Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength

Jun Kimura; Ryohei Kawakami; Taro Matsuo; Sho Sasaki

AI summary · arXiv: 2611.05727v1 · astro-ph.EP

Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength

Jun Kimura, Ryohei Kawakami, Taro Matsuo, Sho Sasaki

Original paper Submitted to arXiv on 03 Nov. 2026 37 pages, 11 figures, Accepted for publication in the Icarus License: CC BY 4.0 Also in: fr

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The essentials

Thermochemical models show that Ganymede’s present magnetic field favors large, non-eutectic-sulfur cores, with viable chondritic compositions depending on hydrosphere density.

Question

Can Ganymede’s observed magnetic field strength reduce the degeneracy in interior structures and bulk compositions allowed by gravity data? Which chondritic compositions and core solidification pathways can sustain a field comparable to the observed value?

Method

The authors simulated 4.6 Gyr of coupled thermal, chemical, and magnetic evolution for three-layer interiors constrained by Ganymede’s mass and moment of inertia, varying mantle and core densities, sulfur content, eight chondrite types, and hydrosphere densities of 1,200 and 1,300 kg m-3. A one-dimensional mixing-length mantle model was coupled to core models of entirely molten, Fe-snow, and FeS-layer evolution; dynamo power was converted to surface-field strength and compared with the observed approximately 719 nT.

Findings

Result Models compatible with the observed field preferentially have core radii of about 750 km or greater and sulfur contents away from the eutectic composition.
See the passage in the paper
The models capable of sustaining the observed field strength preferentially require a relatively large core radius (approximately 750 km or larger) and a bulk core sulfur concentration distinct from the eutectic composition.
Result For a hydrosphere density of 1,200 kg m-3, L and CI chondrite models produce field strengths within the adopted compatibility range.
See the passage in the paper
Specifically, L and CI chondrites yield parameter spaces capable of sustaining comparable field strengths under a low-density hydrosphere scenario (1,200 kg m-3)
Result For a hydrosphere density of 1,300 kg m-3, an H chondrite model can meet the field-strength criterion, with a representative case reaching 1,095 nT.
Authors’ interpretation The authors argue that determining hydrosphere density can help distinguish among possible bulk compositions despite the remaining non-uniqueness of the models.
See the passage in the paper
Therefore, characterizing the hydrosphere density provides the critical independent constraint required to break the degeneracy of Ganymede’s primordial origin.

Limitations

The one-dimensional mixing-length model omits lateral variations in core-mantle heat flux and may misestimate the absolute efficiency of mantle heat transport.
See the passage in the paper
this 1D formulation inherently neglects three-dimensional lateral heterogeneities in the core-mantle boundary heat flux.
The model simplifies the hydrosphere by prescribing a fixed boundary temperature and representative bulk densities rather than resolving its thermodynamic density structure.
See the passage in the paper
In this study, we do not explicitly compute the heat transfer within the H2O hydrosphere.
Predictions for dynamo timing and viability depend on uncertain core thermodynamic parameters, including the Fe-FeS liquidus representation and core thermal conductivity.
See the passage in the paper
These thermal conductivity uncertainties consequently introduce a quantitative uncertainty window of several hundred million years in the predicted dynamo lifetime and the remaining buoyancy flux at 4.6 Gyr.

Why it matters

The authors present magnetic-field strength as an additional constraint on Ganymede’s otherwise degenerate interior models and identify hydrosphere density as an independent way to narrow the possible chondritic origins. They propose that JUICE gravity, tidal, and magnetic measurements can test these predictions and help distinguish core regimes.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Ganymede magnetic field interior structure chondritic composition core dynamo

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Comprehensive Summary
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Original abstract

