Jupiter's ocean moons raise tides on each other

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The moon-moon tides likely generate lots of heat inside these frigid worlds.

The Jupiter moon Europa, seen here by NASA's Galileo spacecraft, harbors a huge ocean beneath its icy shell. Europa's fellow Galilean moons raise powerful tides in that subsurface sea, new research suggests.
The Jupiter moon Europa, seen here by NASA's Galileo spacecraft, harbors a huge ocean beneath its icy shell. Europa's fellow Galilean moons raise powerful tides in that subsurface sea, new research suggests. (Image credit: NASA/JPL-Caltech/SETI Institute)

Jupiter's "ocean world" moons may have strong gravitational effects on each other, raising big tides in each others' subsurface seas, a new study suggests.

Surprisingly, these moon-moon tidal forces might generate more heat in the satellites' oceans than the gravitational tugs of giant Jupiter, study team members found.

"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," lead author Hamish Hay, who performed the work while at the University of Arizona's Lunar and Planetary Laboratory, said in a statement.

Related: The Galilean moons of Jupiter (photos)

Hay and his colleagues modeled the gravitational interactions among Jupiter's four large Galilean moons — Io, Europa, Callisto and Ganymede. The latter three are thought to harbor huge oceans of liquid water beneath their icy shells, whereas powerfully volcanic Io might have a subsurface sea of molten rock.

The researchers determined that the Galilean moons have an outsized influence on each other thanks to "tidal resonance" — basically, a reinforcing sync-up of a gravitational tug and the natural rocking of the satellites' oceans. The moons are more tidally resonant with each other than with Jupiter, which explains why the giant planet's powerful pull doesn't translate into bigger tidal effects.

As an example: Hay and his team calculated that Jupiter's tug could generate a tidal wave in Europa's buried ocean if that sea were about 660 feet (200 meters) deep. Little Io, by contrast, could get a strong wave going in a Europan ocean 50 miles (80 kilometers) deep.

Astronomers suspect that Europa's sea, one of the most promising abodes for alien life in the solar system, is more than 50 miles deep, but nobody knows the actual figure. Tidal resonance among the Galilean moons could help them nail that measurement down, however. If moon-moon tides are strong enough, the icy surfaces of Europa, Callisto and Ganymede could pulse in and out, study team members said. 

"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, who's now at NASA's Jet Propulsion Laboratory in Southern California, said in the statement.

The new study was published last month in the journal Geophysical Research Letters.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook. 

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Mike Wall
Mike Wall
Senior Space Writer

Michael Wall is a Senior Space Writer with Space.com and joined the team in 2010. He primarily covers exoplanets, spaceflight and military space, but has been known to dabble in the space art beat. His book about the search for alien life, "Out There," was published on Nov. 13, 2018. Before becoming a science writer, Michael worked as a herpetologist and wildlife biologist. He has a Ph.D. in evolutionary biology from the University of Sydney, Australia, a bachelor's degree from the University of Arizona, and a graduate certificate in science writing from the University of California, Santa Cruz. To find out what his latest project is, you can follow Michael on Twitter.

23 Comments Comment from the forums
  • rod
    My observation. 116 years of observations since 1891 show Io has moved towards Jupiter by 55 km while Europa moved outward by 125 km and Ganymede moved outward by 365 km from Jupiter. The Laplace resonance in their orbits today is breaking. Io mean distance from Jupiter is 350,200 km. At the present rate of movement toward Jupiter since 1891, it would fall into Jupiter in < 1E+6 years. As the S&T report on chaos in the solar system indicated, it came as a surprise to astronomers that the Laplace resonance of the Jovian moons is slowing breaking.

    ref - Hanging in the Balance, Sky & Telescope 119(4):30-31, 2010, April 2010 issue.
    Reply
  • Helio
    If Jupiter rotates 4x or so faster than Io orbits, the tidal action should move Io outward, right? It would be the dominant force in an in or out radial movement. So what am I missing?
    Reply
  • rod
    Helio, you said in post #3, "So what am I missing?"

    My information cited a specific source from the April 2010 issue of Sky & Telescope. Other sources know this too about the Galilean moons.

    "Tidal dissipation within Io and Jupiter leads to a migration of the Galilean satellites. In fact, the resonant interaction between Io, Europa and Ganymede spreads the dissipative effects from Io to the orbits of the other moons. The amount of the loss of energy and the consequent rate of the orbital evolution was determined in (Lainey et al., 2009), along with the dissipative parameters k2 / Q of Io and Jupiter...", Chaotic orbit determination in the context of the JUICE mission, https://ui.adsabs.harvard.edu/abs/2019P%26SS..17604679L/abstract, October 2019
    Reply
  • rod
    Helio, I had to fact-check this statement here :) "If Jupiter rotates 4x or so faster than Io orbits" MS BING shows Jupiter rotates about 12.6 km/s, Io orbits Jupiter at about 17 km/s.
    Reply
  • Helio
    rod said:
    Helio, I had to fact-check this statement here :) "If Jupiter rotates 4x or so faster than Io orbits" MS BING shows Jupiter rotates about 12.6 km/s, Io orbits Jupiter at about 17 km/s.
    But it is the difference in Jupiter's rotation period compared to Io's orbital period. Jupiter rotates in just under 10 hours, and Io takes 42.5 hours to make one orbit. So if we were on Io, we would see Jupiter spin around 4 x before we got back to where we started.

    Thus, the tidal action from a faster spinning host will cause the moon to migrate outward, as does our own Moon.

    Here's a paper about Io's outward motion.
    Reply
  • rod
    Helio, if you have the sources to show your are correct, then Sky & Telescope should publish a correction to Io migrating and the other Galilean moons changing at Jupiter too. I can only point to the sources I referred to and the 116 years of documented observations of Io and its distance change at Jupiter.
    Reply
  • Helio
    During the formation period of the moons, they migrated inward as there was balance between the outward from of gas and dust from Jupiter's equatorial region ( but flow inward at the polar regions) and gravity. Io would have formed first and produced gravity waves, of some kind, that contributed to inward migration of the sister moons to the point of their mutual resonance. The Sun removed the gas and dust too soon for Callisto to migrate inward enough, apparently.
    Reply
  • Helio
    rod said:
    Helio, if you have the sources to show your are correct, then Sky & Telescope should publish a correction to Io migrating and the other Galilean moons changing at Jupiter too. I can only point to the sources I referred to and the 116 years of documented observations of Io and its distance change at Jupiter.
    I hit the wrong button when I was trying to add that link. But the paper is there now.

    Also, your link doesn't work for me.
    Reply
  • rod
    Helio, your post #8 has nothing to do with the observed changes of the Galilean moons, now tracked and documented for the past 116 years at Jupiter, starting in 1891 as Sky & Telescope reported.
    Reply
  • rod
    116 years of observations since 1891 show Io has moved towards Jupiter by 55 km while Europa moved outward by 125 km and Ganymede moved outward by 365 km from Jupiter. The Laplace resonance in their orbits today is breaking.

    This is the issue, not what the Galilean moons were extrapolated to be doing 4.5 billion years ago, but what they are observed doing today in the solar system. We see apparent young age now at the moons, just like an apparent young age for the ring system of Saturn, etc. I can go on and on here :)
    Reply