The gravitational push and pull JupiterThe moons of may explain greater warming than just the gas giant Jupiter.
The moons of Jupiter are hot.
Well, hotter than it should be, for being so far from the sun. In a process called tidal warming, the gravitational tugs of Jupiter’s moons and the planet itself stretch and squeeze the moons enough to warm them. As a result, some of the icy moons contain interiors warm enough to house oceans of liquid water, and in the case of the rocky moon Io, the warming of the tides melts the rock into magma.
Researchers previously believed that the gas giant Jupiter was responsible for most of the tidal warming associated with the moons’ liquid interiors, but a new study published in Geophysical Research Letters found that moon-moon interactions may be more responsible for warming than Jupiter alone.
“It’s surprising because the moons are so much smaller than Jupiter. You wouldn’t expect them to be able to create such a broad response to tides,” said lead author Hamish Hay, a postdoctoral fellow at the Jet Propulsion Laboratory in Pasadena, California, who did the research when he was a graduate student at the University of Arizona Lunar and Planetary Laboratory.
Understanding how moons affect each other is important because it can shed light on the evolution of the lunar system as a whole. Jupiter has nearly 80 moons, the four largest of which are Io, Europa, Ganymede and Callisto.
“Keeping underground oceans against freezing during geological periods requires a good balance between internal warming and heat loss, and yet we have several evidence that Europa, Ganymede, Callisto and other moons should be ocean worlds,” the co-author said. Antony Trinh, a postdoctoral researcher at the Lunar and Planetary Lab. “Io, the Galilean moon closest to Jupiter, exhibits diffuse volcanic activity, another consequence of tidal warming, but at a greater intensity probably experienced from other terrestrial planets, such as the Earth, in their early history. Ultimately, we want to understand the source of all this heat, both for its influence on the evolution and habitability of many worlds across the solar system and beyond. “
Resonance of the tides
The trick to tidal warming is a phenomenon called tidal resonance.
“The resonance creates a lot more warming,” Hay said. “Basically, if you push an object or a system and let it go, it will oscillate at its natural frequency. If you keep pushing the system to the right frequency, those swings get bigger and bigger, just like when you push a swing. If you push the swing at the right moment, it increases, but the timing is wrong and the swing movement is dampened. “
The natural frequency of each moon depends on the depth of its ocean.
“These tidal resonances were known before this work, but known only for the tides due to Jupiter, which can only create this resonance effect if the ocean is really thin (less than 300 meters or less than 1,000 feet), the which is unlikely, “Hay said. “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.”
According to the researchers ‘model, the influence of Jupiter alone cannot create tides with the right frequency to resonate with the moons because the moons’ oceans are believed to be too thick. It is only when the researchers added the gravitational influence of the other moons that they began to see tidal forces approach the natural frequencies of the moons.
When the tides generated by other objects in Jupiter’s lunar system match the resonant frequency of each moon, the moon begins to experience more warming than that due to the tides raised by Jupiter alone and, in the most extreme cases, this could cause melting ice or rock internally.
For moons to experience tidal resonance, their oceans must be tens to hundreds of kilometers – at most a few hundred miles – thick, which is in the range of scientists’ current estimates. However, there are some caveats to the researchers’ findings.
Their model assumes that tidal resonances never get too extreme, Hay said. He and his team want to go back to this variable in the model and see what happens when they raise that constraint.
Hay also hopes that future studies will be able to infer the true depth of the oceans within these moons.
Reference: “Powering the Galileean Satellites with Moon-Moon Tides” by Hamish C. F. C. Hay, Antony Trinh and Isamu Matsuyama, July 19, 2020, Geophysical Research Letters.
DOI: 10.1029 / 2020GL088317
This study was funded by NASAFrom the Habitable Worlds program.