The Planetary Terrestrial Analogues Library (PTAL) – An exclusive lithological selection of possible martian earth analogues

1Henning Dypvik et al. (>10)
Planetary and Space Science (in Press) Link to Article []
1Department of Geosciences and Department of Technology Systems, Univ. of Oslo, P.O. Box 1047, Blindern, NO 0316, Oslo, Norway

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Fractional crystallization of a basal lunar magma ocean: A dense melt-bearing garnetite layer above the core?

1Giuliano Kraettli,1Max W.Schmidt,1Christian Liebske
Icarus (in Press) link to Article []
1Department of Earth Sciences, ETH, 8092 Zurich, Switzerland
Copyright Elsevier

In the wake of its accretion, the Moon was likely partially or fully molten, forming a lunar magma ocean (LMO). A fully molten Moon may crystallize neutrally buoyant olivine, whose accumulation may form a barrier leading to a separation into two independently evolving melt reservoirs. This study investigates the crystallization of such a putative basal lunar magma ocean, ranging from the core mantle boundary (with radius r = 380 km) to the level of neutral olivine buoyancy (r = 600 km). Consecutive crystallization experiments determine liquidus temperatures and crystallize 10–30 wt% minerals, before conceptually segregating these according to their buoyancy, and then stepping to a new bulk composition that corresponds to the residual melt after removal of the cumulate minerals.

The first olivine to crystallize from a Taylor Whole Moon composition (twm, XMg = Mg/(Mg + Fe2+) = 0.83) has XMg = 0.94 and is neutrally buoyant at 3.8 GPa according to the melt density model of Lange and Carmichael (1990). Crystallization begins at the core mantle boundary, but early olivine floats and re-dissolves until the magma ocean cools to the liquidus temperature at the depth of neutral buoyancy (1850 °C). At this point a > 500 km thick olivine-only layer could form, mainly fed from the upper magma shell. The crystallization sequence in the basal magma ocean is olivine-only at 1850–1675 °C, 0–26 pcs (wt% percent solidified) → olivine + opx (to 1600 °C, 42 pcs) → opx + cpx + garnet (to 1580 °C, 60 pcs) → cpx + garnet (to 1520 °C, 73 pcs) → cpx + garnet + olivine leaving 11 wt% residual melt at 1450 °C.

Cooling this last melt to 1300 °C leads to 80% garnet + cpx + olivine + Ti-spinel, the residual liquid corresponding to 2.2% of the basal magma ocean. Olivine, opx and cpx remain buoyant over the entire crystallization interval and various pyroxenite layers are added to the olivine layer. Garnet and the final cotectic cpx + garnet + olivine + FeTi-oxide assemblage instead form a 70 km thick basal layer on the core-mantle boundary. Such a layer provides a gravitationally stable high-density lowermost lunar mantle, which would concur with a recent re-analysis of the lunar seismic data. The lowest temperature experiment at 1300 °C, not much above the 1250 °C proposed for the core-mantle boundary from inversion of geophysical data, had about 20% of a highly evolved, dense Fe-rich melt. It is feasible that this melt has remained in its liquid state to present day, providing an explanation for the proposed low vp and vs layer and high dissipation just above the core-mantle boundary.

Simulated SPHEREx spectra of asteroids and their implications for asteroid size and reflectance estimation

1Željko Ivezić,2Vedrana Ivezić,1Joachim Moeyens,3Carey M.Lisse,4Schelte J.Bus,1Lynne Jones,5Brendan P.Crill,5,6Olivier Doré,7Joshua P.Emery
Icarus (in Press) Link to Article []
1Department of Astronomy and the DiRAC Institute, University of Washington, 3910 15th Avenue, NE, Seattle, WA 98195, USA
2Department of Computer Science, Princeton University, 35 Olden St, Princeton, NJ 08540, USA
3JHU-APL, SES/SRE, Bldg 200/E206, 11100 Johns Hopkins Road, Laurel, MD 20723, USA
4Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822 USA
5Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove, Pasadena, CA 91109, USA
6California Institute of Technology, Pasadena, CA 91125, USA
7Department of Astronomy and Planetary Science, Northern Arizona University, 527 S Beaver Street, Flagstaff, AZ 86011, USA
Copyright Elsevier

We describe the construction and analysis of simulated SPHEREx spectra of Main Belt and Trojan asteroids. SPHEREx will deliver the first all-sky spectral survey at 96 spectral channels between 0.75 m and 5.0 m. We have developed a method for correcting SPHEREx asteroid spectra for intrinsic rotational variability that does not require light curves and can enable studies before LSST light curves become available for this purpose. Using these spectra, we predict that SPHEREx will deliver meaningful flux measurements for about 100,000 asteroids, including close to 10,000 objects with high-quality spectra; this dataset will represent an increase over our current sample size by more than an order of magnitude. The main SPHEREx contribution to asteroid science will be derived from taxonomic classifications, detailed spectroscopic analyses involving a number of diagnostic spectral features associated with olivine, pyroxene, hydroxyl, water ice, and organics, and constraints on thermal properties. We argue that all asteroids with currently known orbits, about a million objects, should be included in the SPHEREx forced photometry object list to maximize its science impact. Our tools and the library of simulated spectra are made publicly available.

