Jupiter’s moons could keep each other warm by raising tidal waves

It takes a certain amount of heat to keep an ocean wet. For Jupiter’s largest moons, a new analysis suggests a surprising source for some of that heat: each other.

Three of the gas giant’s four largest moons, Ganymede, Callisto and Europa, are thought to harbor oceans of liquid water beneath their icy shells (SN: 5/14/18). The fourth, the volcanic moon Io, may contain an inner magma ocean (SN: 8/6/14).

One of the primary explanations for how these small worlds stay warm enough to harbor liquid water or magma is gravitational kneading, or tidal forces, from their giant planetary host. Jupiter’s huge mass stretches and squishes the moons as they orbit, which creates friction and generates heat.

But no studies had seriously considered how much heat the moons could get from gravitationally squishing each other.

“Because [the moons are] so much smaller than Jupiter, you’d think basically the tides raised by Io on Europa are just so small that they’re not even worth thinking about,” says planetary scientist Hamish Hay of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Together with planetary scientists Antony Trinh and Isamu Matsuyama, both of the University of Arizona in Tucson, Hay calculated the size of the tides that Jupiter’s moons would raise on each other’s oceans. The team reported the results July 19 in Geophysical Research Letters.

The researchers found that the significance of the tides depends on how thick the ocean is. But with the right-sized ocean, neighboring moons could push and pull tidal waves on each other at the right frequency to build resonance. It’s a similar effect to pumping your legs on a swing, or synchronized footfalls making a bridge wobble, Hay says.

“When you get into one of these resonances, those tidal waves start to get bigger,” he says. Those waves would then rush around the moon’s interior and generate heat through friction, the researchers calculated. If the conditions are right, heat from the gushing tidal waves could exceed heat from Jupiter.

The effect was biggest between Io and Europa, the team found.

“Basically everyone neglected these moon-moon effects,” says planetary scientist Cynthia Phillips of NASA’s Jet Propulsion Laboratory, who was not involved in the new work. “I was just astonished … at the amount of heating” that the moons may give each other, she says.

The extra infusion of energy into Europa’s ocean could be good news for the possibility of alien life. Europa’s subsurface ocean is thought to be one of the best places in the solar system to look for extraterrestrial life (SN: 4/8/20). But anything living needs fuel, and the sun is too far away to be useful, Phillips says.

“You have to find other sources of energy,” she says. “Any kind of frictional or heating energy is really exciting for life.”