The Galilean Moons and the Nature of their Magnetic Fields

The Galileo spacecraft’s magnetometer data allowed for an understanding of the Galilean satellites’ internal magnetic fields. My objective was to learn the nature of these fields and see if they are consistent with geological and observational data by reviewing all relevant papers and seeking parallels or inconsistencies between them. The results show that Europa and Callisto have internal brine oceans responsible for their oscillating fields, Ganymede either generates its own field through a weak dynamo process, or possesses significant remnant magnetization from a strong dynamo in the past, and Io has no appreciable internal magnetic field. INTRODUCTION The Galilean Satellites, also known as Jupiter’s four largest moons, Io, Europa, Ganymede, and Callisto, were discovered by Galileo Galilei between January 7 and March 2, 1610. Galileo would later reveal to the Ptolemaic world that not all objects revolve around the Earth, and that these Galilean satellites instead orbit Jupiter, promoting the Copernican theory of a heliocentric universe. This was the first solid evidence proving that the Earth was not the center of the universe. The moons themselves were named by Simon Marius, who claimed to have independently discovered them along with Galileo, and would ironically be identified as lovers or captors of Zeus. Parameter Io Europa Ganymede Callisto Mass (kg) (×1023) 0.8932 0.480 1.482 1.076 Density (g/cm) 3.518-3.549 3.014 1.936 1.839 Mean Radius (km) 1818.1 ± 0.1 1560.7 ± 0.7 2634.1 ± 0.3 2408.4 ± 0.3 Jovian Ambient Field (nT) 1835 420 120 35 Orbital radius in Jupiter Radii (RJ) 5.9 9.4 15.0 26.3 Table . Physical characteristics of the Galilean Moons. 2 The timing of the orbits of these special moons would significantly help in the development of longitudinal measurements, and would eventually be used by Ole Christensen Rømer in experiments to calculate a rough estimate for the speed of light in the 1670’s. Io, Europa, and Ganymede, the innermost moons, orbit in a 4:2:1 Laplacian resonance, a remarkable phenomenon not yet fully understood. Even the orbits of Ganymede and Callisto display a 7:3 resonance, the least probable of all known resonant orbits. Early spacecraft would provide geological detail of the moons, but data on their magnetic fields was not a priority. Magnetometer data from Pioneer 10 and 11, as well as Voyagers 1 and 2, focused mainly on the Jovian magnetosphere, since close flybys of the Galilean moons were not possible. The imaging of the Galilean moons only sparked further curiosity as to their origins and details. The idea that Jupiter formed through a process of concurrent accretion of gasses and solids provided rough estimates for the relative composition of the moons. The formation of the Galilean satellites through a “gas-starved” accretion model, provide a reasonable explanation for the observed composition of ices, rock, and metals within the moons and the ratios between them. This formation method involves slow accretion of gas onto Jupiter, and long mass accretion times for the satellites, allowing for sufficiently low temperatures for ice and hydrated silicate stability within the moons. Yet, the data obtained from the quick flybys of Pioneer and Voyager missions had limitations in constructing an overall picture of the Jovian system, requiring a more focused mission. Shortly after the launches of Voyagers 1 and 2 in 1977, funding was approved for the Galileo spacecraft, which would carry the latest designs in remote sensing