Observations of Phobos and Deimos with SpeX at NASA infrared telescope facility

Icarus (in Press) Link to Article []
1Jacobs, NASA Johnson Space Center, Houston, TX 77058, USA
2Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), Sagamihara, Japan
3Physics and Astronomy, University of Hawai’i at Hilo, HI 96720, USA
Copyright Elsevier

We measured near-infrared (NIR) reflectance spectra of Phobos and Deimos, using the prism (0.7–2.52 μm) and long-wavelength cross dispersed (LXD: 1.9–4.2 μm) modes of NASA Infrared Telescope Facility (IRTF)’s SpeX instrument. The goal of this study is to investigate the surface composition of Phobos and Deimos and search for any mineralogical absorption signatures that may be present on their surfaces, especially in the LXD spectral range. Prism spectra of Phobos showed significant slope variation at shorter wavelengths (λ < 1.3 μm), which indicates surface heterogeneity possibly due to regolith’s composition and grain size, and/or space weathering. Deimos’ prism spectra were found to be consistent with the more red-sloped prism spectra of Phobos. The measured LXD spectra of Deimos revealed evidence of hydration with 3-μm band depths at 2.90 μm of 4–5%. The 3-μm band in Deimos could be attributed to exogenic sources such as solar wind implantation or OH-bearing impactors, or to an endogenic source and the presence of carbonaceous material on its surface. Phobos’ and Deimos’ prism and LXD spectra, however, show no indications for absorption signatures of mafic silicates (i.e., pyroxene, olivine), organics nor carbonates.

Heterogeneity within refractory organic matter from CM2 Carbonaceous Chondrites: Evidence from Raman spectroscopy

1Christian Potiszil,1Wren Montgomery,1Mark A.Sephton
Earth and Planetary Science Letters 574, 117149 Link to Article []
1Impacts and Astromaterials Research Centre, Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, United Kingdom
Copyright Elsevier

CM2 chondrites experienced widespread aqueous and short term thermal alteration on their parent bodies. Whilst previous Raman spectroscopic investigations have investigated insoluble organic matter (IOM), they have not taken into account the binary nature of IOM. Studies employing mass spectrometry have indicated that IOM also known as macromolecular organic matter (MOM) is in fact composed of two distinct fractions: labile organic matter (LOM) and refractory organic matter (ROM). The ROM component represents the aromatic rich and heteroatom poor component of IOM/MOM, whilst the LOM fraction represents a more heteroatom and aliphatic rich component. Here we report Raman 2D maps and spectroscopic data for Murchison and Mighei, both before and after chemical degradation, which attacks and liberates LOM. The removal of LOM simulates the effects of aqueous alteration, where ester and ether bonds are broken and is thought to release some components to the soluble organic matter (SOM) fraction, also known as the free organic matter fraction (FOM). Raman spectroscopy can be used to reveal the nature of bonding (sp2 and sp3) within carbonaceous materials such as meteoritic organic matter, through evaluation of the D and G band peak centres and FWHM values from the recorded data. The presence of sp3 orbitals indicates that the organic materials contain aliphatic linkages and/or heteroatoms. Statistical analysis of the Raman parameters obtained here indicates that the organic matter originating the Raman response is indistinguishable between the bulk (chemically untreated) and chemically degraded (treated with KOH and HI) samples. Such an observation indicates that the ROM fraction is the major contributor to the Raman response of meteoritic organic matter and thus Raman spectroscopy is unlikely to record any aqueous alteration processes that have affected meteoritic organic matter. Therefore, studies which use Raman to probe the IOM are investigating just one of the components of IOM and not the entire fraction. Studies that aim to investigate the effects of aqueous alteration on meteoritic organic matter should use alternate techniques to Raman spectroscopy. Furthermore, the indistinguishable nature of the Raman response of ROM from Murchison and Mighei suggests these meteorites inherited a ROM component that is chemically similar, reflecting either a common process for the formation of CM2 meteoritic ROM and/or that these meteorites probed the same ROM reservoir.

