Tracing the ingredients for a habitable earth from interstellar space through planet formation

Inefficient nitrogen transport to the lower mantle by sediment subduction

The fate of sedimentary nitrogen during subduction is essential for understanding the origin of nitrogen in the deep Earth. Here we study the behavior of nitrogen in slab sediments during the phengite to K-hollandite transition at 10-12 GPa and 800-1100 °C. Phengite stability is extended by 1-3 GPa in the nitrogen (NH4+)-bearing system. The phengite-fluid partition coefficient of nitrogen is 0.031 at 10 GPa, and K-hollandite-fluid partition coefficients of nitrogen range from 0.008 to 0.064, showing a positive dependence on pressure but a negative dependence on temperature. The nitrogen partitioning data suggest that K-hollandite can only preserve ~43% and ~26% of the nitrogen from phengite during the phengite to K-hollandite transition along the cold and warm slab geotherms, respectively. Combined with the slab sedimentary nitrogen influx, we find that a maximum of ~1.5 × 108 kg/y of nitrogen, representing ~20% of the initial sedimentary nitrogen influx, could be transported by K-hollandite to the lower mantle. We conclude that slab sediments may have contributed less than 15% of the lower mantle nitrogen, most of which is probably of primordial origin.

The origin and evolution of Earth's nitrogen

Nitrogen is a vital element for life on Earth. Its cycling between the surface (atmosphere + crust) and the mantle has a profound influence on the atmosphere and climate. However, our understanding of the origin and evolution of Earth's nitrogen is still incomplete. This review presents an overview of the current understanding of Earth's nitrogen budget and the isotope composition of different reservoirs, laboratory constraints on deep nitrogen geochemistry, and our understanding of the origin of Earth's nitrogen and the deep nitrogen cycle through plate subduction and volcanism. The Earth may have acquired its nitrogen heterogeneously during the main accretion phase, initially from reduced, enstatite-chondrite-like impactors, and subsequently from increasingly oxidized impactors and minimal CI-chondrite-like materials. Like Earth's surface, the mantle and core are also significant nitrogen reservoirs. The nitrogen abundance and isotope composition of these three reservoirs may have been fundamentally established during the main accretion phase and have been insignificantly modified afterwards by the deep nitrogen cycle, although there is a net nitrogen ingassing into Earth's mantle in modern subduction zones. However, it is estimated that the early atmosphere of Earth may have contained ∼1.4 times the present-day atmospheric nitrogen (PAN), with ∼0.4 PAN being sequestered into the crust via biotic nitrogen fixation. In order to gain a better understanding of the origin and evolution of Earth's nitrogen, directions for future research are suggested.

Nitrogen isotope evidence for Earth's heterogeneous accretion of volatiles

The origin of major volatiles nitrogen, carbon, hydrogen, and sulfur in planets is critical for understanding planetary accretion, differentiation, and habitability. However, the detailed process for the origin of Earth's major volatiles remains unresolved. Nitrogen shows large isotopic fractionations among geochemical and cosmochemical reservoirs, which could be used to place tight constraints on Earth's volatile accretion process. Here we experimentally determine N-partitioning and -isotopic fractionation between planetary cores and silicate mantles. We show that the core/mantle N-isotopic fractionation factors, ranging from -4‰ to +10‰, are strongly controlled by oxygen fugacity, and the core/mantle N-partitioning is a multi-function of oxygen fugacity, temperature, pressure, and compositions of the core and mantle. After applying N-partitioning and -isotopic fractionation in a planetary accretion and core-mantle differentiation model, we find that the N-budget and -isotopic composition of Earth's crust plus atmosphere, silicate mantle, and the mantle source of oceanic island basalts are best explained by Earth's early accretion of enstatite chondrite-like impactors, followed by accretion of increasingly oxidized impactors and minimal CI chondrite-like materials before and during the Moon-forming giant impact. Such a heterogeneous accretion process can also explain the carbon-hydrogen-sulfur budget in the bulk silicate Earth. The Earth may thus have acquired its major volatile inventory heterogeneously during the main accretion phase.

Searching For the t=0 of Planetary System Formation

The composition of bodies in the solar system points to strong gradients in the volatile content within solid bodies hinting at the presence of gas-ice transitions across sublimation fronts in the young formative stages when the gas-rich disk was present. Terrestrial worlds are constructed out of the disk solids which are primarily silicate and water, but might also contain a significant fraction of organic material. These refractory organics are the source of carbon to Earth-like worlds, but have the potential to be destroyed if temperatures exceed 300-500 K (depending on pressure). These temperatures are most readily prevalent during the early stages of planetary system formation where the seeds of terrestrial worlds are potentially assembled. Here we present an ongoing observational search for refractory carbon grain destruction. We also discuss the implications on the overall gas phase chemistry within sublimation zones and on the ultimate composition of planetary bodies forming from available materials.

