Free Astronomy Magazine May-June 2015

the properties of magnetic fields and of the structures and mechanisms generating them. Since auroras are governed by the interaction between charged particles and the magnetic field, by observing the first with appropriate techniques it is possible to deduce the conformation of the latter and obtain information about the internal structure of the planetary body generating it. For producing auroras, the charged particles (mostly elec- trons and protons of solar or- igin) captured by the lines of force of the magnetic field must however impact the gas of an atmosphere, and, as it happens, Ganymede has a very tenuous atmosphere of oxy- gen, the most suitable chem- ical element for producing auroral light. In expecting to be able to bet- ter comprehend that phenom- enon in the ultraviolet (a radi- ation mostly absorbed by the atmosphere), Saur and some of his colleagues decided to set up their own observations In the absence of an in-situ probe, suitably designed and equipped to study in the long term the behaviour of Ganymede’s magnetic field, it would seem impossible to provide a definitive answer to this ques- tion. But since scientists are accustomed to find startling solutions, it is not surprising that Joachim Saur, professor for Geophysics at the University of Cologne in Germany, came up with this fine idea: studying the internal structure of Ganymede through the observation of its auroras. As known, magnetic fields are invisible and their lines of force, as well as their polarity, are basi- cally recognizable by the presence of trac- ers of various kinds, such as iron filings, charged particles, particular forms of radia- tion and so on. It is through the behaviour of tracers that it is possible to determine A bove, a very simplified re- presentation of the lines of force of Ganymede’s magnetic field, generated by the rotation of the fluid iron core. On the side, com- parison between the amount of water contained in Ganymede’s ocean and that in the Earth's oceans: the first exceeds the sum of the latter. Bot- tom left, diagram of Ganymede’s internal structure based on Saur and colleagues’ recent work, pub- lished in March in the Journal of Geophysical Re- search. The salt water ocean lay- ing beneath its icy crust, sits on top a layer of ice, a rocky mantle and the iron core. [NASA, ESA, and A. Feild (STScI)]