The effect of carbon concentration on its core-mantle partitioning behavior in inner Solar System rocky bodies

1Damanveer S.Grew,1Rajdeep Dasgupta,1Sanath Aithala
Earth and Planetary Science Letters 571, 117090 Link to Article []
1Department of Earth, Environmental, and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USA
Copyright Elsevier

Partitioning of carbon (C) into the cores of rocky protoplanets and planets is one of the primary causes of its depletion in their bulk silicate reservoirs. Most of the experimental studies that determined the alloy to silicate melt partition coefficient of carbon () have been conducted in graphite-saturated conditions. Because carbon is a minor element in all known protoplanetary and planetary cores, it is not known whether graphite-saturated values are applicable to core-mantle differentiation in rocky bodies which likely occurred in C-poor conditions. In this study we experimentally determined in MgO capsules with variable bulk C contents between oxygen fugacity (fO2) of IW–6.35 and IW–2.59 at a fixed P (3 GPa)-T (1700 °C). A mafic-ultramafic (NBO/T = 1.23-1.72) and mildly hydrous (bulk H = 44-161 ppm) nature of the silicate melts caused anhydrous C species ( + CO) to dominate over a wider fO2 range (>IW–4.2) in comparison to previous studies. This resulted in an increase in with decreasing fO2 from IW–2.6 to IW–4.2 followed by a drop at more reduced conditions due to the formation of C-H species. Importantly, increases with increasing bulk C content of the system at a given fO2. Partitioning of C between alloy and silicate melts follows non-Henrian behavior (i.e., it depends on bulk C content) because the activity coefficient of C in the alloy melt () varies with C content in the alloy. Therefore, in addition to other intensive (P, T, fO2) and extensive variables (alloy and silicate melt compositions), also depends on the bulk C content available during core-mantle differentiation. Consequently, previously determined for C-rich alloys are not directly applicable for core-mantle differentiation in relatively C-poor magma oceans (MOs). Because the experiments from the present study more realistically simulate C-poor cores and mildly hydrous, mafic-ultramafic silicate MOs, our data can be used to more accurately predict C fractionation between MOs and cores in inner Solar System rocky bodies. Our study suggests that closed system MO-core equilibration should have led to less severe depletion of C in the silicate reservoirs of inner Solar System rocky bodies than previously predicted.

A model for evolving crust on 4 Vesta through combined compositional and thermal modelling

1Jennifer T.Mitchell,1Andrew G.Tomkins,2Christopher Newton,3Tim E.Johnson
Earth and Planetary Science Letters (in Press) Link to Article []
1School of Earth, Atmosphere & Environment, Monash University, Melbourne, Australia
2School of Physics & Astronomy, Monash University, Melbourne, Australia
3School of Earth & Planetary Sciences, The Institute for Geoscience Research, Curtin University, Perth, Australia
Copyright Elsevier

Combined phase equilibrium and thermal modelling has been used to investigate the evolution of asteroid 4 Vesta. Orthopyroxene compositions of 200 natural diogenite meteorites are used as a basis for constructing a staged mantle melting model for Vesta, which is then used to develop a staged thermal evolution model. Our pMELTS models find that removal of 15–20% of a mean eucrite component from an initial Vestan mantle composition allows a second stage of melting that crystallises low-calcium orthopyroxenes that match the observed compositions of those in natural diogenites, whereas single stage melting produces orthopyroxenes that are too calcic. Using the compositions generated by the pMELTS modelling, THERMOCALC models were created for an initial Vestan mantle composition and an evolved composition generated by a melt extraction stage. These models suggest that melt production for second-stage diogenite generation required considerably hotter temperatures (>1340 °C) than for eucrites (<1240 °C). Staged and layered thermal evolution models developed using these composition and temperature constraints, based on the decay of 26Al and 60Fe, suggest that Vesta accreted 1.50 to 1.75 Myr after calcium-aluminium inclusion (CAI) formation. Earlier accretion results in conditions that are inconsistent with the petrology of the HED meteorites, whereas later accretion predicts temperatures that are insufficient to produce diogenites. We suggest that upward migration of 26Al-rich melt initially created a convecting shallow magma ocean of <20 km depth that rapidly crystallised to form a 26Al-rich eucritic crust that acted as a hot insulating lid. The second stage of crust formation began once the depleted mantle residue reached high enough temperatures to produce diogenite-forming magmas. These results further support the view that diogenites likely formed as crustal intrusions rather than as magma ocean cumulates.