The paper asks whether Ganymede’s present-day magnetic field can help constrain its uncertain interior structure and bulk composition. Gravity measurements show that Ganymede is strongly differentiated, but its moment of inertia does not uniquely determine the sizes and densities of its metallic core, rocky mantle, and water-rich outer layer. The authors therefore investigate which interior models, associated with different chondritic precursor compositions, can generate a magnetic field comparable to the observed surface strength of about 719 nanoteslas. The authors model Ganymede’s thermal and chemical evolution over 4.6 billion years. First, they identify structures consistent with its mass and normalized moment-of-inertia factor of 0.3156. They consider mantle densities of 3,000–4,000 kilograms per cubic metre, core densities corresponding to sulfur concentrations of 0–36 percent, and two representative hydrosphere densities: 1,200 and 1,300 kilograms per cubic metre. They then associate candidate structures and sulfur budgets with eight chondrite classes. A one-dimensional mixing-length model estimates heat transfer through the rocky mantle. The metallic core begins fully molten and may either remain molten or solidify through iron-snow formation or the development of a solid FeS layer. The authors estimate dynamo power from thermal and compositional sources, then use a scaling law to calculate the surface magnetic field. They do not model heat transfer through the hydrosphere explicitly; instead, they fix the mantle–hydrosphere boundary temperature at 300 kelvins. The simulations show that all three core regimes can generate dynamos, but matching the observed field narrows the range of viable models. Successful cases generally have relatively large cores—about 750 kilometres or more—and sulfur concentrations that differ from the Fe–FeS eutectic composition. Larger cores imply thinner rocky mantles, which contain less radiogenic heat and may allow the core to cool more effectively. Sulfur concentration alone does not determine field strength; core size, mantle cooling, and the timing and type of solidification also matter. Hydrosphere density affects which chondritic compositions are compatible with the observations. At a density of 1,200 kilograms per cubic metre, L-chondrite models produce present-day fields of 534–574 nanoteslas, while a CI-chondrite model reaches 573 nanoteslas. These values fall within the authors’ adopted compatibility interval of 503–1,222 nanoteslas. At a density of 1,300 kilograms per cubic metre, an H-chondrite model with an 830-kilometre core and 12.9 percent sulfur produces a field of 1,095 nanoteslas. Another H-chondrite model, with 9.4 percent sulfur, enters the iron-snow regime too early, causing its dynamo to cease before the present. A representative CM model forms an FeS layer but produces a field of only 406 nanoteslas. Some models with entirely molten cores also generate magnetic fields, but none reaches 719 nanoteslas when the adopted dipole attenuation factor is 10. Their compatibility therefore depends on uncertainty in that factor. The authors conclude that magnetic-field strength, combined with geodetic constraints, can reduce—but not eliminate—uncertainty about Ganymede’s composition. Measurements of the hydrosphere’s density and structure could help distinguish among the preferred scenarios, making future JUICE gravity, tidal, and magnetic measurements important tests. These predictions remain conditional on assumptions that include a fully differentiated hot start, a simplified water layer, one-dimensional mantle convection, uncertain Fe–FeS phase relations and thermal properties, and uncertain dynamo-scaling parameters.

Can Ganymede’s Magnetic Field Reveal What Lies Beneath?

Jupiter’s moon Ganymede is the only moon known to generate its own magnetic field. That field offers more than a striking planetary feature: it may also provide clues about the moon’s hidden interior. In “Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength,” Jun Kimura, Ryohei Kawakami, Taro Matsuo, and Sho Sasaki investigate whether the field can help narrow down what Ganymede is made of.

Ganymede is known to be strongly differentiated, with a metallic core, rocky mantle, and water-rich outer layer. But gravity measurements, including its moment of inertia, do not uniquely reveal the sizes and densities of those layers. The researchers therefore ask a complementary question: which plausible interior structures can produce a magnetic field close to the observed surface strength of about 719 nanoteslas?

From Interior Models to Magnetic Fields

The team modeled Ganymede’s thermal and chemical evolution across 4.6 billion years. They first identified interior structures consistent with the moon’s mass and normalized moment-of-inertia factor of 0.3156. The models varied the rocky mantle’s density, the core’s sulfur content, and the density of the water-rich outer layer, or hydrosphere.

