Kent C. Condie^a, Charles K. Shearer^a,b
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.027]
^aDepartment of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA
^bInstitute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Major processes affecting high field strength (HFSE) and rare earth (REE) element ratios in planetary basalts are degree of melting, separation of metal-sulfide melt fractions, addition and loss of silicate melt, ilmenite fractionation, and subduction. Fractional crystallization of planetary magma oceans has left a surviving imprint on only three bodies for which we have data: the Moon, Vesta, and the angrite parent body. Thorium mobilization in aqueous fluids may account for decoupling of Th and Nb in planetary systems, and this is especially notable on Earth but also possible on Mars, the Moon and some asteroids. On Earth, HFSE and REE ratios in young basalts characterize hydrated (HM), enriched (EM) and depleted (DM) mantle sources, associated with, respectively, subduction, mantle plumes and ocean ridges. Terrestrial hydrated and depleted mantle were in existence by at least 4 Ga and possibly they may have been produced in a stagnant lid tectonic regime before 3 Ga. Also, removal of Nb in metal-sulfide melts can force the composition of silicate restitic material into the hydrated mantle field on HFSE-REE graphs, thus not requiring hydration. Such an origin is probable for “hydrated” mantle in primitive achondrites and plutonic angrites. The record of all three types of mantle in basalts from other bodies in the Solar System indicates the three mantle reservoirs are not diagnostic of plate tectonics, but can be produced in stagnant lid settings.

Enriched mantle is thus far recognized only in Earth and possibly Mars. There are at least two enriched mantle reservoirs in Earth: a primordial (> 4 Ga) reservoir, perhaps hidden in the D” layer above the core and rarely sampled by basalts, and a recycled plate reservoir (< 3 Ga), perhaps located in the two LLSVPs commonly sampled by oceanic island basalts. Between 3 and 2 Ga, the recycled enriched mantle reservoir became established in Earth, possibly in response to the widespread propagation of subduction. On Mars enriched mantle shows depleted radiogenic isotopic signatures and requires a multistage process to decouple trace element and isotopic systems.

Although there are several processes by which Nb can be fractionated from Ta in planetary bodies, the low Nb/Ta (<15) characteristic of some planetary and asteroid basalts may reflect separation of a metal-sulfide melt enriched in Nb, which may or may not produce a core. This fractionation must occur early during a relatively reduced stage of planetary evolution (IW-3 to IW-5) such that Nb behaves as a chalcophile or siderophile element. If the average Nb/Ta ratio of both primitive and depleted mantle is equal to 15, production of basaltic magma in the terrestrial mantle through time has not fractionated Nb from Ta. On the other hand, if the Nb/Ta in primitive mantle equals 17, Nb must be fractionated from Ta before 4 Ga, perhaps by partitioning into the core during or soon after planetary accretion when reducing conditions may have existed.

Darryl Seligman¹, Gregory Laughlin¹, and Konstantin Batygin²
Astrophysical Journal Letters 876, L26 Link to Article [DOI: 10.3847/2041-8213/ab0bb5]
¹Dept. of Astronomy, Yale University, New Haven, CT 06517, USA
²Division of Geological and Planetary Sciences, Caltech, Pasadena, CA 91125, USA

We show that the P ~ 8 hr photometric period and the astrometrically measured A _ng ~ 2.5 × 10⁻⁴cm s⁻² non-gravitational acceleration (at r ~ 1.4 au) of the interstellar object 1I/2017 (‘Oumuamua) can be explained by a nozzle-like venting of volatiles whose activity migrated to track the subsolar location on the object’s surface. Adopting the assumption that ‘Oumuamua was an elongated a × b × c ellipsoid, this model produces a pendulum-like rotation of the body and implies a long semi-axis . This scale agrees with the independent estimates of ‘Oumuamua’s size that stem from its measured brightness, assuming an albedo of p ~ 0.1, which is appropriate for ices that have undergone long-duration exposure to the interstellar cosmic-ray flux. Using ray tracing, we generate light curves for ellipsoidal bodies that are subject to both physically consistent subsolar torques and to the time-varying geometry of the Sun–Earth–’Oumuamua configuration. Our synthetic light curves display variations from chaotic tumbling and changing cross-sectional illumination that are consistent with the observations, while avoiding significant secular changes in the photometric periodicity. If our model is correct, ‘Oumuamua experienced mass loss that wasted ~10% of its total mass during the ~100 days span of its encounter with the inner solar system and had an icy composition with a very low [C/O]  0.003. Our interpretation of ‘Oumuamua’s behavior is consistent with the hypothesis that it was ejected from either the outer regions of a planetesimal disk after an encounter with an embedded M _p ~ M _Nep planet, or from an exo-Oort cloud.

