The Density of Liquid FeO and the Oxygen Contents of the Cores of Earth and Mars
Abstract
Oxygen is a possible light element in the cores of Earth and Mars. Its effect on the density and elastic properties of liquid iron is, therefore, important for estimating its abundance in planetary cores, from which conditions of core‐mantle equilibration could be constrained. To address this, a self‐consistent thermodynamic model for the Fe–FeO system has been refined by incorporating new X‐ray absorption measurements of liquid FeO density, multi‐anvil experiments on Fe–FeO liquid immiscibility and data on Fe and FeO equations of state and melting relations. The measurements show that liquid FeO at ∼2.3 GPa has a density of 5.2–5.4 g/cm 3 , significantly higher than implied by previous 1 bar experimental data. The model accounts for the pressure dependence of the Fe–FeO excess volume of mixing ( V ex ), allowing its effect on the bulk modulus and P‐wave velocity (Vp) to be estimated. This effect is found to be small at conditions of the top of Earth's core. Although ∼5–7 wt.% oxygen could explain the outer‐core density deficit, no oxygen content results in a Vp high enough to match that of the core, implying that it cannot be the dominant light element. At Martian core conditions, Fe–FeO non‐ideality is larger, causing oxygen to raise the Vp of Fe liquid. Although this effect will be smaller in the presence of sulfur, oxygen could shift the Vp of a sulfur‐dominated Martian core into agreement with recent InSight‐based estimates. A sulfur‐dominated outer‐core would contain 5–7 wt.% oxygen if proposals for an FeO Martian inner core are correct.
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BibTeX
@article{frost2026density,
title = {{The Density of Liquid FeO and the Oxygen Contents of the Cores of Earth and Mars}},
author = {Daniel J Frost and Lianjie Man and Rémy Pierru and Laura Henry and Adrien Néri},
journal = {Journal of Geophysical Research : Solid Earth},
year = {2026},
month = sep,
doi = {10.1029/2025jb033478},
url = {https://lilloa.hal.science/hal-05751329v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The Density of Liquid FeO and the Oxygen Contents of the Cores of Earth and Mars
AU - Daniel J Frost
AU - Lianjie Man
AU - Rémy Pierru
AU - Laura Henry
AU - Adrien Néri
PY - 2026
DA - 2026/09/15
JO - Journal of Geophysical Research : Solid Earth
DO - 10.1029/2025jb033478
UR - https://lilloa.hal.science/hal-05751329v1
AB - Oxygen is a possible light element in the cores of Earth and Mars. Its effect on the density and elastic properties of liquid iron is, therefore, important for estimating its abundance in planetary cores, from which conditions of core‐mantle equilibration could be constrained. To address this, a self‐consistent thermodynamic model for the Fe–FeO system has been refined by incorporating new X‐ray absorption measurements of liquid FeO density, multi‐anvil experiments on Fe–FeO liquid immiscibility and data on Fe and FeO equations of state and melting relations. The measurements show that liquid FeO at ∼2.3 GPa has a density of 5.2–5.4 g/cm 3 , significantly higher than implied by previous 1 bar experimental data. The model accounts for the pressure dependence of the Fe–FeO excess volume of mixing ( V ex ), allowing its effect on the bulk modulus and P‐wave velocity (Vp) to be estimated. This effect is found to be small at conditions of the top of Earth's core. Although ∼5–7 wt.% oxygen could explain the outer‐core density deficit, no oxygen content results in a Vp high enough to match that of the core, implying that it cannot be the dominant light element. At Martian core conditions, Fe–FeO non‐ideality is larger, causing oxygen to raise the Vp of Fe liquid. Although this effect will be smaller in the presence of sulfur, oxygen could shift the Vp of a sulfur‐dominated Martian core into agreement with recent InSight‐based estimates. A sulfur‐dominated outer‐core would contain 5–7 wt.% oxygen if proposals for an FeO Martian inner core are correct.
SN - 2169-9313
SN - 2169-9356
ER -