Scientists have said that Saturn’s famous rings may be in the wake of a moon torn apart by the planet’s gravity.
The research, based on data from the final stage of NASA’s Cassini mission, suggests that Saturn may have been ringless throughout its roughly 4.5 billion years of existence. But about 160 million years ago, an inner moon strayed too close to the gas giant to tear it apart, drawing its orbit in a path of scattered icy bits.
The lost virtual moon was named Chrysalis.
“Just like a butterfly cocoon, this satellite had been dormant for a long time and suddenly became active, and the rings appeared,” said Jack Wisdom, professor of planetary sciences at MIT and lead author of the study.
Professor Scott Tremaine, of the Institute for Advanced Study in Princeton, who was not involved in the research, called the findings “brilliant” because of their ability to solve many separate mysteries about Saturn with “one bold but plausible hypothesis”.
Wisdom’s team first set out to explain why Saturn is tilted about 27 degrees on its axis. Theoretical models indicated that the tilt was likely due to Saturn’s entrapment in its gravitational resonance with Neptune. But these types of models are often sensitive to small changes in a wide range of variables.
As the Cassini mission, which orbited Saturn from 2004 to 2017, filled in details of everything from Saturn’s internal composition to the dynamics of the planet’s 83 moons, the original explanation fell apart. These new details indicate that Saturn had, at some point in the past, fallen outside the grasp of Neptune.
This led scientists to search for potential disruptive events that could have caused it. The Lost Moon scenario provided an elegant, unexpected fit for the data. “We set out to try to explain the tilt of Saturn,” Wisdom said. “But we found we had to propose an additional satellite and then get rid of the satellite again.”
Wisdom and colleagues ran simulations to characterize the hypothetical moon. These studies indicate that, between 100 and 200 million years ago, Chrysalis entered a chaotic tropical region and experienced a number of close encounters with Saturn’s moons Iapetus and Titan. It eventually gets close to Saturn, and this dramatic encounter rips the moon to pieces, leaving a ring of debris in its wake.
The loss of the cocoon explains Saturn’s present-day tilt and rings. It would also be consistent with measurements of the chemical properties of the rings, which date to about 100 million years ago, but were rejected by some because it was not clear how the rings would have materialized so late in the planet’s history. “I think we make a very persuasive argument,” said Wisdom.
“We will never know for sure if an additional satellite is in the Saturn system, but we will explain it [several] “Puzzles with a single premise are a very good return on investment,” Tremaine said.
Saturn’s rings weigh about 15 million trillion kilograms, and are made almost entirely of ice – about 95% of it is pure water while the rest is rocks and minerals.
The results were published in the journal Sciences.