Gravitational kneading from mammoth planets isn’t the only source of heat for satellites

It takes a certain amount of heat to keep the ocean moist. For Jupiter’s largest moon, a new study suggests a surprising source of some of that heat: each.

Three of gas giant Ganymede’s four largest moons, Callisto and Europe, are believed to have an ocean port with liquid water under their icy shells (SN: 5/14/18). The fourth, the volcanic moon Io, may contain an interior ocean of magma (SN: 6/8/14).
One of the main explanations for how these tiny worlds stay hot to handle liquid water or magma is gravitational kneading or the tidal forces of their large planet. Jupiter’s vast mass stretches and crushes the moon as it orbits, creating friction and generating heat.

But no study seriously thinks about how much heat the moon can get by gravitating towards each other.
“Because [the moon] is smaller than Jupiter, one would think that the sea level rise caused by Io in Europe is really small even if it is not worth thinking about,” the scientist said. Hamish Hay at the NASA Propulsion Laboratory in Pasadena, California.

Working with planetary scientists Antony Trinh and Isamu Matsuyama, both from the University of Arizona at Tucson, Hay calculates the tidal magnitudes that will magnify Jupiter’s moons in each ocean. The team reported the results to Geophysical Research Letters on July 19.

The researchers found that the importance of the tides depends on the size of the ocean. But in the right-sized ocean, the surrounding moons can push and pull the water waves at the right frequency to create resonance. It’s a similar effect to pumping your legs up on a swing or synchronized steps to make a bridge loosen, says Hay.

“When you step into one of these resonances, those waves get stronger,” he says. These waves then rush around the moon and generate heat through friction, the researchers calculated. When the conditions are right, the heat of precipitated water can exceed the heat of Jupiter.

The team found that the impact was greatest between Io and Europe.

“Usually everyone ignores the effects of this monthly,” says planetologist Cynthia Phillips of NASA’s Jet Propulsion Laboratory, which is not among the new work. “I was just amazed … at how much heat” the moon could give itself, he said.

More energy in Europe’s oceans could be good news for the possibility of extraterrestrial life. The European underground ocean is considered one of the best places in the solar system to search for extraterrestrial life (SN: 4/8/20). But all living things need fuel, and the sun is too far away to be of use, says Phillips.

“You have to look for other sources of energy,” he says. “Any kind of friction or heating energy is truly exciting for life.”


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