Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis

1,2Makiko K. Haba,1,3Yi-Jen Lai,1Jörn-Frederik Wotzlaw,4Akira Yamaguchi,5,6,7Maria Lugaro,1Maria Schönbächler
Proceedings of the National Academy of Sciences of teh United States of America (PNAS) (in Press) Link to Article []
1Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland;
2Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan;
3Macquarie GeoAnalytical, Department of Earth and Environmental Sciences, Macquarie University, Sydney, NSW 2109, Australia;
4Antarctic Meteorite Research Center, National Institute of Polar Research, 190-8518 Tokyo, Japan;
5 Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), 1121 Budapest, Hungary;
6Institute of Physics, ELTE Eötvös Loránd University, 1117 Budapest, Hungary;
7Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, VIC 3800, Australia

The niobium-92–zirconium-92 (92Nb–92Zr) decay system with a half-life of 37 Ma has great potential to date the evolution of planetary materials in the early Solar System. Moreover, the initial abundance of the p-process isotope 92Nb in the Solar System is important for quantifying the contribution of p-process nucleosynthesis in astrophysical models. Current estimates of the initial 92Nb/93Nb ratios have large uncertainties compromising the use of the 92Nb–92Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial 92Nb abundance is determined to high precision by combining the 92Nb–92Zr systematics of cogenetic rutiles and zircons from mesosiderites with U–Pb dating of the same zircons. The mineral pair indicates that the 92Nb/93Nb ratio of the Solar System started with (1.66 ± 0.10) × 10−5, and their 92Zr/90Zr ratios can be explained by a three-stage Nb–Zr evolution on the mesosiderite parent body. Because of the improvement by a factor of 6 of the precision of the initial Solar System 92Nb/93Nb, we can show that the presence of 92Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light p-process nuclei.

129I and 247Cm in meteorites constrain the last astrophysical source of solar r-process elements

1,2,3Benoit Côté et al. (>10)
Science 371, 945-948 Link to Article [DOI: 10.1126/science.aba1111]
1Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network, Konkoly Observatory, 1121 Budapest, Hungary.
2Institute of Physics, Eötvös Loránd University, 1117 Budapest, Hungary.
3National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA.
Reprinted with permission from AAAS

The composition of the early Solar System can be inferred from meteorites. Many elements heavier than iron were formed by the rapid neutron capture process (r-process), but the astrophysical sources where this occurred remain poorly understood. We demonstrate that the near-identical half-lives (≃15.6 million years) of the radioactive r-process nuclei iodine-129 and curium-247 preserve their ratio, irrespective of the time between production and incorporation into the Solar System. We constrain the last r-process source by comparing the measured meteoritic ratio 129I/247Cm = 438 ± 184 with nucleosynthesis calculations based on neutron star merger and magneto-rotational supernova simulations. Moderately neutron-rich conditions, often found in merger disk ejecta simulations, are most consistent with the meteoritic value. Uncertain nuclear physics data limit our confidence in this conclusion.

Earth and Mars – Distinct inner solar system products

1Takashi Yoshizaki,1,2,3William F.McDonough
Geochemistry [Chemie der Erde] (in Press) Link to Article []
1Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
2Department of Geology, University of Maryland, College Park, MD 20742, USA
3Research Center for Neutrino Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
Copyright Elsevier

