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Jugales OP t1_jb8dbme wrote

Reply to comment by ChickenChimneyChanga in TIL the largest moon in our solar system, Ganymede, is larger than the planet of Mercury by Jugales

You got it bosmang

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JoeTheFingerer t1_jb8gh4f wrote

is there a minimum volume or mass required for a celestial body to have a magnetic field?

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Hattix t1_jb8nsuc wrote

Yes, and it isn't constant over time.

A magnetic field comes from a conductive fluid core which generates electric currents via convection and the core's rotation, so the object needs to be large enough to have a dense, fluid core and maintain it over time.

In the very early solar system most larger objects (non-asteroidal) would have had fluid cores and so likely magnetic fields, but these cool over time for smaller objects and they lose the field. Ganymede's core is on the lower edge of what could retain a magnetic field until the present day.

Coupled with differences based on core composition, we therefore believe Ganymede's core to be slightly larger than Mars'.

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gryfter_13 t1_jb9lpez wrote

Don't tidal forces also play a role in keeping Ganymede's core liquid? Or is that Io's volcanic activity?

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MrLucky13 t1_jb9vexg wrote

I believe that's mostly Io. Isn't Ganymede the farthest of the big 4 from jupiter?

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Hattix t1_jbav6rq wrote

Callisto is the outer moon. Tidal effects on Ganymede are smaller than the other two of the Laplace resonance, it's both the outer (and so slower) and the least eccentric (0.0011).

Callisto isn't part of that resonance.

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Hattix t1_jbauvo9 wrote

Tidal effects from the other large moons (mostly Europa) would help, as would tides from Jupiter.

The Laplace resonance, however, dumps most of its excess energy into the inner moon, which orbits the fastest.

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