A single doomed moon could clear up some mysteries about Saturn.
Researchers suggest that this lost hypothetical moon, nicknamed Chrysalis, could have helped tilt Saturn on September 15. Sciences. The ensuing orbital chaos may have led to the Moon’s demise, tearing it apart to form the iconic rings that surround the planet today.
“We liked it because it’s a scenario that explains two or three different things that were not previously thought to be related to each other,” says Jack Wisedom, one of the study’s authors, a planetary scientist at the Massachusetts Institute of Technology. “The rings are related to the tilt, who would have guessed that?”
Saturn’s rings are surprisingly young, only 150 million years old or so (SN: 12/14/17). If dinosaurs had telescopes, you might have seen Saturn without rings. Another mysterious feature of the gas giant is its tilt of about 27 degrees relative to its orbit around the Sun. This tendency is too large to form when Saturn occurred or be the result of collisions that destroyed the planet.
Planetary scientists have long suspected that the tilt is related to Neptune, due to the coincidence in timing between the way the two planets move. Saturn’s axis oscillates, or kicks off, like a rotating top. The entire orbit of Neptune also oscillates around the Sun, like a faltering hoop.
The periods of both magnitudes are nearly the same, a phenomenon known as resonance. Scientists hypothesized that gravity from Saturn’s moons — particularly the largest moon, Titan — helped align the planets. But some features of Saturn’s internal structure were not well known enough to prove that the two timings are related.
Wisdom and colleagues used precise measurements of Saturn’s gravitational field from the Cassini spacecraft, which plunged into Saturn in 2017 after 13 years of orbiting the gas giant, to learn the details of its internal structure (SN: 9/15/17). Specifically, the team came up with Saturn’s moment of inertia, a measure of how much force is needed to flip the planet. The team found that the moment of inertia is close, but not exactly, than it would be if Saturn’s rotation were in perfect resonance with Neptune’s orbit.
“We’re arguing that it’s too close, and it can’t happen by chance,” Wisdom says. “This is where this Chrysalis satellite came in.”
After considering a host of other explanations, Wisdom and his colleagues realized that another small moon would have helped Titan resonate with Saturn and Neptune by adding its own gravitational locomotives. Titan drifted away from Saturn until its orbit coincided with that of the chrysalis. Gravity’s enhanced kicks from the larger moon pushed the doomed smaller moon into a chaotic dance. Eventually, Chrysalis swooped so close to Saturn that it flew past the giant planet’s cloud tops. Saturn tore the moon apart, slowly expelling its pieces into rings.
Calculations and computer simulations showed that the scenario worked, but not all the time. Of the 390 simulated scenarios, only 17 ended with the cocoon disintegrating to create the episodes. Then again, massive beating rings like Saturn’s are rare as well.
The name Chrysalis came from that startling ending: “The cocoon is the cocoon of a butterfly,” says Wisdom. “The Chrysalis satellite has been dormant for 4.5 billion years, probably. Then suddenly Saturn’s rings appeared from it.”
Planetary scientist Larry Esposito of the University of Colorado Boulder, who was not involved in the new work, says the story hangs together. But he is not completely convinced. “I think that’s all reasonable, but maybe not very likely,” he says. “If Sherlock Holmes is solving a case, even an unlikely explanation might be the correct one. But I don’t think we’re there yet.”