Composition of terrestrial planets records planetary accretion, core–mantle and crust–mantle differentiation, and surface processes. Here we compare the compositional models of Earth and Mars to reveal their characteristics and formation processes. Earth and Mars are equally enriched in refractory elements (1.9 × CI), although Earth is more volatile-depleted and less oxidized than Mars. Their chemical compositions were established by nebular fractionation, with negligible contributions from post-accretionary losses of moderately volatile elements. The degree of planetary volatile element depletion might correlate with the abundances of chondrules in the accreted materials, planetary size, and their accretion timescale, which provides insights into composition and origin of Mercury, Venus, the Moon-forming giant impactor, and the proto-Earth. During its formation before and after the nebular disk’s lifetime, the Earth likely accreted more chondrules and less matrix-like materials than Mars and chondritic asteroids, establishing its marked volatile depletion. A giant impact of an oxidized, differentiated Mars-like (i.e., composition and mass) body into a volatile-depleted, reduced proto-Earth produced a Moon-forming debris ring with mostly a proto-Earth’s mantle composition. Chalcophile and some siderophile elements in the silicate Earth added by the Mars-like impactor were extracted into the core by a sulfide melt (∼0.5% of the mass of the Earth’s mantle). In contrast, the composition of Mars indicates its rapid accretion of lesser amounts of chondrules under nearly uniform oxidizing conditions. Mars’ rapid cooling and early loss of its dynamo likely led to the absence of plate tectonics and surface water, and the present-day low surface heat flux. These similarities and differences between the Earth and Mars made the former habitable and the other inhospitable to uninhabitable.

Micro-distribution of Oxygen Isotopes in Unequilibrated Enstatite Chondrites

1,2,3Michael K.Weisberg,4Noriko T.Kita,4Kohei Fukuda,4Guillaume Siron,2,3Denton S.Ebel
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Dept. Physical Sci, Kingsborough College CUNY, Brooklyn, NY 11235
2Dept. Earth and Environmental Sci, CUNY Graduate Center, New York, NY 10016
3Dept. Earth and Planetary Sci, American Museum of Natural History, New York, NY 10024
4WiscSIMS, Dept. of Geoscience, University of Wisconsin-Madison, Madison, WI 53706
Copyright Elsevier

We report petrology and high precision, in situ oxygen isotope analyses of silicates in chondrules, fragments, metal-rich nodules, refractory inclusions from the ALH 81189 (EH3), ALH 85159 (paired with ALH 81189) and from the MAC 88136 (EL3) chondrite. This is the first report of oxygen isotope ratios for individual objects in an EL3 and for the silicates associated with the metal-rich nodules that are characteristic of unequilibrated enstatite (E3) chondrites. The oxygen isotopic data from the chondrules and other objects form a trend, on a 3-isotope plot, that coincides with the slope∼1 primitive chondrule mineral (PCM) line (initially defined by chondrules from the Acfer 094 primitive carbonaceous chondrite), with most objects clustering at the intersection of the PCM line with the terrestrial fractionation (TF) line, near whole rock E3. The data from EH3 and EL3 overlap and show a similar distribution, suggesting they formed from a similar pool of precursors or in similar gaseous environments, but their mineral compositions suggest differences in their nebular environments and/or parent bodies. Silicates in the metal-rich nodules we analyzed (in both EH3 and EL3) have oxygen isotope ratios (as well as mineral compositions) similar to the silicate (metal-free) chondrules. This is consistent with formation of the metal-rich nodules prior to chondrite accretion, in an environment and from a process similar to that which formed the coexisting chondrules, but from more metal-rich mixtures of precursors. Olivine in an AOA from ALH 81189 is 16O-rich with δ18O = –46.5‰, δ17O = –48.0‰, similar to the AOAs and refractory inclusions previously reported in E3 and in all other chondrite groups. There is a clear distinction in oxygen isotopic compositions between the chondrules in the E3 chondrites and those in the LL and R as well as those in CV and CM chondrite groups. Chondrules from CR and E chondrites plot closer to the PCM line than all other chondrite groups with E3 chondrules having a different distribution toward more 16O-poor compositions. Chondrules in other chondrite groups form trends above and below the PCM. From the distribution of EC chondrules along the PCM line, we propose that similar pools of chondrule precursors were present in the different (carbonaceous, CR and Acfer 094 and non-carbonaceous, E) chondrule forming regions in the protoplanetary disk but with different amounts of 16O-rich refractory materials, prior to development of the postulated Jupiter divide that potentially separated inner (non-carbonaceous) from outer (carbonaceous chondrite) Solar System materials or the Jupiter barrier was inefficient in completely separating these materials.

