How gravitational tides help explain the puzzle of Jupiter's hot moons

A new study from the University of Arizona indicates that Jupiter's four largest moons are as warm as they are thanks to tidal forces caused by the moons' gravitational fields tugging on one another. This tidal heating may help to explain how the Jovian moon system evolved.

Ever since NASA's Pioneer 10 flew by Jupiter in 1973, the giant planet has provided a growing list of surprises. One of the more perplexing is that the Galilean Moons Io, Europa, Ganymede, and Callisto, the largest four of Jupiter's 80 moons, are not the frozen balls of rock and ice that one would expect 484 million mi (778 million km) from the Sun. Instead, three of the moons are warm enough to have subsurface global oceans and the fourth's interior is so hot it is riddled with active volcanoes.

The obvious explanation for this is the tidal forces set up by the pull of Jupiter that, as they stretch and press on the moons, heats them up enough to have liquid interiors that don't cool off over geological time. However, according to the Arizona team led by Hamish Hay, a postdoctoral fellow at the Jet Propulsion Laboratory in Pasadena, California, these Jovian tides aren't strong enough to account for so much heating.

This is because the Galilean Moons are too small to produce the sort of large tidal responses needed for such significant heating, but adding in the gravitational pull of the other moons in the Jovian system help to balance the thermal books. This is due to a phenomenon known as tidal resonance.

"Resonance creates loads more heating," says Hay. "Basically, if you push any object or system and let go, it will wobble at its own natural frequency. If you keep on pushing the system at the right frequency, those oscillations get bigger and bigger, just like when you're pushing a swing. If you push the swing at the right time, it goes higher, but get the timing wrong and the swing's motion is dampened.

"These tidal resonances were known before this work, but only known for tides due to Jupiter, which can only create this resonance effect if the ocean is really thin (less than 300 meters or under 1,000 feet), which is unlikely. When tidal forces act on a global ocean, it creates a tidal wave on the surface that ends up propagating around the equator with a certain frequency, or period."

By running computer models, the Arizona team showed that Jupiter alone isn't enough to produce the right resonance frequency in the moons, but when the other moons are inserted into the equations, the tidal forces start to match each moon's resonant frequency. This produces more heat and the interior water or rock melts if the subsurface oceans are within the right range of thickness, which is much closer to the current estimates.

According to Hay, the current model assumes that the tidal resonances remain relatively moderate, so the next step will be to remove that limit. In addition to providing a mechanism to explain the heating of the Galilean Moons, the model may also help to calculate the true depth of their oceans.

The research was published in Geophysical Research Letters.

Source: University of Arizona