M. A. Cordiner^1,2 et al. (>10)
Astrophysical Journal Letters 8709, L26 Link to Article [DOI: 10.3847/2041-8213/aafb05]
¹NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
²Department of Physics, Catholic University of America, Washington, DC 20064, USA

The Atacama Large Millimeter/submillimeter Array (ALMA) is a powerful tool for high-resolution mapping of comets, but the main interferometer (comprised of 50 × 12 m antennas) is insensitive to the largest coma scales due to a lack of very short baselines. In this Letter, we present a new technique employing ALMA autocorrelation data (obtained simultaneously with the interferometric observations), effectively treating the entire 12 m array as a collection of single-dish telescopes. Using combined autocorrelation spectra from 28 active antennas, we recovered extended HCN coma emission from comet C/2012 S1 (ISON), resulting in a fourteen-fold increase in detected line brightness compared with the interferometer. This resulted in the first detection of rotational emission from H¹³CN in this comet. Using a detailed coma radiative transfer model accounting for optical depth and non-local thermodynamic equilibrium excitation effects, we obtained an H¹²CN/H¹³CN ratio of 88 ± 18, which matches the terrestrial value of 89. This is consistent with a lack of isotopic fractionation in HCN during comet formation in the protosolar accretion disk. The possibility of future discoveries in extended sources using autocorrelation spectroscopy from the main ALMA array is thus demonstrated.

Christopher Spalding
Astrophysical Journal Letters 869, L17 Link to Article [DOI: 10.3847/2041-8213/aaf478]
Department of Astronomy, Yale University, New Haven, CT 06511, USA

Our solar system is almost entirely devoid of material interior to Mercury’s orbit, in sharp contrast to the multiple Earth masses of material commonly residing within the analogous region of extrasolar planetary systems. Recent work has suggested that Jupiter’s orbital migration early in the solar system’s history fragmented primordial planetary material within the inner solar system. However, the reason for the absence of subsequent planet formation within 0.4 au remains unsolved. Here, we show that leftover debris interior to Mercury’s current orbit was susceptible to outward migration driven by the early Solar wind, enhanced by the Sun’s primordial rapid rotation and strong magnetic field. The ram pressure arising from azimuthal motion of the Solar wind plasma transported ~100 m-sized objects and smaller from 0.1 au out to the terrestrial planet-forming zone within the suspected ~30–50 Myr timespan of the Earth’s formation. The mass of material within this size class typically exceeds Mercury, and can rival that of Earth. Consequently, the present-day region of terrestrial planets and the asteroid belt has been supplied by a large mass of material from the innermost, hot solar system, providing a potential explanation for the evidence of high-temperature alteration within some asteroids and the high iron content of Mercury.

Gerrit Budde, Christoph Burkhardt & Thorsten Kleine
Nature Astronomy Link to Article [https://www.nature.com/articles/s41550-019-0779-y]
Institut für Planetologie, University of Münster, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany

Earth grew through collisions with Moon-sized to Mars-sized planetary embryos from the inner Solar System, but it also accreted material from greater heliocentric distances^1,2, including carbonaceous chondrite-like bodies, the likely source of Earth’s water and highly volatile species^3,4. Understanding when and how this material was added to Earth is critical for constraining the dynamics of terrestrial planet formation and the fundamental processes by which Earth became habitable. However, earlier studies inferred very different timescales for the delivery of carbonaceous chondrite-like bodies, depending on assumptions about the nature of Earth’s building materials^{5,6,7,8,9,10,11}. Here we show that the Mo isotopic composition of Earth’s primitive mantle falls between those of the non-carbonaceous and carbonaceous reservoirs^12,13,14,15, and that this observation allows us to quantify the accretion of carbonaceous chondrite-like material to Earth independently of assumptions about its building blocks. As most of the Mo in the primitive mantle was delivered by late-stage impactors¹⁰, our data demonstrate that Earth accreted carbonaceous bodies late in its growth history, probably through the Moon-forming impact. This late delivery of carbonaceous material probably resulted from an orbital instability of the gas giant planets, and it demonstrates that Earth’s habitability is strongly tied to the very late stages of its growth.

