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How Ganymede’s magnetic field constrains its interior

A measured surface field filters interior models, but assumptions leave multiple core compositions possible.

By GreenSpacealien Editorial1 min read
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Infrared image of Ganymede against black space, with broad blue, brown and pale regions across its disk.
An infrared view of Ganymede captured by SPHERE on ESO’s Very Large Telescope on 5 September 2021. The interior models discussed here draw on magnetic-field and gravity constraints.ESO/King & Fletcher · The moon Ganymede seen by SPHERELicense terms · CC BY 4.0. Resized and converted from the original TIFF; no crop or overlay.View full image

The surface field is a model constraint

Ganymede’s measured equatorial surface field is about 719 nanotesla. The authors treat it as a constraint on models of the moon’s interior, asking which possibilities can generate a comparable field. It does not directly identify the core’s composition.[1]

Several interiors can fit

A dynamo sustains a magnetic field through motion in electrically conducting liquid. The models combine constraints on Ganymede’s mass and its distribution with a metallic core, rocky mantle and water-rich shell. Starting with a differentiated interior and molten core, they follow cooling and dynamo activity for 4.6 billion years. Different assumed shell densities allow different meteorite-like starting mixtures to fit.[2]

Why the answer remains uncertain

Those results depend on assumptions. The model gives the water-rich shell fixed densities rather than a full depth profile; high-pressure ice and ocean salinity could shift that profile. Its simplified one-dimensional mantle-convection treatment may move heat too efficiently, changing core cooling and dynamo timing. Iron-sulfur phase-diagram choices affect whether iron snow forms or the core freezes from below. An uncertain attenuation factor also changes how modeled dynamo power maps to the surface field.[2]

A future test

The authors point to JUICE as a future test of the model predictions. For now, the 719-nanotesla field narrows possible interiors, but it does not identify Ganymede’s exact core composition or origin.[1]

Sources & further reading

  1. arXiv record and version historyarXivAccessed November 5, 2026

    The record lists v1 submitted 3 Nov 2026 and an acceptance comment for Icarus; it does not show a journal publication date. The Nov 5 listing block is separate from the manuscript submission and from any observation date.

  2. Constraining Ganymede's Interior Structure and Bulk Composition from Its Present-Day Magnetic Field StrengtharXiv; original research by Jun Kimura, Ryohei Kawakami, Taro Matsuo and Sho SasakiAccessed November 5, 2026

    The exact v1 manuscript was inspected in the prior source assessment. Its 3 Nov 2026 submission and Icarus-acceptance comment do not establish a journal publication date or version-of-record comparison.

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