Ever since Voyager 1 spacecraft flew past Jupiter in March, 1979 (where a day lasts 10 hours), NASA scientists have been captivated by the mysteries that have been unveiled. A new study argues that the four largest moons of the largest planet in the solar system –three of them harboring oceans believed to 100 kilometers deep or more–may have a bigger influence on each other’s tides than the gas giant itself does. The findings suggest that oceans on these moons could then generate more heat from friction and could be more suitable to hosting life than previously thought.
Rethinking the Galilean Moons
Researchers might have to reconsider their understanding of how these ocean moons evolved and where most of their heat is coming from according to a new study in AGU’s journal Geophysical Research Letters suggests that these Galilean moons, might play a bigger role in the ebb and flow of each other’s tides than the big planet they orbit does because of a phenomenon called tidal resonance. This newly discovered relationship between the moons means researchers might have to reconsider their understanding of how these ocean moons evolved and where most of their heat is coming from.
The three icy Galilean moons, Europa, Ganymede, and Callisto, are thought to contain liquid water oceans beneath their surface, while the innermost moon Io ––the volcanic epicenter of our solar system—may contain an internal ocean of magma first detected by rising volcanic plumes by the Voyager 1 spacecraft in 1979. Jupiter’s gravity stretches and squeezes these moons as they orbit the gas giant, heating their interiors through friction, just as Earth’s oceans are tugged at by our Moon.
A Miniature Earth May Lie Beneath
Beneath the six-mile thick ice shell of Europa, for example, exists a miniature Earth, with plate tectonics, continents, deep trenches, and active spreading centers some 15 miles below its surface, according to Caltech’s Mike Brown, based on earlier unrelated studies.
“Think about mid-ocean ridges on Earth,” Brown says, “with their black smokers belching scalding nutrient-rich waters into a sea floor teaming with life that is surviving on these chemicals. It doesn’t take much of an imagination to picture the same sort of rich chemical soup in Europa’s ocean leading to the evolution of some sort of life, living off of the internal energy generated inside of Europa’s core. The long, linear ridges and bands that crisscross Europa’s surface are thought to be related to the response of Europa’s icy surface crust as it is stretched and pulled by Jupiter’s strong gravity and the immense tidal forces below.
Although researchers have long considered the gravitational effects that Jupiter has on its ocean moons, they have until now neglected the potential tides raised by the moons on each other.
(NASA’s Juno spacecraft revealed that rather than casting one “shadow” in Jupiter’s aurorae (above), the moon Io – Jupiter’s fifth – casts several, in a double wing-shaped pattern, while Jupiter’s largest moon, Ganymede, casts a double shadow)
Tidal Resonance –Larger Tides than Jupiter
The new paper, by researchers with the Lunar and Planetary Laboratory in Tucson, Ariz., argues for the first time that Galilean moons’ gravitational pulls on each other, though smaller, could be producing larger tides than Jupiter does. That’s because they are more tidally resonant with each other.
Resonance is more about timing than size. If you’re pushing someone on a swing, for instance, timing your shove with the natural forward momentum of the swing will ensure their next swing is higher. Jupiter does the gravitational equivalent of giving the swing a big push when it’s coming at you, whereas the moons give each other the equivalent of little pushes on the upswing.
According to Hamish Hay, lead author of the new study and now a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., for Jupiter to produce dramatic tides on its moons, the planet would have to be tidally resonant with the oceans of those moons.
Warm, Dynamic Oceans –Habitats for Alien Octopus?
In the new study, Hay and his colleagues calculated how tidal forces of Jupiter and other moons would affect the flow of oceans of different depths. For Europa, for instance, they assumed an ice shell 10 kilometers (6 miles) thick.
They found that Jupiter’s pull could raise a tidal wave if Europa’s ocean were just 200 meters (660 feet) thick, which researchers think is unlikely. Io’s weaker gravitational influence, on the other hand, could induce a sizable westward tidal wave in oceans up to 80 kilometers (50 miles) thick. The researchers saw this trend for all the Galilean satellites, with moon-moon tides capable of producing many times more heat than Jupiter’s tidal forces could possibly generate.
A warmer, more dynamic ocean is one that could potentially be more suitable to life.“That’s kind of interesting, because Jupiter is the biggest mass in that system, so its tidal forces are much bigger than one moon on another,” Hay said.
Without tidal resonance, most of the heat within these oceans would be generated from radioactive decay of elements and Jupiter-raised tides in the rocky portions of the moons. But if dramatic tides due to the other moons are also in play, the ocean could be generating its own heat from friction with the icy shell it’s trapped under. A warmer, more dynamic ocean is one that could potentially be more suitable to life.
Tidal resonance could also help scientists pinpoint exactly how thick the Galilean moons’ oceans are. If the tides under the ice are strong enough, the whole moon’s surface would pulse in and out, as though the moon were breathing.
“If you can measure the rate at which the moon’s surface is moving up and down, then that would be a way to tell you how thick the ocean might be,” Hay said.
However, much of this research is still theoretical, he cautioned. The models used in the paper take only horizontal motion into account, just as they would on Earth. If these oceans are actually over 100 kilometers (60 miles) deep, as most researchers think they are, there would also be a significant amount of vertical motion to account for.
In addition, it isn’t possible to calculate the exact resonant frequency without knowing precisely how thick the moons’ oceans are. Although the moons could have been tidally resonant with each other in the past, they might not be today.
In the future, Hay said, researchers could build on this work by modeling the way the oceans and their icy shells evolved together on these moons in light of the potential for tidal resonance. This could completely change scientists’ views of the history of Galilean moons, Hay said.
Image at top of page: NASA shows triple eclipses that occur on Jupiter. Galileo spotted the planet’s four largest moons—Europa, Ganymede, Io, and Callisto—four centuries ago.