Magnetic field of Ganymede

As part of this months Naked Astronomy podcast I answered a question on “How does Ganymede’s magnetic field arise?”. This is a rather interesting question and I thought this would be an interesting blog post too. If you want to listen to my answer on the podcast go here.

Lets start of with a bit of background; Ganymede is the 7th moon of Jupiter. It is also the largest and most massive moon in the Solar System. It has a mass that is about 2 times that of the Earth’s Moon. Ganymede is mostly made out of silicate rock and ice water, though it has an Iron-rich liquid core.


This natural color view of Ganymede was taken from the Galileo spacecraft during its first encounter with the Jovian moon. North is to the top of the picture and the sun illuminates the surface from the right. The dark areas are the older, more heavily cratered regions and the light areas are younger, tectonically deformed regions.

Ganymede orbits around Jupiter, which  has a strong magnetic filed and it was only due to observations by the Galileo spacecraft (between 1995-2000) that we were able to determine that Ganymede has it own magnetic field. This magnetic filed is 3 times stronger than Mercury. This creates a tiny magnetosphere inside Jupiters.

There is even some evidence for aurora being observed around Ganymede’s poles.

The magnetic field is likely generated in a similar way to the Earth’s. To generate this magnetic field there must be: a conducting fluid, enough energy to cause the fluid to move, and a “seed” magnetic field. In the case of the Earth there is, a molten iron core which is a great conductor, convective flow from heat with the Earth’s rotation moving it in the right manner, and an existing magnetic field from the Sun.

As molten Iron steams through the magnetic field an electric current is formed (this is called magnetic induction). Due to the presence of an electric field a magnetic field is created and continues assuming there is a flow of fluid.

So Ganymede needs to have a conducting material moving in the interior!


HST/STIS Ultraviolet Imaging of Polar Aurora on Ganymede (Feldman et al. 2000)


How this magnetic field is generated is still a bit of a mystery. There is a thought that the Iron core will have cooled and fluid motions and hence the magnetic field will have not been sustained. There are other suggestions that the tidal forces from Jupiter disrupted the cooling. The tidal energy being dissipated into the ice shell / silicate mantle, thus insulating the core.

Much more will be learnt about the composition and magnetic filed of Ganymede with JUICE (Jupiter Icy Moons Explorer) which has a planned launch data of 2022 and arrival at Ganymede for 2033.

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