A single doomed moon could clear up a couple of mysteries about Saturn.
This hypothetical missing moon, nicknamed Chrysalis, could have helped tilt saturnresearchers suggest on September 15 in Sciences. The resulting orbital chaos could have led to the moon’s demise, shattering it to form the iconic rings that surround the planet today.
“We like it because it’s a scenario that explains two or three different things that were previously not thought to be related,” says study co-author Jack Wisdom, a planetary scientist at MIT. “Rings are related to tilt, who would have guessed?”
saturn’s rings look surprisingly youngbarely 150 million years old more or less (Serial number: 12/14/17). If dinosaurs had telescopes, they might have seen a ringless Saturn. Another mysterious feature of the gas giant is its tilt of nearly 27 degrees relative to its orbit around the sun. That tilt is too large to have formed when Saturn did or to be the result of collisions that brought the planet down.
Planetary scientists have long suspected that the tilt is related to Neptune, due to a coincidence in timing between the way the two planets move. Saturn’s axis wobbles, or precesses, like a top. Neptune’s entire orbit around the sun also wobbles, like a struggling hula hoop.
The periods of both precessions are almost equal, a phenomenon known as resonance. Scientists theorized that the gravity of Saturn’s moons, especially its largest moon Titan, helped align planetary precessions. But some features of Saturn’s internal structure were not known well enough to show that the two times were related.
Wisdom and his colleagues used precision measurements of Saturn’s gravitational field from the Cassini spacecraft, which submerged in saturn in 2017 after 13 years orbiting the gas giant, to find out the details of its internal structure (Serial number: 09/15/17). Specifically, the team calculated Saturn’s moment of inertia, a measure of how much force is needed to tip the planet over. The team found that the moment of inertia is close to, but not exactly, what it would be if Saturn’s spin were in perfect resonance with Neptune’s orbit.
“We argue that it’s so close that it couldn’t have happened by chance,” says Wisdom. “That’s where this Chrysalis satellite came in.”
After considering a number of other explanations, Wisdom and his colleagues realized that another, smaller moon would have helped Titan bring Saturn and Neptune into resonance by adding its own gravitational pulls. Titan moved away from Saturn until its orbit was synchronized with that of Chrysalis. The larger moon’s enhanced gravitational kicks sent the damn smaller moon into a chaotic dance. Eventually, Chrysalis swooped so close to Saturn that it grazed the giant planet’s cloud tops. Saturn tore the moon apart and slowly ground its pieces into the rings.
Calculations and computer simulations showed that the scenario works, although not all the time. Of 390 simulated scenarios, only 17 ended with Chrysalis disintegrating to create the rings. On the other hand, massive and conspicuous rings like Saturn’s are also rare.
The name Chrysalis comes from that spectacular ending: “A chrysalis is a butterfly cocoon,” says Wisdom. “The Chrysalis satellite was dormant for 4.5 billion years, presumably. Then all of a sudden Saturn’s rings emerged from it.”
The story holds together, says planetary scientist Larry Esposito of the University of Colorado Boulder, who was not involved in the new work. But he is not entirely convinced. “I think it’s all plausible, but maybe not that likely,” he says. “If Sherlock Holmes is solving a case, even the unlikely explanation may be the correct one. But I don’t think we’re there yet.”