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Interior Models of Mercury and Conditions for Iron Snow Formation in a Fe‐S‐Si Core.

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Title: Interior Models of Mercury and Conditions for Iron Snow Formation in a Fe‐S‐Si Core.
Authors: Dunnigan, Abigail H.1,2 (AUTHOR) dunnigaa@purdue.edu, Liu, Dunyu3 (AUTHOR), Steinbrügge, Gregor B.4 (AUTHOR), Rivoldini, Attilio5 (AUTHOR), Dumberry, Mathieu6 (AUTHOR), Cao, Hao7 (AUTHOR), Soderlund, Krista M.3 (AUTHOR)
Source: Journal of Geophysical Research. Planets. Apr2026, Vol. 131 Issue 4, p1-16. 16p.
Subject Terms: Moments of inertia, Planetary interiors, Earth's core, Magnetohydrodynamics, Earth's mantle
Abstract: Mercury's unique interior structure and magnetic field generation remain to be fully understood. We construct models to further constrain Mercury's interior and test the hypothesis that iron snow within the liquid core drives the dynamo. We build upon previous models by incorporating an updated iron‐sulfur‐silicon (Fe‐S‐Si) core alloy composition and use a Monte Carlo approach to explore the parameter space consistent with geodetic and geophysical constraints. A high normalized moment of inertia (MoI) of 0.346±0.014 $0.346\pm 0.014$, in combination with thermal and geochemical constraints, favors models with an Earth‐like mantle density of ∼3300 ${\sim} 3300$ kg/m3 ${\mathrm{m}}^{3}$, an inner core ≤1050 ${\le} 1050$ km in radius, and a core silicon content of at least 6 wt%. In contrast, a lower value of MoI =0.333±0.005 ${=}0.333\pm 0.005$ favors models with lower mantle densities of ∼3100 ${\sim} 3100$ kg/m3 ${\mathrm{m}}^{3}$, an inner core radius in the range of 850–1,450 km, and a core silicon content less than 8 wt%. We also show that the formation of iron snow requires a sulfur concentration greater than ∼4 ${\sim} 4$ wt%. However, the expected geochemistry of the core restricts the sulfur content to less than 2 wt%. This inconsistency suggests the absence of snow layers in Mercury's present‐day core and that its dynamo is not driven by sulfur‐induced iron crystallization. Plain Language Summary: Geophysical measurements that provide information on the distribution of mass inside a planet, or its Moment of Inertia (MoI), can be utilized to estimate Mercury's interior structure. We use interior structure models informed by two published MoI values to constrain Mercury's inner core size and test the iron snow hypothesis for dynamo generation, where convection is driven by iron precipitation within the outer core. Our results broadly show that lower MoI values favor models with moderate inner core sizes (850–1,450 km) and lower silicon concentrations (2–7 wt%), while higher MoI values favor models with potentially small inner cores (less than ∼1050 ${\sim} 1050$ km) and higher silicon concentrations (at least 6 wt%). We also find a strong correlation of mantle density with MoI: higher MoI values yield Earth‐like mantle densities, whereas lower MoI values imply lower densities. We find that iron snow forms only when core sulfur exceeds the amount suggested by geochemical data. This mismatch implies that Mercury's core likely lacks snow layers and that its dynamo has a different power source. Key Points: The inner core radius and mantle density of Mercury sensitively depend on the normalized moment of inertia (MoI)A high MoI favors higher mantle densities and inner cores <1,100 km; a low MoI favors lower mantle densities and inner cores 850–1,450 kmAn iron snow layer in the liquid core requires a S concentration >2 wt%, inconsistent with core geochemistry [ABSTRACT FROM AUTHOR]
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Abstract:Mercury's unique interior structure and magnetic field generation remain to be fully understood. We construct models to further constrain Mercury's interior and test the hypothesis that iron snow within the liquid core drives the dynamo. We build upon previous models by incorporating an updated iron‐sulfur‐silicon (Fe‐S‐Si) core alloy composition and use a Monte Carlo approach to explore the parameter space consistent with geodetic and geophysical constraints. A high normalized moment of inertia (MoI) of 0.346±0.014 $0.346\pm 0.014$, in combination with thermal and geochemical constraints, favors models with an Earth‐like mantle density of ∼3300 ${\sim} 3300$ kg/m3 ${\mathrm{m}}^{3}$, an inner core ≤1050 ${\le} 1050$ km in radius, and a core silicon content of at least 6 wt%. In contrast, a lower value of MoI =0.333±0.005 ${=}0.333\pm 0.005$ favors models with lower mantle densities of ∼3100 ${\sim} 3100$ kg/m3 ${\mathrm{m}}^{3}$, an inner core radius in the range of 850–1,450 km, and a core silicon content less than 8 wt%. We also show that the formation of iron snow requires a sulfur concentration greater than ∼4 ${\sim} 4$ wt%. However, the expected geochemistry of the core restricts the sulfur content to less than 2 wt%. This inconsistency suggests the absence of snow layers in Mercury's present‐day core and that its dynamo is not driven by sulfur‐induced iron crystallization. Plain Language Summary: Geophysical measurements that provide information on the distribution of mass inside a planet, or its Moment of Inertia (MoI), can be utilized to estimate Mercury's interior structure. We use interior structure models informed by two published MoI values to constrain Mercury's inner core size and test the iron snow hypothesis for dynamo generation, where convection is driven by iron precipitation within the outer core. Our results broadly show that lower MoI values favor models with moderate inner core sizes (850–1,450 km) and lower silicon concentrations (2–7 wt%), while higher MoI values favor models with potentially small inner cores (less than ∼1050 ${\sim} 1050$ km) and higher silicon concentrations (at least 6 wt%). We also find a strong correlation of mantle density with MoI: higher MoI values yield Earth‐like mantle densities, whereas lower MoI values imply lower densities. We find that iron snow forms only when core sulfur exceeds the amount suggested by geochemical data. This mismatch implies that Mercury's core likely lacks snow layers and that its dynamo has a different power source. Key Points: The inner core radius and mantle density of Mercury sensitively depend on the normalized moment of inertia (MoI)A high MoI favors higher mantle densities and inner cores <1,100 km; a low MoI favors lower mantle densities and inner cores 850–1,450 kmAn iron snow layer in the liquid core requires a S concentration >2 wt%, inconsistent with core geochemistry [ABSTRACT FROM AUTHOR]
ISSN:21699097
DOI:10.1029/2025JE009368