{"id":1252,"date":"2025-04-14T13:54:05","date_gmt":"2025-04-14T04:54:05","guid":{"rendered":"https:\/\/www.grc.ehime-u.ac.jp\/en\/?post_type=project&p=1252"},"modified":"2025-04-14T14:10:33","modified_gmt":"2025-04-14T05:10:33","slug":"2025-4-14","status":"publish","type":"project","link":"https:\/\/www.grc.ehime-u.ac.jp\/en\/archives\/highlights\/2025-4-14","title":{"rendered":"Missing nitrogen: a dramatic game of cosmic hide-and-seek deep within our planet"},"content":{"rendered":"

How the core-mantle differentiation shaped volatile elements in the Earth<\/h3>\n
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Earth\u2019s rocky layers are mysteriously low in nitrogen compared with carbon and argon. A scientific team explored our planet\u2019s molten youth using advanced quantum mechanical simulations, revealing nitrogen\u2019s secret: under extreme pressure, it chose to hide in the iron core 100 times more than the mantle. This solved why Earth\u2019s volatile ratios involving nitrogen look odd. The findings suggest the necessary ingredients for developing a habitable world may have been settled in the early Earth.<\/p>\n<\/div>\n

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Imagine if Earth\u2019s history had a mystery novel, and one of its biggest unsolved puzzles was: Where did all the nitrogen go? Scientists have long known that our planet\u2019s rocky outer layers\u2014the mantle\u2014are oddly poor in nitrogen compared to other volatile elements like carbon or water. Very strangely, the C\/N and\u00a036<\/sup>Ar\/N ratios in the bulk silicate Earth (BSE, the whole Earth minus the metallic core) are far higher than those found in the meteorites that supposedly delivered these ingredients during the planet\u2019s infancy. For decades, this \u201cmissing nitrogen\u201d problem has puzzled researchers. A new study published in Earth and Planetary Science Letters might finally have the answer: a dramatic game of cosmic hide-and-seek deep within our planet.<\/p>\n

To understand this mystery, we need to rewind 4.6 billion years. The Earth was a fiery, molten ball, with a churning magma ocean over one thousand kilometers deep. During this period, heavy metals like iron sank to form the core, while lighter mineral components rose and then solidified to create the silicate mantle. This process, called the core-mantle differentiation, shaped Earth\u2019s layered structure. But it was not just metals and rocks sorting themselves out\u2014volatile elements like nitrogen, carbon, and argon were caught in the crossfire. Where these elements ended up\u2014trapped in the core, dissolved in the mantle, or lost to space\u2014determines why the Earth looks and functions as it does today.<\/p>\n

Nitrogen is particularly enigmatic. While it makes up 78% of the atmosphere today, the total amount in the Earth\u2019s entire rocky mantle is shockingly low\u2014just 1 to 5 parts per million. Carbon and argon are far more abundant relative to nitrogen than in the meteorites that likely delivered these elements. Scientists have proposed many hypotheses: Maybe nitrogen escaped into space, or perhaps it was never delivered in large amounts. But a team of researchers from Geodynamics Research Center, Ehime University in Japan asked a different question: what if the Earth\u2019s core stole most of the nitrogen?<\/p>\n

To test this idea, the scientists recreated the extreme conditions of Earth\u2019s early magma ocean using \u201csupercomputers\u201d. They simulated how nitrogen behaves when squeezed at pressures up to 1.35 million times the pressure at the surface (135 GPa) and heated to 5000 K\u2014conditions found thousands of kilometers deep in a young, molten planet. Using a quantum mechanical method called ab initio molecular dynamics combined with the thermodynamic integration method based on statistical physics, which calculates atomic interactions from fundamental physics principles, they tracked nitrogen\u2019s preferences: did it bond with the iron-rich core or dissolve into the silicate mantle?<\/p>\n

The results were striking. Under the intense heat and pressure of a deep magma ocean, nitrogen became a \u201cmetal lover.\u201d At 60 GPa, nitrogen was over 100 times more likely to join the core than stay in the mantle after its solidification. As pressure increased, this preference grew\u2014but not in a straight line. Instead, the relationship was curved. This nonlinear effect had never been clearly shown before and helps explain why earlier experiments produced conflicting results.<\/p>\n

But why does nitrogen behave this way? The simulations revealed a microscopic mechanism. In the molten silicate of magma ocean, nitrogen atoms initially bonded with themselves or hydrogen atoms like ammonium ions (NH4<\/sub>+<\/sup>). But under increasing pressures, they broke apart. Nitrogen instead bonded with silicon atoms, integrating into the silicate network as nitride ions (N\u00b3\u207b). Meanwhile, in the metallic core, nitrogen slipped into gaps between iron atoms, behaving more like a neutral atom. This behavior caused the more nitrogen to abandon the molten silicate for the core\u2019s embrace.<\/p>\n

The study didn\u2019t stop at nitrogen. Combing with previous studies, Huang and Tsuchiya found that carbon, while somewhat siderophile (metal-loving), was less than nitrogen under deep magma ocean conditions. Argon, an inert element, didn\u2019t care for metals at all. This hierarchy\u2014nitrogen > carbon > argon in core preference\u2014may solve two mysteries.<\/p>\n