They then connected possible structures and sulfur budgets to eight types of chondritic meteorites, which serve as indicators of potential starting materials. A one-dimensional mixing-length model estimated how heat moves through the rocky mantle. Meanwhile, the metallic core began fully molten and could follow different paths: staying liquid, partially solidifying through iron-snow formation, or developing a solid layer of iron sulfide, known as FeS.

By estimating the energy available from cooling and chemical changes in the core, the researchers calculated whether a dynamo could operate and used a scaling law to estimate the resulting surface field. The model simplifies some aspects of the moon: it does not explicitly simulate heat transfer through the hydrosphere and instead sets the temperature at the mantle–hydrosphere boundary to 300 kelvins.

Core Size and Cooling Matter

All three modeled core regimes can produce dynamos. However, matching Ganymede’s present-day field narrows the range of viable models. Successful cases generally have relatively large cores, around 750 kilometres in radius or more, and sulfur concentrations different from the iron–iron sulfide eutectic composition.

A larger core also means a thinner rocky mantle. In these models, a thinner mantle contains less heat-producing radioactive material and may let the core cool more effectively. That cooling can help sustain the energy needed for a dynamo. The results therefore caution against treating sulfur content as a stand-alone predictor: core dimensions, mantle cooling, and the timing and type of core solidification all influence magnetic-field strength.

Hydrosphere Density Changes the Picture

The density assumed for Ganymede’s hydrosphere affects which candidate compositions fit both the interior constraints and the magnetic observations. With a hydrosphere density of 1,200 kilograms per cubic metre, modeled L-chondrite cases produce fields of 534–574 nanoteslas, while a CI-chondrite case reaches 573 nanoteslas. These values fall within the study’s adopted compatibility interval of 503–1,222 nanoteslas.

At a hydrosphere density of 1,300 kilograms per cubic metre, one H-chondrite model—with an 830-kilometre core and 12.9 percent sulfur—produces a field of 1,095 nanoteslas, also within that interval. But a different H-chondrite model, with 9.4 percent sulfur, enters the iron-snow regime too early. Its dynamo shuts down before the present day.

A representative CM-chondrite model forms an FeS layer but generates a weaker field of 406 nanoteslas. Models in which the core remains entirely molten can also produce magnetic fields. Yet, under the assumed dipole attenuation factor of 10, none reaches the observed value of 719 nanoteslas. Whether those models remain compatible depends on uncertainty in that factor.

What the Results Can—and Cannot—Tell Us

The study’s central message is that magnetic-field strength can add a valuable constraint to gravity-based estimates of Ganymede’s interior, but it cannot uniquely identify the moon’s composition. Several combinations of core size, sulfur content, mantle properties, and hydrosphere density can remain plausible. Better knowledge of the water-rich layer’s density and structure could help distinguish among them.

The conclusions are also conditional on important modeling choices. These include a fully differentiated, initially hot Ganymede; a simplified treatment of the water layer; one-dimensional mantle convection; uncertain iron–iron sulfide phase relationships and thermal properties; and uncertainties in the scaling used to connect dynamo power to magnetic-field strength.

Future observations may test these predictions. The European Space Agency’s JUICE mission is expected to make gravity, tidal, and magnetic measurements that can improve our understanding of Ganymede’s interior. Combining those data with magnetic-field modeling could help narrow the options—and reveal how much of the moon’s hidden structure can truly be read from the field it generates today.

Created on 05 Nov. 2026

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Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field Strength

The essentials

Question

Method

Findings

Limitations

Why it matters

Read further

Definitions

Can Ganymede’s Magnetic Field Reveal What Lies Beneath?

From Interior Models to Magnetic Fields

Core Size and Cooling Matter

Hydrosphere Density Changes the Picture

What the Results Can—and Cannot—Tell Us

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