Francesco C. Pignatale^1,2, Sébastien Charnoz¹, Marc Chaussidon¹, and Emmanuel Jacquet²
Astrophysical Journal Letters 867, L23 Link to Article [DOI: 10.3847/2041-8213/aaeb22]
¹Institut de Physique du Globe de Paris (IPGP) 1 rue Jussieu, F-75005, Paris, France
²Muséum national d’Histoire naturelle, UMR 7590, CP52 57 rue Cuvier, F-75005, Paris, France

Chondritic meteorites, the building blocks of terrestrial planets, are made of an out-of-equilibrium assemblage of solids formed at high and low temperatures, either in our Solar system or previous generations of stars. For decades this was considered to result from large-scale transport processes in the Sun’s isolated accretion disk. However, mounting evidence suggests that refractory inclusions in chondrites formed contemporaneously with the disk building. Here we numerically investigate, using a 1D model and several physical and chemical processes, the formation and transport of rocky materials during the collapse of the Sun’s parent cloud and the consequent assembling of the Solar Nebula. The interplay between the cloud collapse, the dynamics of gas and dust, vaporization, recondensation, and thermal processing of different species in the disk results in a local mixing of solids with different thermal histories. Moreover, our results also explain the overabundance of refractory materials far from the Sun and their short-formation timescales, during the first tens of kyr of the Sun, corresponding to class 0-I, opening new windows into the origin of the compositional diversity of chondrites.

Judit Szulágyi^1,2, Marco Cilibrasi¹, and Lucio Mayer¹
Astrophysical Journal Letters 868, L13 Link to Article [DOI: 10.3847/2041-8213/aaeed6]
¹Center for Theoretical Astrophysics and Cosmology, Institute for Computational Science, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
²Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, 8093, Zurich, Switzerland

Satellites of giant planets have been thought to form in gaseous circumplanetary disks (CPDs) during the late planet-formation phase, but it was unknown whether or not smaller-mass planets such as the ice giants could form such disks, and thus moons, there. We combined radiative hydrodynamical simulations with satellite population synthesis to investigate the question in the case of Uranus and Neptune. For both ice giants we found that a gaseous CPD is created at the end of their formation. The population synthesis confirmed that Uranian-like, icy, prograde satellite system could form in these CPDs within a couple of 10⁵ yr. This means that Neptune could have a Uranian-like moon system originally that was wiped away by the capture of Triton. Furthermore, the current moons of Uranus can be reproduced by our model without the need for planet–planet impact to create a debris disk for the moons to grow. These results highlight that even ice giants—among the most common mass category of exoplanets—can also form satellites, opening a way to a potentially much larger population of exomoons than previously thought.

Nienke van der Marel¹, Jonathan P. Williams², and Simon Bruderer³
Astrophysical Journal Letters 867, L14 Link to Article [DOI: 10.3847/2041-8213/aae88e]
¹Herzberg Astronomy & Astrophysics Programs, National Research Council of Canada, 5071 West Saanich Road, Victoria BC V9E 2E7, Canada
²Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, 96822 Honolulu, HI, USA
³Max-Planck Institut für Extraterrestrische Physik, Giessenbachstrasse 2, D-85741 Garching bei München, Germany

High-resolution Atacama Large Millimeter/submillimeter Array observations of protoplanetary disks have revealed that many, if not all, primordial disks consist of ring-like dust structures. The origin of these dust rings remains unclear, but a common explanation is the presence of planetary companions that have cleared gaps along their orbit and trapped the dust at the gap edge. A signature of this scenario is a decrease of gas density inside these gaps. In a recent work, Isella et al. derived drops in gas density that are consistent with Saturn-mass planets inside the gaps in the HD 163296 disk through spatially resolved CO isotopologue observations. However, as CO abundance and temperature depends on a large range of factors, the interpretation of CO emission is non-trivial. We use the physical–chemical code DALI to show that the gas temperature increases inside dust density gaps, implying that any gaps in the gas, if present, would have to be much deeper, consistent with planet masses >M _Jup. Furthermore, we show that a model with increased grain growth at certain radii, as expected at a snowline, can reproduce the dust rings in HD 163296 equally well without the need for companions. This scenario can explain both younger and older disks with observed gaps, as gaps have been seen in systems as young <1 Myr. While the origin of the rings in HD 163296 remains unclear, these modeling results demonstrate that care has to be taken when interpreting CO emission in protoplanetary disk observations.

