Free Astronomy Magazine May-June 2015

A rtist’s depic- tion of Gan- ymede in orbit around Jupiter, with highlighted the two auroral ovals discovered at mid-latitudes in the northern and southern hemispheres. Hubble's mea- surement of the fluctuations that affect those structures has led to the discovery of a huge under- ground ocean. [NASA, ESA, G. Bacon (STScI), J. Saur (University of Cologne, Ger- many)] ocean. But it was not possible to go fur- ther, because the measurements, perform- ed for short periods at intervals of 20 min- utes, were too brief to highlight with cer- tainty a peculiar interference in the be- haviour of Ganymede's magnetic field, ex- pected in case of the ocean’s existence. That interference is part of a broader con- text that has as its dominant element the Jovian magnetic field, which “solidly” ro- tates with the planet over a period of about 10 hours. For half that cycle the lines of force of the field point approximately towards Ganymede, while for the remain- ing half they point in the opposite direc- tion (with respect to the planet’s rotation axis). This behaviour affects the structure of the magnetic field of Ganymede (origi- nated by the rotation of its iron core), whose axis is forced to oscillate, making in turn oscillate in latitude the phenomena associated with it (in both hemispheres). The models predict that in Ganymede's case, those oscillations are of 6 degrees to- wards north and then of 6 degrees towards south, in a cycle of about 10 hours (which coincides with Jupiter's rotation). This is the expected scenario in the event of the large moon being entirely solid and thus without a global underground ocean. Conversely, in the presence of an enormous reservoir of liquid salt water, its mass, flu- idity and electrical conductivity, would al- ter the dynamics of Ganymede’s magnetic field by introducing an inhibiting effect in the influence of the Jovian magnetosphere and significantly reducing the magnitude of the oscillations. By succeeding in measuring a migration in latitude below the theoretical 6 degrees would have in fact validated the hypothesis of the global ocean, and the amplitude of the difference would have provided valu- able information on its main characteristics.