Ceres, a wet planet: The view after Dawn

1Thomas B.McCord,1Jean-Philippe Combe,2Julie C.Castillo-Rogez,3Harry Y.McSween,4Thomas H.Prettyman
Geochemistry [Chemie der Erde] (in Press) Link to Article []
1The Bear Fight Institute, 22 Fiddler’s Road, P.O. Box 667, Winthrop, WA, 98862 USA
2Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109 USA
3Department of Earth & Planetary Sciences, University of Tennessee, Knoxville, TN, 37996-1410, USA
4Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ, 85719-2395, USA
Copyright Elsevier

Ceres, a nearly 1000-km diameter body located in the Solar System’s main asteroid belt, has been classified under many categories: planet, comet, asteroid, minor planet and, presently, dwarf planet. No matter what the designation, Ceres has experienced major planetary processes. Its evolution has been controlled by water, making it a most unusual, interesting and accessible inner Solar System object that can inform the evolution of outer Solar System moons and other dwarf planets. Early telescopic observations suggested a hydroxylated mineralogy similar to carbonaceous chondrite meteorites and a size and mass indicating a bulk density that implied a water content of 17−27 wt%. Thermodynamic modeling of Ceres’ evolution indicated that thermal aqueous evolution likely occurred. The Dawn Mission produced a huge increase in our understanding of Ceres, confirming but vastly extending the early knowledge. Dawn, carrying multispectral cameras, a visible-infrared imaging spectrometer and a nuclear spectrometer, orbited Ceres between 2015–2018 (after orbiting Vesta) at a number of different altitudes, ultimately reaching 35 km from the surface at periapsis. Observations of almost the entire surface and gravity field mapping revealed multiple geological and internal features attributed to the effects of water. The surface displays cryovolcanic-like and flow structures, exposed phyllosilicates, carbonates, evaporites and water ice. The subsurface shows partial differentiation, decreasing viscosity with depth, and lateral density heterogeneity. Ceres appears to be geologically active today and possesses liquid water/brine pockets or even an extended liquid layer in the interior, confirming an “Ocean World” designation in today’s vernacular.

Lava worlds: From early earth to exoplanets

1Keng-Hsien Chao,1Rebecca deGraffenried,1Mackenzie Lach,1William Nelson,1Kelly Truax,1Eric Gaidos
Geochemistry [Chemie der Erde] (in Press) Link to Article []
1Department of Earth Sciences, University of Hawai’i at Mānoa, Honolulu, HI 96822, USA
Copyright Elsevier

The magma ocean concept was first conceived to explain the geology of the Moon, but hemispherical or global oceans of silicate melt could be a widespread “lava world” phase of rocky planet accretion, and could persist on planets on short-period orbits around other stars. The formation and crystallization of magma oceans could be a defining stage in the assembly of a core, origin of a crust, initiation of tectonics, and formation of an atmosphere. The last decade has seen significant advances in our understanding of this phenomenon through analysis of terrestrial and extraterrestrial samples, planetary missions, and astronomical observations of exoplanets. This review describes the energetic basis of magma oceans and lava worlds and the lava lake analogs available for study on Earth and Io. It provides an overview of evidence for magma oceans throughout the Solar System and considers the factors that control the rocks these magma oceans leave behind. It describes research on theoretical and observed exoplanets that could host extant magma oceans and summarizes efforts to detect and characterize them. It reviews modeling of the evolution of magma oceans as a result of crystallization and evaporation, the interaction with the underlying solid mantle, and the effects of planetary rotation. The review also considers theoretical investigations on the formation of an atmosphere in concert with the magma ocean and in response to irradiation from the host star, and possible end-states. Finally, it describes needs and gaps in our knowledge and points to future opportunities with new planetary missions and space telescopes to identify and better characterize lava worlds around nearby stars.