Numerous chondritic impactors and oxidized magma ocean set Earth's volatile depletion

Earth’s surface environment is largely influenced by its budget of major volatile elements: carbon (C), nitrogen (N), and hydrogen (H). Although the volatiles on Earth are thought to have been delivered by chondritic materials, the elemental composition of the bulk silicate Earth (BSE) shows depletion in the order of N, C, and H. Previous studies have concluded that non-chondritic materials are needed for this depletion pattern. Here, we model the evolution of the volatile abundances in the atmosphere, oceans, crust, mantle, and core through the accretion history by considering elemental partitioning and impact erosion. We show that the BSE depletion pattern can be reproduced from continuous accretion of chondritic bodies by the partitioning of C into the core and H storage in the magma ocean in the main accretion stage and atmospheric erosion of N in the late accretion stage. This scenario requires a relatively oxidized magma ocean (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\log _{10} f_{{\mathrm{O}}_2}$$\end{document}log10fO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gtrsim$$\end{document}≳\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{IW}}$$\end{document}IW\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-2$$\end{document}-2, where \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{\mathrm{O}}_2}$$\end{document}fO2 is the oxygen fugacity, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{IW}$$\end{document}IW is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\log _{10} f_{{\mathrm{O}}_2}^{\mathrm{IW}}$$\end{document}log10fO2IW, and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{\mathrm{O}}_2}^{\mathrm{IW}}$$\end{document}fO2IW is \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$f_{{\mathrm{O}}_2}$$\end{document}fO2 at the iron-wüstite buffer), the dominance of small impactors in the late accretion, and the storage of H and C in oceanic water and carbonate rocks in the late accretion stage, all of which are naturally expected from the formation of an Earth-sized planet in the habitable zone.

Exploring the link between star and planet formation with Ariel

The goal of the Ariel space mission is to observe a large and diversified population of transiting planets around a range of host star types to collect information on their atmospheric composition. The planetary bulk and atmospheric compositions bear the marks of the way the planets formed: Ariel's observations will therefore provide an unprecedented wealth of data to advance our understanding of planet formation in our Galaxy. A number of environmental and evolutionary factors, however, can affect the final atmospheric composition. Here we provide a concise overview of which factors and effects of the star and planet formation processes can shape the atmospheric compositions that will be observed by Ariel, and highlight how Ariel's characteristics make this mission optimally suited to address this very complex problem.

Earth's carbon deficit caused by early loss through irreversible sublimation

Carbon is an essential element for life, but its behavior during Earth's accretion is not well understood. Carbonaceous grains in meteoritic and cometary materials suggest that irreversible sublimation, and not condensation, governs carbon acquisition by terrestrial worlds. Through astronomical observations and modeling, we show that the sublimation front of carbon carriers in the solar nebula, or the soot line, moved inward quickly so that carbon-rich ingredients would be available for accretion at 1 astronomical unit after the first million years. On the other hand, geological constraints firmly establish a severe carbon deficit in Earth, requiring the destruction of inherited carbonaceous organics in the majority of its building blocks. The carbon-poor nature of Earth thus implies carbon loss in its precursor material through sublimation within the first million years.

Early volatile depletion on planetesimals inferred from C-S systematics of iron meteorite parent bodies

During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration, and inferred parent body C concentrations range from 0.0004 to 0.11 wt%. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed-system extraction to degassing of a wholly molten body, show that significant open-system silicate melting and volatile loss are required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos.

Did a Complex Carbon Cycle Operate in the Inner Solar System?

Solids in the interstellar medium consist of an intimate mixture of silicate and carbonaceous grains. Because 99% of silicates in meteorites were reprocessed at high temperatures in the inner regions of the Solar Nebula, we propose that similar levels of heating of carbonaceous materials in the oxygen-rich Solar Nebula would have converted nearly all carbon in dust and grain coatings to CO. We discuss catalytic experiments on a variety of grain surfaces that not only produce gas phase species such as CH₄, C₂H₆, C₆H₆, C₆H₅OH, or CH₃CN, but also produce carbonaceous solids and fibers that would be much more readily incorporated into growing planetesimals. CH₄ and other more volatile products of these surface-mediated reactions were likely transported outwards along with chondrule fragments and small Calcium Aluminum-rich Inclusions (CAIs) to enhance the organic content in the outer regions of the nebula where comets formed. Carbonaceous fibers formed on the surfaces of refractory oxides may have significantly improved the aggregation efficiency of chondrules and CAIs. Carbonaceous fibers incorporated into chondritic parent bodies might have served as the carbon source for the generation of more complex organic species during thermal or hydrous metamorphic processes on the evolving asteroid.