To quantify this, the researchers built a model of Earth\u2019s accretion 4.6 billion years ago. Suppose Earth gained volatiles from carbonaceous chondrites, meteorites with compositions similar to the early solar system. Delivering just 5\u201310% of Earth\u2019s mass from these rocks would supply enough nitrogen, carbon, and argon. If the core formation happened in a deep magma ocean (e.g., 60 GPa), over 80% of nitrogen would sink into the core, leaving the mantle with 1\u20137 ppm\u2014matching observations. Carbon, less eager to leave, would stay in the mantle, creating the observed high C\/N ratio. Argon, rejected by both the core and mantle, would be disproportionately concentrated in the atmosphere, explaining the high\u00a036<\/sup>Ar\/N of the BSE.<\/p>\n

This discovery reshapes our understanding of Earth\u2019s volatile origins. For years, scientists debated whether Earth\u2019s weird ratios meant it accreted unusual meteorites or lost nitrogen to space. This study argues for a simpler story: Earth\u2019s volatiles came from carbonaceous chondrites, but their fates were sealed by the extreme physics of the core formation. The differentiation depth mattered most\u2014shallow magma oceans could not produce the observed ratios, but deep ones perfectly replicate Earth\u2019s volatile fingerprint. This further links to an argument that the distinct volatile ratios of the BSE compared to chondrites may reflect different accretion times rather than different sources.<\/p>\n

This core formation process has determined how much nitrogen was retained in the BSE, one of prerequisites for the abundance of bioessential elements in the Earth\u2019s atmosphere and rocky layers. Despite it took Earth a long time to become habitable, the conditions essential for life may have been set billions of years ago when the core and mantle separated.<\/p>\n

In the end, Earth\u2019s nitrogen was not lost. It has been hiding in plain sight, locked away in the core for billions of years. This discovery reminds us that our planet\u2019s history is written not just in rocks and fossils, but in the cryptic preferences of atoms under unimaginable pressures.<\/p>\n<\/div>\n\n\n

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Calculated nitrogen metal\/silicate partition coefficients (DFe\/silicate<\/sup>N<\/sub>) under high-pressure and high-temperature<\/strong>
The ratio that indicates whether an element is more soluble in liquid iron or molten silicate is called the partition coefficient. This figure shows that nitrogen has a greater affinity for liquid iron than for molten silicate, and that this affinity is enhanced by pressure but compromised by temperature.<\/figcaption><\/figure>\n\n\n\n
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Modeling of C\/N and\u00a036<\/sup>Ar\/N ratios for the BSE placed during the core formation<\/strong>
Relationship between the size of the core and the C\/N and\u00a036<\/sup>Ar\/N ratios of the BSE obtained from modeling using the results of the present calculations. The C\/N ratio decreases when the core-mantle separation occurs at low pressure (5-20 GPa) and increases at high pressure (60 GPa). The\u00a036<\/sup>Ar\/N ratio slightly increases when the core-mantle separation occurs at low pressure (5-20 GPa) and significantly increases at high pressure (60 GPa).<\/figcaption><\/figure>\n\n\n\n
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Schematic images for volatile delivery to the proto-Earth during the Earth\u2019s formation<\/strong>
The C\/N and\u00a036<\/sup>Ar\/N ratios of the BSE change strongly depending on the differentiation depth and redox conditions in the proto-Earth. Only the differentiation at deep magma ocean conditions could increase two ratios simultaneously to match the observations for the present Earth.<\/figcaption><\/figure>\n\n\n\n
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Nitrogen-carbon-argon features of the silicate Earth established by deep core-mantle differentiation, Shengxuan Huang and Taku Tsuchiya, Earth and Planetary Science Letters, 657, 119291, doi:10.1016\/j.epsl.2025.119291<\/a><\/p>\n

Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP22KF0281, JP22F22020, JP21K03703, and JP22H01327<\/p>","protected":false},"excerpt":{"rendered":"How the core-mantle differentiation shaped volatile elements in the Earth Earth\u2019s rocky layers are mysteriousl […]","protected":false},"featured_media":1254,"template":"","project_category":[],"class_list":["post-1252","project","type-project","status-publish","has-post-thumbnail","hentry"],"_links":{"self":[{"href":"https:\/\/www.grc.ehime-u.ac.jp\/en\/wp-json\/wp\/v2\/project\/1252","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.grc.ehime-u.ac.jp\/en\/wp-json\/wp\/v2\/project"}],"about":[{"href":"https:\/\/www.grc.ehime-u.ac.jp\/en\/wp-json\/wp\/v2\/types\/project"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.grc.ehime-u.ac.jp\/en\/wp-json\/wp\/v2\/media\/1254"}],"wp:attachment":[{"href":"https:\/\/www.grc.ehime-u.ac.jp\/en\/wp-json\/wp\/v2\/media?parent=1252"}],"wp:term":[{"taxonomy":"project_category","embeddable":true,"href":"https:\/\/www.grc.ehime-u.ac.jp\/en\/wp-json\/wp\/v2\/project_category?post=1252"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}