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composition of outer core
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Short answer
The outer core is a mostly liquid metallic layer composed predominantly of iron (Fe) with a significant nickel (Ni) component and several percent of light elements (S, O, Si, C, and/or H). Rough mass/atom fractions commonly quoted are ~80–90% Fe, ~5–10% Ni, and ~5–10% combined light elements (exact amounts are uncertain).
Key details
- Physical state: liquid (this is why S‑waves do not propagate through it).
- Main constituents: Fe (major), Ni (minor), plus light elements (sulfur, oxygen, silicon, carbon, hydrogen are the main candidates).
- Typical compositional estimates: Fe ≈ 80–90 wt%, Ni ≈ 5–10 wt%, light elements ≈ 5–10 wt% (the identity and amounts of the light elements are debated).
- Density and conditions: pressure ≈ 135–330 GPa (top to bottom of outer core); temperature ≈ 4000–6000 K; density from seismic models (e.g., PREM) is ~9.9 g/cm3 at the top to ~12.2 g/cm3 near the inner core boundary.
- Functionally important: convection of the liquid Fe–Ni alloy (driven by thermal and compositional buoyancy as the inner core grows) powers the geodynamo that produces Earth’s magnetic field.
How we know this
- Seismology: disappearance of S-waves and slowed P-wave speeds give the liquid layer and its density/velocity profile.
- Mineral physics and lab experiments: high‑P/T experiments and first‑principles calculations constrain which elements alloy with Fe and how physical properties (density, sound speed, melting point) change.
- Cosmochemistry and meteorite analogs: composition of primitive meteorites constrains likely planetary building blocks.
- Geodynamo and energetics: models require a conductive, convecting metallic liquid consistent with an Fe‑rich alloy.
Uncertainties
- The exact mix and proportions of light elements remain uncertain. Different studies favor different combinations (e.g., some favor sulfur‑rich models, others oxygen‑ or silicon‑rich, and some include measurable hydrogen or carbon).
- Quantitative numbers (wt% of each light element) vary with the data sets and modeling assumptions.
If you want, I can:
- summarize recent papers arguing for a particular light‑element mix (e.g., Fe–Ni–Si–O vs Fe–Ni–S models), or
- show how seismic observables constrain different mixtures.
The outer core is a mostly liquid metallic layer composed predominantly of iron (Fe) with a significant nickel (Ni) component and several percent of light elements (S, O, Si, C, and/or H). Rough mass/atom fractions commonly quoted are ~80–90% Fe, ~5–10% Ni, and ~5–10% combined light elements (exact amounts are uncertain).
Key details
- Physical state: liquid (this is why S‑waves do not propagate through it).
- Main constituents: Fe (major), Ni (minor), plus light elements (sulfur, oxygen, silicon, carbon, hydrogen are the main candidates).
- Typical compositional estimates: Fe ≈ 80–90 wt%, Ni ≈ 5–10 wt%, light elements ≈ 5–10 wt% (the identity and amounts of the light elements are debated).
- Density and conditions: pressure ≈ 135–330 GPa (top to bottom of outer core); temperature ≈ 4000–6000 K; density from seismic models (e.g., PREM) is ~9.9 g/cm3 at the top to ~12.2 g/cm3 near the inner core boundary.
- Functionally important: convection of the liquid Fe–Ni alloy (driven by thermal and compositional buoyancy as the inner core grows) powers the geodynamo that produces Earth’s magnetic field.
How we know this
- Seismology: disappearance of S-waves and slowed P-wave speeds give the liquid layer and its density/velocity profile.
- Mineral physics and lab experiments: high‑P/T experiments and first‑principles calculations constrain which elements alloy with Fe and how physical properties (density, sound speed, melting point) change.
- Cosmochemistry and meteorite analogs: composition of primitive meteorites constrains likely planetary building blocks.
- Geodynamo and energetics: models require a conductive, convecting metallic liquid consistent with an Fe‑rich alloy.
Uncertainties
- The exact mix and proportions of light elements remain uncertain. Different studies favor different combinations (e.g., some favor sulfur‑rich models, others oxygen‑ or silicon‑rich, and some include measurable hydrogen or carbon).
- Quantitative numbers (wt% of each light element) vary with the data sets and modeling assumptions.
If you want, I can:
- summarize recent papers arguing for a particular light‑element mix (e.g., Fe–Ni–Si–O vs Fe–Ni–S models), or
- show how seismic observables constrain different mixtures.
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