Exoplanet secondary atmosphere loss and revival

The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus-high-molecular-weight (secondary) atmospheres-on rocky exoplanets. Many rocky exoplanets are born with thick (>10 kbar) H2-dominated atmospheres but subsequently lose their H2; this process has no known Solar System analog. We study the consequences of early loss of a thick H2 atmosphere for subsequent occurrence of a high-molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H2 Our results imply that most rocky exoplanets orbiting closer to their star than the habitable zone that were formed with thick H2-dominated atmospheres lack high-molecular-weight atmospheres today. During early magma ocean crystallization, high-molecular-weight species usually do not form long-lived high-molecular-weight atmospheres; instead, they are lost to space alongside H2 This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, water worlds). Our results imply that in order to find high-molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H2-poor, that have anomalously large radii, or that orbit less active stars.

Redox control on nitrogen isotope fractionation during planetary core formation

The present-day nitrogen isotopic compositions of Earth's surficial (15N-enriched) and deep reservoirs (15N-depleted) differ significantly. This distribution can neither be explained by modern mantle degassing nor recycling via subduction zones. As the effect of planetary differentiation on the behavior of N isotopes is poorly understood, we experimentally determined N-isotopic fractionations during metal-silicate partitioning (analogous to planetary core formation) over a large range of oxygen fugacities (ΔIW -3.1 < logfO2 < ΔIW -0.5, where ΔIW is the logarithmic difference between experimental oxygen fugacity [fO2] conditions and that imposed by the coexistence of iron and wüstite) at 1 GPa and 1,400 °C. We developed an in situ analytical method to measure the N-elemental and -isotopic compositions of experimental run products composed of Fe-C-N metal alloys and basaltic melts. Our results show substantial N-isotopic fractionations between metal alloys and silicate glasses, i.e., from -257 ± 22‰ to -49 ± 1‰ over 3 log units of fO2 These large fractionations under reduced conditions can be explained by the large difference between N bonding in metal alloys (Fe-N) and in silicate glasses (as molecular N2 and NH complexes). We show that the δ15N value of the silicate mantle could have increased by ∼20‰ during core formation due to N segregation into the core.

Selenium isotopes as tracers of a late volatile contribution to Earth from the outer Solar System

The origin of Earth's volatiles has been attributed to a late addition of meteoritic material after core-mantle differentiation. The nature and consequences of this 'late veneer' are debated, but may be traced by isotopes of the highly siderophile, or iron-loving, and volatile element selenium. Here we present high-precision selenium isotope data for mantle peridotites, from double spike and hydride generation multi-collector inductively coupled plasma mass spectrometry. These data indicate that the selenium isotopic composition of peridotites is unaffected by petrological processes, such as melt depletion and melt-rock reaction, and thus a narrow range is preserved that is representative of the silicate Earth. We show that selenium isotopes record a signature of late accretion after core formation and that this signature overlaps only with that of the CI-type carbonaceous chondrites. We conclude that these isotopic constraints indicate the late veneer originated from the outer Solar System and was of lower mass than previously estimated. Thus, we suggest a late and highly concentrated delivery of volatiles enabled Earth to become habitable.

