![]() That arrangement gives them an advantage for tidal changes. Whereas Earth’s oceans have an average depth of about 3.7 kilometers-and plunge to 11 kilometers at their deepest point in the Mariana Trench-the Galilean satellites’ oceans are thought to reach hundreds of kilometers. The oceans of Jupiter’s moons differ from those found on our planet. Ocean worlds seem to be widespread, and many scientists suspect Ganymede and Callisto house their own deep, watery inner layers. In recent years such oceans have been found on many other worlds-even tiny, cold Pluto may boast one. Further observations from Voyager 1 and other missions showed telltale fissures on the surface of Europa, revealing the icy moon’s crust was more active and mobile than previously realized, likely because of a subsurface ocean. When NASA’s Voyager 1 spacecraft arrived at Jupiter in 1979, it spotted volcanic plumes rising from Io that hinted at a buried magma ocean. Beuthe reviewed Hay’s study but was not directly involved in the work. “It's quite interesting because the effects can be strong,” says Mikael Beuthe, who studies subsurface oceans at the Royal Observatory of Belgium. Under those special conditions, known as resonances, the tidal effects could grow significantly larger, even exceeding those produced by the much more massive Jupiter. To his surprise, he found that when the orbits of the natural satellites lined up just right, their relatively small pull could be amplified by the behavior of their oceans. Hay, now a researcher at NASA’s Jet Propulsion Laboratory, decided to look at the tides of the ocean-rich Galilean moons. When Hamish Hay, then a graduate student at the University of Arizona, found himself working on a side project involving the tidal pull of exoplanets, he began to wonder more about the way an ocean layer could change the equation. Because of their small size, the Galilean satellites’ tug on one another has long been considered irrelevant as well. Jupiter, for instance, pulls on Earth but is so far away that the effect is negligible. All objects in the universe exert a gravitational tug in proportion to their mass, and it attenuates across distances. Less attention has been paid to how the moons pull at one another. As the moons move around the planet, its gravitational tugging on their interior and the heat from the slow decay of radioactive elements have kept the liquid material from solidifying. Three of the moons are thought to have a water-rich mantle, while Io may instead have a magma ocean driving its extreme volcanic activity. Observations of Jupiter’s Galilean satellites-Europa, Callisto, Ganymede and Io-have long hinted at the presence of subsurface liquid layers. Measuring such tides can provide insights about the depths of these lunar abysses-environments that may offer the best chances for finding extraterrestrial life in our solar system. Long thought to arise from heat generated by the crust-flexing pull of Jupiter, these oceans may also owe their existence to immense subsurface tidal waves generated by gravitational interactions among the moons. Jupiter’s four largest moons may be conspiring to maintain their subsurface oceans.
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