An old Saturn moon might have put a ring on it

Saturn's rings are 100 million years old and made of chunks of water ice.

Saturn’s rings are 100 million years old and made of chunks of water ice.
Image: NASA/JPL-Caltech/SSI

Saturn’s rings are one of the most iconic structures in the solar system, but their genesis has long been the subject of debate. New research suggests the spectacular rings may have been born from the death of an icy moon.

Saturn is a dynamic system. Besides these fascinating rings, one of its moons, Titan, is rapidly receding from the planet at around 4 inches (11 centimeters) per year (our own Moon is moving away from Earth at a slow rate of 1.5 inches per year). Saturn is also tilted at an angle of 26.7 degrees to the plane of its orbit, and while that’s not all that uncommon in our solar system, the mechanism that caused this tilt is shrouded in mystery. However, an article published yesterday in Science may point to the missing link that could connect all these phenomena: an icy moon of Saturn, now extinct, called Chrysalis.

“If you throw a top on a table, after an initial period of oscillation, it settles into a motion where this axis of rotation of the top regularly circles around the vertical. It’s ‘precession’ from the top,” Jack Wisdom said in a phone call with Gizmodo. Wisdom is a professor of planetary sciences at the Massachusetts Institute of Technology and is the lead author of the new study. Wisdom has said that the precession of Saturn and its distant neighbor Neptune was at one time very close or resonant. This resonance, together with Titan’s migration away from Saturn, could have explained why the planet tilted.

But as Wisdom and his colleagues examined gravitational data from the Cassini spacecraft, they noticed that Saturn and Neptune were no longer in resonance and began to wonder what mechanism might be causing it. “We came across the idea that Saturn had another satellite,” Wisdom said. “If this satellite were suddenly lost, then [Saturn] could come out of resonance.

They call this hypothetical lost moon Chrysalis and believe it existed somewhere between the orbits of the moons Titan and Iapetus. Through computer modeling, they discovered that when Titan spirals outward, it will destabilize Chrysalis’ orbit. Chrysalis would then move toward Saturn, where it would be torn apart by the planet’s gravitational field, creating the rings we see today while pushing Saturn out of Neptune’s resonance. All this drama would have taken place about 100 million years ago.

“It all fits together,” Wisdom said. “Even though it is a chain of events, each element of the chain is not an improbable thing.

Henry Throop, a program scientist in NASA’s Planetary Science Division who is not affiliated with the new paper, was intrigued by the results. “What is compelling about their hypothesis is that it connects so many unexplained aspects of Saturn’s current state: the apparently young age of the rings, Saturn’s steep tilt on its axis, and the great eccentricity of Titan’s orbit. If correct, the concept presented here expands our understanding that the solar system continues to evolve significantly,” Throop wrote in an email to Gizmodo. Throop short the Cassini data analysis program, which funded part of the research for the article.

Luke Dones, an unaffiliated researcher with the Planetary Science Branch of the Southwest Research Institute in Boulder, Colorado, said more research is needed to fully flesh out the existence and lifespan of Chrysalis. “There is a lot of space between [Iapetus and Titan] where there could have been a satellite, but there is also no independent evidence that such a moon existed,” Dones said in an email. “Perhaps the rings of Saturn formed as they describe, but they did not provide a definitive answer. The authors did not simulate the formation of the rings, but relied on previous work in their brief discussion at the end of the article.

The origin of Saturn’s rings is one of the great mysteries of our little corner of the Milky Way. The untimely demise of Chrysalis, assuming it ever existed, could be why Saturn is a favorite target for amateur and professional astronomers.

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