The fate of nitrogen during core-mantle separation on Earth

Nitrogen, the most dominant constituent of Earth's atmosphere, is critical for the habitability and existence of life on our planet. However, its distribution between Earth's major reservoirs, which must be largely influenced by the accretion and differentiation processes during its formative years, is poorly known. Sequestration into the metallic core, along with volatility related loss pre- and post-accretion, could be a critical process that can explain the depletion of nitrogen in the Bulk Silicate Earth (BSE) relative to the primitive chondrites. However, the relative effect of different thermodynamic parameters on the alloy-silicate partitioning behavior of nitrogen is still poorly known. Here we present equilibrium partitioning data of N between alloy and silicate melt ( D N alloy / silicate ) from 67 new high pressure (P = 1-6 GPa)-temperature (T = 1500-2200 °C) experiments under graphite saturated conditions at a wide range of oxygen fugacity (logfO₂ ~ΔIW - 4.2 to - 0.8), mafic to ultramafic silicate melt compositions (NBO/T = 0.4 to 2.2), and varying chemical composition of the alloy melts (S and Si contents of 0-32.1 wt.% and 0-3.1 wt.%, respectively). Under relatively oxidizing conditions (~ΔIW - 2.2 to - 0.8) nitrogen acts as a siderophile element ( D N alloy / silicate between 1.1 and 52), where D N alloy / silicate decreases with decrease in fO₂ and increase in T, and increases with increase in P and NBO/T. Under these conditions D N alloy / silicate remains largely unaffected between S-free conditions and up to ~17 wt.% S content in the alloy melt, and then drops off at > ~20 wt.% S content in the alloy melt. Under increasingly reduced conditions (< ~ ΔIW - 2.2), N becomes increasingly lithophile ( D N alloy / silicate between 0.003 and 0.5) with D N alloy / silicate decreasing with decrease in fO₂ and increase in T. At these conditions fO₂, along with Si content of the alloy under the most reduced conditions (< ~ΔIW - 3.0), is the controlling parameter with T playing a secondary role, while, P, NBO/T, and S content of the alloy have minimal effects. A multiple linear least-squares regression parametrization for D N alloy / silicate based on the results of this study and previous studies suggests, in agreement with the experimental data, that fO₂ (represented by Si content of the alloy melt and FeO content of the silicate melt), followed by T, has the strongest control on D N alloy / silicate . Based on our modeling, to match the present-day BSE N content, impactors that brought N must have been moderately to highly oxidized. If N bearing impactors were reduced, and/or there was significant disequilibrium core formation, then the BSE would be too N-rich and another mechanism for N loss, such as atmospheric loss, would be required.

Delivery of carbon, nitrogen, and sulfur to the silicate Earth by a giant impact

Earth's status as the only life-sustaining planet is a result of the timing and delivery mechanism of carbon (C), nitrogen (N), sulfur (S), and hydrogen (H). On the basis of their isotopic signatures, terrestrial volatiles are thought to have derived from carbonaceous chondrites, while the isotopic compositions of nonvolatile major and trace elements suggest that enstatite chondrite-like materials are the primary building blocks of Earth. However, the C/N ratio of the bulk silicate Earth (BSE) is superchondritic, which rules out volatile delivery by a chondritic late veneer. In addition, if delivered during the main phase of Earth's accretion, then, owing to the greater siderophile (metal loving) nature of C relative to N, core formation should have left behind a subchondritic C/N ratio in the BSE. Here, we present high pressure-temperature experiments to constrain the fate of mixed C-N-S volatiles during core-mantle segregation in the planetary embryo magma oceans and show that C becomes much less siderophile in N-bearing and S-rich alloys, while the siderophile character of N remains largely unaffected in the presence of S. Using the new data and inverse Monte Carlo simulations, we show that the impact of a Mars-sized planet, having minimal contributions from carbonaceous chondrite-like material and coinciding with the Moon-forming event, can be the source of major volatiles in the BSE.

The geobiological nitrogen cycle: From microbes to the mantle

Nitrogen forms an integral part of the main building blocks of life, including DNA, RNA, and proteins. N₂ is the dominant gas in Earth's atmosphere, and nitrogen is stored in all of Earth's geological reservoirs, including the crust, the mantle, and the core. As such, nitrogen geochemistry is fundamental to the evolution of planet Earth and the life it supports. Despite the importance of nitrogen in the Earth system, large gaps remain in our knowledge of how the surface and deep nitrogen cycles have evolved over geologic time. Here, we discuss the current understanding (or lack thereof) for how the unique interaction of biological innovation, geodynamics, and mantle petrology has acted to regulate Earth's nitrogen cycle over geologic timescales. In particular, we explore how temporal variations in the external (biosphere and atmosphere) and internal (crust and mantle) nitrogen cycles could have regulated atmospheric pN₂ . We consider three potential scenarios for the evolution of the geobiological nitrogen cycle over Earth's history: two in which atmospheric pN₂ has changed unidirectionally (increased or decreased) over geologic time and one in which pN₂ could have taken a dramatic deflection following the Great Oxidation Event. It is impossible to discriminate between these scenarios with the currently available models and datasets. However, we are optimistic that this problem can be solved, following a sustained, open-minded, and multidisciplinary effort between surface and deep Earth communities.