The research, based on data from the final stage of Nasa’s Cassini mission, suggests that Saturn may have been ringless for nearly all of its 4.5 billion-year existence. But about 160 meters ago, an inner moon drifted too close to the gas giant, causing it to drift away, painting its own orbit in a trail of broken icy fragments. The hypothetical lost moon is nicknamed Chrysalis. “Just like the chrysalis of a butterfly, this satellite was very dormant and suddenly became active and the rings appeared,” said Jack Wisdom, professor of planetary science at the Massachusetts Institute of Technology and lead author of the study. Professor Scott Tremaine of the Institute for Advanced Study in Princeton, who was not involved in the research, described the findings as “remarkable” because of their potential to resolve several separate puzzles about Saturn with “one bold but plausible hypothesis”. Wisdom’s team initially set out to explain why Saturn is tilted about 27 degrees on its axis. Theoretical models had shown that the tilt was likely due to Saturn being locked in a gravitational resonance with Neptune. But these kinds 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 on everything from Saturn’s internal composition to the dynamics of the planet’s 83 moons, the original explanation fell apart. These new details suggest that Saturn had, at some point in the past, fallen outside of Neptune. This led scientists to look for possible disruptive events that could have caused it. The lost moon scenario provided an unexpectedly neat fit to the data. “We set out to try to explain the tilt of Saturn,” Wisdom said. “But we found that we had to propose an additional satellite and then get rid of the satellite again.” Wisdom and his colleagues ran simulations to determine the properties of the hypothetical moon. These suggest that, between 100 and 200 m years ago, Chrysalida entered a chaotic orbital zone and experienced a series of close encounters with Saturn’s moons Iapetus and Titan. It eventually came very close to Saturn, and this dramatic encounter shattered the moon into fragments, leaving a debris-strewn ring in its wake. The loss of Chrysalis would explain Saturn’s current declination and its rings. It would also be consistent with measurements of the chemical properties of the rings, which have been dated to about 100 meters old, but which some had dismissed because it was unclear how the rings would have materialized so late in the planet’s history. “I think we provide a pretty compelling argument,” Wisdom said. “We will never know for sure whether there once was an extra satellite in the Saturn system, but explaining [several] puzzle with a premise is a pretty good return on investment,” Tremaine said. Saturn’s rings weigh about 15 million trillion kilograms and are composed almost entirely of ice – about 95% of which is pure water, the rest being rock and minerals. The findings are published in the journal Science.
