle mineralogies should be rare to absent; many exoplanets will have a peridotite mantle like Earth, but pyroxenite planets should also be quite common. In addition, we find that half or more of the range of exoplanet mantle mineralogy is possibly controlled by core formation, which we model using αFe = FeBSP/FeBP, where FeBSP is Fe in a Bulk Silicate Planet (bulk planet, minus core), on a cation weight percent basis (elemental weight proportions, absent anions) and FeBP is the cation weight percent of Fe for a Bulk Planet. This ratio expresses, in this case for Fe, the fraction of an element that is partitioned into the silicate mantle relative to the total amount available upon accretion. In our solar system, αFe varies from close to 0 (Mercury) to about 0.54 (Mars). Remaining variations in theoretical exoplanet mantle mineralogy result from non-trivial variations in star compositions. But we also find that Earth is decidedly non-solar (non-chondritic); this is not a new result, but appears worth re-emphasizing, given that current discussions often still use carbonaceous or enstatite chondrites as models of Bulk Earth. While some studies emphasize the close overlap of some isotope ratios between certain meteoritic and terrestrial (Earth-derived) samples, we find that major oxides of chondritic meteorites do not precisely explain bulk Earth. To allow Earth to be chondritic (or solar), there is the possibility that Earth contains a hidden component that, added to known reservoirs, would yield a solar/chondritic bulk Earth. We test that idea using a mass balance of major oxides using known reservoirs, so that the sum of upper mantle, metallic core, and crust, plus a hidden component, yields a solar bulk composition. In this approach, the fractions of crust and core are fixed and the hidden mantle component, Fh, is some unknown fraction of the entire mantle (so if FDM is the fraction of depleted mantle, then Fh + FDM = 1). Such mass balance shows that if a hidden mantle component were to exist, it must comprise >28% of Earth’s mantle, otherwise it would have negative abundances of TiO2 and Al2O3. There is no clear upper limit for such a component, so it could comprise the entire mantle. But all estimates from Fh = 0.28 to Fh = 1.0 yield a hidden fraction that does not match the inferred sources of ocean island or mid-ocean ridge basalts, and would be geologically unusual, having higher Na2O, Cr2O3, and FeO and lower CaO, MgO, and Al2O3 compared to familiar mantle components. We conclude that such a hidden component does not exist.

^1,2Renyu Hu
Earth and Planetary Science Letters 519, 192-201 Link to Article [https://doi.org/10.1016/j.epsl.2019.05.017]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Division of Planetary and Geological Science, California Institute of Technology, Pasadena, CA 91125, USA
Copyright Elsevier

Regolith on Mars exchanges water with the atmosphere on a diurnal basis and this process causes significant variation in the abundance of water vapor at the surface. While previous studies of regolith-atmosphere exchange focus on the abundance, recent in-situ experiments and remote sensing observations measure the isotopic composition of the atmospheric water. We are therefore motivated to investigate isotopic water exchange between the atmosphere and the regolith and determine its effect on the deuterium to hydrogen ratio (D/H) of the atmosphere. We model transport of water in the regolith and regolith-atmosphere exchange by solving a transport equation including regolith adsorption, condensation, and diffusion. The model calculates equilibrium fractionation between HDO and H2O in each of these processes. The fractionation in adsorption is caused by the difference in the latent heat of adsorption, and that of condensation is caused by the difference in the vapor pressure. Together with a simple, bulk-aerodynamic boundary layer model, we simulate the diurnal variation of the D/H near the planetary surface. We find that the D/H can vary by 300–1400‰ diurnally in the equatorial and mid-latitude locations, and the magnitude is greater at a colder location or season. The variability is mainly driven by adsorption and desorption of regolith particles, and its diurnal trend features a drop in the early morning, a rise to the peak value during the daytime, and a second drop in the late afternoon and evening, tracing the water vapor flow into and out from the regolith. The predicted D/H variation can be tested with in-situ measurements. As such, our calculations suggest stable isotope analysis to be a powerful tool in pinpointing regolith-atmosphere exchange of water on Mars.

¹A.Néri,¹J.Guignard,¹M.Monnereau,¹M.J.Toplis,¹G.Quitté
Earth and Planetary Science Letters 518, 40-52 Link to Article [https://doi.org/10.1016/j.epsl.2019.04.049]
¹IRAP, Université de Toulouse, CNRS, CNES, UPS, Toulouse, France
Copyright Elsevier

