Jupiter’s Biggest Moons Started as Tiny Grains of Hail

https://www.nytimes.com/2020/05/18/science/jupiter-moons-europa.html

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Konstantin Batygin did not set out to solve one of the solar system’s most puzzling mysteries when he went for a run up a hill in Nice, France. Dr. Batygin, a Caltech researcher, best known for his contributions to the search for the solar system’s missing “Planet Nine,” spotted a beer bottle. At a steep, 20 degree grade, he wondered why it wasn’t rolling down the hill.

He realized there was a breeze at his back holding the bottle in place. Then he had a thought that would only pop into the mind of a theoretical astrophysicist: “Oh! This is how Europa formed.”

Europa is one of Jupiter’s four large Galilean moons. And in a paper published Monday in the Astrophysical Journal, Dr. Batygin and a co-author, Alessandro Morbidelli, a planetary scientist at the Côte d’Azur Observatory in France, present a theory explaining how some moons form around gas giants like Jupiter and Saturn, suggesting that millimeter-sized grains of hail produced during the solar system’s formation became trapped around these massive worlds, taking shape one at a time into the potentially habitable moons we know today.

Dr. Batygin and Dr. Morbidelli say earlier theories explain only a part of how the solar system’s many objects formed. The two researchers set out to present the rest of the story with equations explaining how a new planet transitions from being surrounded by its disk of matter, to creating satellite building blocks, all the way to the formation of moons like Europa.

When Dr. Batygin and Dr. Morbidelli ran computer simulations of their proposed theory, they found that they’d accidentally re-created Jupiter’s small innermost moons as well as the four Galilean satellites, much as we see them today.

“I thought I was still dreaming when I saw the results,” Dr. Batygin said.

The equations amount to a recipe for how to make a moon. It starts with a mix of hydrogen and helium gas raining down onto Jupiter from above. Some of the gas gets swept out and away, spreading viscously as it goes into orbit around Jupiter in a process called decretion.

At this point in Jupiter’s formation, the only solid particles that orbited it were smaller than one millimeter across. Because this dust is very small — tiny grains about two parts ice to one part rock — it can couple itself to the gas washing away from Jupiter.

“The disk around Jupiter acts a little bit like a vacuum cleaner, where it sources small dust from the protoplanetary disk,” Dr. Batygin said.

As this material builds up over the course of about a million years, he says, it eventually reaches a mass that approximately matches Io, Europa, Ganymede and Callisto today.

The dust clumps together into a massive carpet of icy asteroids, some of which slow down, growing larger as they consume some of the other objects.

“Once the moon is big enough to ship, it gets on the conveyor belt,” Dr. Batygin said, and eventually moves in closer to Jupiter, parking into its orbit around the planet.

In this model, Io was formed in about 1,000 years and then quickly got ejected from the satellite feeding zone, leaving behind a mess of remaining icy asteroids in wonky orbits. Around 10,000 years later, Europa grows over about the course of a millennium and does the same thing. After a 30,000-year break, Ganymede begins to form, but takes 2,000 years to grow. Callisto, however, begins to form when the material from Jupiter is nearly depleted, so it takes much longer, around eight million years.

The model offers a similar explanation for Saturn and its largest moon, Titan.

Jonathan Lunine, an astronomer at Cornell University who has studied the Galilean satellites’ formation, says the paper “sketches out a scenario more like the formation of the terrestrial planets,” than other theories. But he thinks that “it doesn’t solve head-on the curious fact that Ganymede, Callisto and Titan (Titan being the big moon of Saturn) all have very similar sizes and densities and yet totally different geologic histories.”

Closer study will be needed to fully explain these moons’ history. Luckily, missions planned to Saturn’s moon Titan and Jupiter’s moons Callisto, Europa and Ganymede in the next 20 years will yield more data to test theories like this one. And this research may aid our understanding about whether life is possible around other stars.

“If we’re going to find life, arguably the best place to look are the icy satellites of the giant planets,” Dr. Batygin said. If similar moons are likely to form around other stars’ gas giants, it raises the question of whether “life in the universe is actually pretty common” he said. “I don’t know, of course, but it’s an exciting thing to think about.”