N. Mari^a, A.J.V. Riches^b, L.J. Hallis^a, Y. Marrocchi^c, J. Villeneuve^c, P. Gleissner^d, H. Becker^d M.R. Lee^a
Geochimica et Cosmochimcia Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.025]
^aSchool of Geographical and Earth Sciences, University of Glasgow, University Avenue, Glasgow, G12 8QQ, UK
^bDepartment of Earth Sciences, Durham University, Stockton Road, Durham, DH1 3LE, UK
^cCentre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
^dFreie Universität Berlin, Institut für Geologische Wissenschaften, Kaiserswerther Str. 16-18, 14195, Berlin, Germany
Copyright Elsevier

Martian lava flows likely acquired S-rich material from the regolith during their emplacement on the planet’s surface. We investigated five of the twenty known nakhlites (Nakhla, Lafayette, Miller Range (MIL) 090032, Yamato 000593, and Yamato 000749) to determine whether these lavas show evidence of regolith assimilation, and to constrain the potential implications that this process has on chemical tracing of martian mantle source(s). To establish the proportionate influence of atmospheric, hydrothermal, and volcanic processes on nakhlite isotopic systematics we obtained in situ sulphur isotope data (Δ³³S and δ³⁴S) for sulphide grains (pyrrhotite and pyrite) in all five nakhlite samples. For Nakhla, Lafayette, and MIL 090032, these data are integrated with highly siderophile element (HSE) abundances and Os-isotope compositions, as well as textural information constrained prior to isotopic analysis. This work thereby provides the first Re-Os isotope systematics for two different nakhlites, and also the first Re-Os isotope data for martian sample for which detailed petrographic information was constrained prior to digestion.

We report the largest variation in δ³⁴S yet found in martian meteorites (-13.20 ‰ to +15.16 ‰). The relatively positive Δ³³S and δ³⁴S values of MIL 090032 (δ³⁴S = +10.54 ± 0.09 ‰; Δ³³S = -0.67 ± 0.10 ‰) indicate this meteorite assimilated sulphur affected by UV-photochemistry. In contrast, the strongly negative values of Lafayette (δ³⁴S = -10.76 ± 0.14 ‰; Δ³³S = -0.09 ± 0.12 ‰) are indicative of hydrothermal processes on Mars. Nakhla, Yamato 000593, and Yamato 000749 sulphides have a narrower range of sulphur isotope compositions (Δ³³S and δ³⁴S ∼ 0) that is consistent with no assimilation of martian surface materials during lava flow emplacement. Consequently we used this second group of Δ³³S values to approximate the Δ³³S of the nakhlite source, yielding a Δ³³S value of -0.1 ‰.

Nakhlite HSE patterns result from a sulphide-saturated melt where Ru-Os-Ir alloys/sulphide were likely crystallized during earlier phases of magmatic processing in Mars to result in the fractionated HSE patterns of the nakhlites. Our data, alongside a synthesis of previously published data, suggest assimilation of an enriched component to the primary nakhlite melt, potentially a late-stage crystallization cumulate from the martian magma ocean stage. In the context of this model, and within large uncertainties, our data hint at perturbation and potential decoupling of nakhlite Re-Os isotope systematics from other isotopic systems as a result of small degrees of assimilation of a regolith component with highly radiogenic ¹⁸⁷Os/¹⁸⁸Os.

John M. Brewer^1,2, Songhu Wang¹, Debra A. Fischer¹, and Daniel Foreman-Mackey³
Astrophysical Journal Letters 867, L3 Link to Article [DOI: 10.3847/2041-8213/aae710]
¹Department of Astronomy, Yale University, 52 Hillhouse Avenue, New Haven, CT 06511, USA
²Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
³Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA

In systems with detected planets, hot Jupiters and compact systems of multiple planets are nearly mutually exclusive. We compare the relative occurrence of these two architectures as a fraction of detected planetary systems to determine the role that metallicity plays in planet formation. We show that compact multi-planet systems occur more frequently around stars of increasingly lower metallicities using spectroscopically derived abundances for more than 700 planet hosts. At higher metallicities, compact multi-planet systems comprise a nearly constant fraction of the planet hosts despite the steep rise in the fraction of hosts containing hot and cool Jupiters. Since metal-poor stars have been underrepresented in planet searches, this implies that the occurrence rate of compact multis is higher than previously reported. Due to observational limits, radial velocity planet searches have focused mainly on high-metallicity stars, where they have a higher chance of finding giant planets. New extreme-precision radial velocity instruments coming online that can detect these compact multi-planet systems can target lower-metallicity stars to find them.