High temperature experiments have been performed to constrain interfacial energies in a three-phase system (metal–forsterite–silicate melt) representative of partially differentiated planetesimals accreted early in the solar system history, with the aim of providing new insights into the factors affecting the interconnection threshold of metal-rich phases. Experiments were run under controlled oxygen fugacity (ΔNi-NiO=−3) at 1440 °C, typically for 24 h. Quantification of the true dihedral angles requires a resolution of at least 30 nm per pixel in order to reveal small-angle wedges of silicate melt at crystal interfaces. At this level of resolution, dihedral angle distributions of silicate melt and olivine appear asymmetric, an observation interpreted in terms of anisotropy of olivine crystals. Based upon the theoretical relation between dihedral angles and interfacial energies in a three-phase system, the relative magnitudes of interfacial energies have been determined to be: γMelt-Ol<γMelt-Ni<γOl-Ni. This order differs from that obtained with experiments using an iron sulfide liquid close to the Fe–FeS eutectic for which γMelt-Sulfide<γMelt-Ol<γOl-Sulfide, implying a lower interconnection threshold for sulfur-rich melts than for pure metallic phases. This dependence of the interconnection threshold on the sulfur content will affect the drainage of metallic phases during melting of small bodies. Assuming a continuous extraction of silicate melt, evolution of the metal volume fraction has been modeled. Several sulfur-rich melts extraction events are possible over a range of temperatures relevant with thermometric data on primitive achondrites (1200–1400 °C and 25% of silicate melt extracted). These successive events provide novel insight into the variability of sulfur content in primitive achondrites, which are either representative of a region that experienced sulfide extraction or from a region that accumulated sulfide melt from overlying parts of the parent body.

Yasuhiro Hasegawa¹, Bradley M. S. Hansen², and Gautam Vasisht¹
Astrophysical Journal Letters 876, L32 Link to Article [DOI: 10.3847/2041-8213/ab1b5a]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Mani L. Bhaumik Institute for Theoretical Physics, Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA

Identification of the main planet formation site is fundamental to understanding how planets form and migrate to their current locations. We consider the heavy-element content trend of observed exoplanets, derived from improved measurements of mass and radius, and explore how this trend can be used as a tracer of their formation sites. Using gas accretion recipes obtained from hydrodynamical simulations, we confirm that the disk-limited gas accretion regime is most important for reproducing the trend. Given that such a regime is specified by two characteristic masses of planets, we compute these masses as a function of the distance (r) from the central star, and then examine how the regime appears in the mass–semimajor axis diagram. Our results show that a plausible solid accretion region emerges at r  0.6 au and expands with increasing r, using the conventional disk model. Given that exoplanets that possess the heavy-element content trend distribute currently near their central stars, our results imply the importance of planetary migration that would occur after solid accretion onto planets might be nearly completed at r ≥ 0.6 au. Self-consistent simulations would be needed to verify the predictions herein.

¹Robert M. Hazen
American Mineralogist 104, 810-816 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2019/open_access/AM104P0810.pdf]
¹Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A. Orcid: 0000-0003-4163-8644
Copyright: The Mineralogical Society of America

Minerals reveal the nature of the co-evolving geosphere and biosphere through billions of years of Earth history. Mineral classification systems have the potential to elucidate this rich evolutionary story; however, the present mineral taxonomy, based as it is on idealized major element chemistry and crystal structure, lacks a temporal aspect, and thus cannot reflect planetary evolution. A complementary evolutionary system of mineralogy based on the quantitative recognition of “natural kind clustering” for a wide range of condensed planetary materials with different paragenetic origins has the potential to amplify, though not supersede, the present classification system.

¹Bernard J. Wood,¹Duane J. Smythe,¹Thomas Harrison
American Mineralogist 104. 844-856 Link to Article [http://www.minsocam.org/MSA/AmMin/TOC/2019/open_access/AM104P0844.pdf]
¹Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, U.K.
Copyright: The Mineralogical Society of America

As part of a project to investigate the volatilities of so-called “moderately volatile elements” such as Zn, In, Tl, Ga, Ag, Sb, Pb, and Cl during planetary formation, we began by re-calculating the condensation temperatures of these elements from a solar gas at 10-4 bar. Our calculations highlighted three areas where currently available estimates of condensation temperature could be improved. One of these is the nature of mixing behavior of many important trace elements when dissolved in major condensates such as silicates, Fe-rich metals, and sulfides. Nonideal solution of the trace elements can alter (generally lower) condensation temperatures by up to 500 K. Second, recent measurements of the halogen contents of CI chondrites (Clay et al. 2017) indicate that the solar system abundance of chlorine is significantly overestimated, and this affects the stabilities of gaseous complexes of many elements of interest. Finally, we have attempted to improve on previous estimates of the free energies of chlorine-bearing solids since the temperature of chlorine condensation has an important control on the condensation temperatures of many trace elements. Our result for the 50% condensation temperature of chlorine, 472 K is nearly 500 K lower than the result of Lodders (2003), and this means that the HCl content of the solar gas at temperatures <900 K is higher than previously estimated. We based our calculations on the program PHEQ (Wood and Hashimoto 1993), which we modified to perform condensation calculations for the elements H, O, C, S, Na, Ca, Mg, Al, Si, Fe, F, Cl, P, N, Ni, and K by free energy minimization. Condensation calculations for minor elements were then performed using the output from PHEQ in conjunction with relevant thermodynamic data. We made explicit provision for nonidealities using information from phase diagrams, heat of solution measurements, partitioning data and by using the lattice strain model for FeS and ionic solids and the Miedema model for solutions in solid Fe. We computed the relative stabilities of gaseous chloride, sulfide, oxide, and hydroxide species of the trace elements of interest and used these, as appropriate in our condensation calculations. In general, our new 50% condensation temperatures are similar to or, because of the modifications noted above, lower than those of Lodders (2003).