Thermal evolution of water and hydrogen from Apollo lunar regolith grains

1,2Brant M.Jones,1Aleksandr Aleksandrov,3Charles A.Hibbitts,1,2,4Thomas M.Orlando
Earth and Planetary Science Letters (in Press) Link to Article []
1School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA, United States of America
2Center for Space Technology and Research, Georgia Institute of Technology, Atlanta, GA, United States of America
3John Hopkins Applied Physics Laboratory, Laurel, MD, United States of America
4School of Physics, Georgia Institute of Technology, Atlanta, GA, United States of America
Copyright Elsevier

The evolution of water and molecular hydrogen from Apollo lunar sample 15221, a mature mare soil, was examined by temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions. Desorption at the grain/vacuum interface with re-adsorption as water transports though the void space of the grains and activated sub-surface diffusion were found to reproduce the experimental TPD signal. Signal from the grain/vacuum interface yielded the second order desorption activation energies and site probability distributions. Water from sample 15221 exhibited a broad distribution of activation energies peaking at 130 kJ mol−1 extending up to 350 kJ mol−1 at zero coverage limit with an onset of 110 kJ mol−1 at full coverage. Our results suggest that water and hydrogen originating from lunar regolith contributes a minor amount to the observed mass in the LCROSS impact event. The abnormal amount of molecular hydrogen observed in the ejecta plume of the LCROSS impact may indicate that the radiolytic production of H2 from electron and galatic cosmic rays of physisorbed water is a contributor to the vast quantity of molecular hydrogen detected.

Crystal chemistry of schreibersite, (Fe,Ni)3P

1Sergey N. Britvin,1Maria G. Krzhizhanovskaya, 1Andrey A. Zolotarev, 1Liudmila A. Gorelova,3Edita V. Obolonskaya,4Natalia S. Vlasenko,4,5Vladimir V. Shilovskikh,1Mikhail N. Murashko
The American Mineralogist 106, 1520–1529 Link to Article []
1Institute of Earth Sciences, St. Petersburg State University, Universitetskaya Nab. 7/9, 199034 St. Petersburg, Russia
2Kola Science Center, Russian Academy of Sciences, Fersman Str. 14, 184209 Apatity, Russia
3The Mining Museum, Saint Petersburg Mining University, 2, 21st Line 199106 St. Petersburg, Russia
4Centre for Geo-Environmental Research and Modelling, St. Petersburg State University, Ulyanovskaya ul. 1, 198504 St. Petersburg, Russia
5Institute of Mineralogy, Urals Branch of Russian Academy of Science, Miass 456317, Russia
Copyright: The Mineralogical Society of America

Schreibersite, (Fe,Ni)3P, the most abundant cosmic phosphide, is a principal carrier of phosphorus in the natural Fe-Ni-P system and a likely precursor for prebiotic organophosphorus compounds at the early stages of Earth’s evolution. The crystal structure of the mineral contains three metal sites allowing for unrestricted substitution of Fe for Ni. The distribution of these elements across the structure could serve as a tracer of crystallization conditions of schreibersite and its parent celestial bodies. However, discrimination between Fe (Z = 26) and Ni (Z = 28) based on the conventional X-ray structural analysis was for a long time hampered due to the proximity of their atomic scattering factors. We herein show that this problem has been overcome with the implementation of area detectors in the practice of X-ray diffraction. We report on previously unknown site-specific substitution trends in schreibersite structure. The composition of the studied mineral encompasses a Ni content ranging between 0.03 and 1.54 Ni atoms per formula unit (apfu): the entire Fe-dominant side of the join Fe3P-Ni3P. Of 23 schreibersite crystals studied, 22 comprise magmatic and non-magmatic iron meteorites and main group pallasites. The near end-member mineral (0.03 Ni apfu) comes from the pyrometamorphic rocks of the Hatrurim Basin, Negev desert, Israel. It was found that Fe/Ni substitution in schreibersite follows the same trends in all studied meteorites. The dependencies are nonlinear and can be described by second-order polynomials. However, the substitution over the M2 and M3 sites within the most common range of compositions (0.6 < Ni <1.5 apfu) is well approximated by a linear regression: Ni(M2) = 0.84 × Ni(M3) – 0.30 apfu (standard error 0.04 Ni apfu). The analysis of the obtained results shows a strong divergence between the variation of unit-cell parameters of natural schreibersite and those of synthetic (Fe,Ni)3P. This indicates that Fe/Ni substitution trends in the mineral and its synthetic surrogates are different. A plausible explanation might be related to the differences in the system equilibration time of meteoritic schreibersite (millions of years) and synthetic (Fe,Ni)3P (~100 days). However, regardless of the reason for the observed difference, synthetic (Fe,Ni)3P cannot be considered a structural analog of natural schreibersite, and this has to be taken into account when using synthetic (Fe,Ni)3P as an imitator of schreibersite in reconstructions of natural processes