A new estimate for the age of highly-siderophile element retention in the lunar mantle from late accretion

Icarus (in Press) Link to Article []
1Earth Life Science Institute, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8550, Japan
2Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA
3Origins Research Institute, Research Centre for Astronomy and Earth Sciences, H-1112 Budapest, Hungary
4Centre for Earth Evolution and Dynamics, University of Oslo, N-0315 Oslo, Norway
5Planetary Science Institute, Tucson, AZ 85719, USA
Copyright Elsevier

Subsequent to the Moon’s formation, late accretion to the terrestrial planets strongly modified the physical and chemical nature of silicate crusts and mantles. This alteration came in the form of melting through impacts, as well as the belated addition of volatiles and the highly siderophile elements (HSEs). Even though late accretion is well established as having been an important process in the evolution of the young solar system, its intensity and temporal decline remain subject to debate. Much of this deliberation hinges on what can be inferred about late accretion to the Moon from its computed mantle HSE abundances. Current debate centres on whether the lunar HSE record is representative of its whole late accretion history or alternatively that these were only retained in the mantle and crust after a particular time, and if so, when. Here we employ improved Monte Carlo impact simulations of late accretion onto the Moon and Mars and present an updated chronology based on new dynamical simulations of leftover planetesimals and the E-belt – a now-empty hypothesised inner extension of the asteroid belt (Bottke et al., 2012). We take into account the inefficient retention of colliding material. The source of impactors on both planetary bodies is assumed to be the same, hence we use constraints from both objects simultaneously. We compute the crater and basin densities on the Moon and Mars, the largest objects to strike these planets and the amount of material they accreted. Outputs are used to infer the mass in leftover planetesimals at a particular time period, which is then compared to the lunar HSE abundance. From this estimate we calculate a preferred lunar HSE retention age of ca. 4450 Ma which means that the modelled lunar mantle HSE abundances trace almost all of lunar late accretion. Based on our results, the surface ages of the lunar highlands are at least 4370 Ma. We find that the mass of leftover planetesimals with diameters Di < 300 km at 4500 Ma that best fits the crater chronology is approximately 2 × 10−3 Earth mass (ME) while the mass of the E-belt was fixed at 4.5 × 10−4 ME. We also find that a leftover planetesimal mass in excess of 0.01 ME results in a lunar HSE retention age younger than major episodes of lunar differentiation and crust formation, which in turn violates geochemical constraints for the timing and intensity of late accretion to the Earth (Mojzsis et al., 2019).

Unmixing Mineral Abundance and Mg# With Radiative Transfer Theory: Modeling and Applications

1Lingzhi Sun,1Paul G. Lucey
Journal of Geophysical Research, Planets (in Press) Link to Article []
1Department of Earth Sciences, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, HI, USA
Published by arrangement with John Wiley & Sons

Mineral abundance and Mg# (100× molar Mg/(Mg + Fe)) are significant in understanding the crustal composition and thermal history of the Moon. In this study, we derive a new set of optical constants for olivine, orthopyroxene, and clinopyroxene using radiative transfer equations that include soil porosity and the opposition effect. Based on the new optical constants, we develop a mineral abundance and Mg# unmixing model, and build a spectral library composed of mineral mixtures of plagioclase, olivine, low‐Ca pyroxene (LCP) and high‐Ca pyroxene (HCP), and Mg# ranging within 40–90. The accuracy of this model in estimating mineral abundance and chemistry is better than 3 vol% for olivine, LCP and HCP, better than 6 vol% for plagioclase, and better than 10 for Mg#. This model is validated using forward and inverse modeling. For the forward modeling, we reproduce the spectra of powdered pure minerals and Lunar Sample Characterization Consortium (LSCC) lunar soils, and the modeled spectra are consistent with those measured in the laboratory. For the inverse modeling, we determined mineral abundances and Mg# of 19 LSCC soil spectra by searching the best match to the spectral library. The modeled mineral abundances of LSCC soils are consistent with those measured by X‐ray digital imaging. We derived a global Mg# map using our model and Moon Mineralogy Mapper images, and our Mg# map shows a peak concentration at 70, consistent with that measured by the Lunar Prospector gamma‐ray spectrometer.

