1610

Ganymede

Galileo Galilei (1564–1642), Pierre-Simon Laplace (1749–1827)

Galileo’s January 8, 1610, drawing of Jupiter’s four bright satellites, with Ganymede’s position on that night highlighted by the red arrow.

The third of Galileo’s newly discovered moons of 1610 is Ganymede, named after the mythological prince, cupbearer of the gods, and lover of Zeus (Ganymede is the only moon of Jupiter named after a male figure). Ganymede takes a little more than seven days to make one orbit of Jupiter. When Galileo and other astronomers worked out the precise orbits of Io, Europa, and Ganymede, they noticed something very interesting: for every single orbit of Ganymede, Europa orbits exactly twice, and Io orbits exactly four times. The satellites are described by astronomers as being in resonances.

Discovery of the 4:2:1 orbital resonances of the three inner Galilean satellites spurred a mini revolution of sorts among mathematicians and physicists trying to understand and explain how those resonances came to be. Key explanations were worked out by the French mathematician and astronomer Pierre-Simon Laplace, and in honor of his achievements these kinds of three-body situations are now called Laplace resonances. Orbital resonances have been found to generate gaps in the main asteroid belt and in Saturn’s rings; even some newly discovered extrasolar planets are in orbital resonances, too.

Ganymede has also now been closely studied by many robotic spacecraft, which have revealed it to be the largest moon in the solar system, at 3,275 miles (5,270 kilometers) in diameter—even larger than the planet Mercury. Its density of 1.9 grams per cubic centimeter implies that it is made of a significantly higher relative percentage of ice than Io and Europa. Brighter, more ice-rich grooves and ridges on Ganymede’s surface—potentially the result of past tectonic activity—appear younger than the darker, more heavily cratered terrains. Ganymede is the only moon in the solar system with its own (weak) magnetic field, indicating that the interior has differentiated into a crust, mantle, and molten iron core. Perhaps most interestingly, the magnetic readings, presence of salty surface minerals, and evidence for past eruption of water into some grooves and ridges could all be consistent with a deep underground layer of liquid water—yet another ocean?—inside Ganymede.

The NASA Voyager 2 spacecraft’s 1996 mosaic of Ganymede, the largest moon in the solar system. The brighter zones are areas of extensive tectonic deformation.