Compositional Mapping of the Nili Patera Feldspathic Unit: Extent and Implications for Formation

1Gabriel L. Eggers,1James J. Wray,2Josef Dufek
Journal of Geophysical Research, Planets (in Press) Link to Article []
1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA, USA
2Department of Earth Sciences, University of Oregon, Eugene, OR, USA
Published by arrangement with John Wiley & Sons

Decades of study of the igneous martian crust concluded that it was primarily basaltic, but a range of new investigations find evidence of evolved compositions. Foremost of these is a highly feldspathic unit within the Nili Patera caldera of Syrtis Major, the only detection with preserved volcanic context but which nonetheless remains ambiguous in exact composition and formation. We conduct compositional mapping of this feldspathic unit via near‐infrared spectroscopy from the Compact Reconnaissance Imaging Spectrometer for Mars instrument and find that the unit occupies at minimum 104 km2 at high confidence and an additional 41 km2 at moderately high confidence, meaning the unit is locally significant. We compare our mapping with that inferred from geomorphology and find that while texture and albedo are useful proxies, they are not perfectly reliable as substitutes for thorough compositional investigation. Study of the boundary between the feldspathic unit and surrounding mafic rock indicates the former formed early and may extend locally in the subsurface. We consider what compositional mixtures could explain the conflicting interpretations derived from visible/near‐infrared and thermal infrared spectroscopy, concluding it is likely due to thermophysical differences between the light‐toned feldspathic unit and the infilling dark mafic sand. We discuss proposed plutonic and volcanic formation scenarios for the feldspathic unit, considering Earth analogs and implications for the parent magmatic system, and offer observations in rock texture and composition that would clarify.

The Fundamental Connections Between the Solar System and Exoplanetary Science

1Stephen R. Kane,2Giada N. Arney,3Paul K. Byrne,1,4Paul A. Dalba,5Steven J. Desch,6Jonti Horner,7Noam R. Izenberg,7Kathleen E. Mandt,8Victoria S. Meadows,9Lynnae C. Quick
Journal of Geophysical Research, Planets (in Press) Link to Article []
1Department of Earth and Planetary Sciences, University of California, Riverside, CA, 92521 USA
2Planetary Systems Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
3Planetary Research Group, Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, NC, 27695 USA
4NSF Astronomy and Astrophysics Postdoctoral Fellow
5School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287 USA
6Centre for Astrophysics, University of Southern Queensland, Toowoomba, QLD, 4350 Australia
7Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 20723 USA
8Department of Astronomy, University of Washington, Seattle, WA, 98195 USA
9Planetary Geology, Geophysics and Geochemistry Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA
Published by arrangement with John Wiley & Sons

Over the past several decades, thousands of planets have been discovered outside of our Solar System. These planets exhibit enormous diversity, and their large numbers provide a statistical opportunity to place our Solar System within the broader context of planetary structure, atmospheres, architectures, formation, and evolution. Meanwhile, the field of exoplanetary science is rapidly forging onward towards a goal of atmospheric characterization, inferring surface conditions and interiors, and assessing the potential for habitability. However, the interpretation of exoplanet data requires the development and validation of exoplanet models that depend on in‐situ data that, in the foreseeable future, are only obtainable from our Solar System. Thus, planetary and exoplanetary science would both greatly benefit from a symbiotic relationship with a two‐way flow of information. Here, we describe the critical lessons and outstanding questions from planetary science, the study of which are essential for addressing fundamental aspects for a variety of exoplanetary topics. We outline these lessons and questions for the major categories of Solar System bodies, including the terrestrial planets, the giant planets, moons, and minor bodies. We provide a discussion of how many of these planetary science issues may be translated into exoplanet observables that will yield critical insight into current and future exoplanet discoveries.