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Destruction of Saturn’s Former Moon May Explain Planet’s Tilt and Young Rings

Wreathed in concentric rings of icy debris, Saturn is one of the most iconic planets in our solar system. However, since their discovery nearly 400 years ago, the origin of Saturn's rings — their age and how they came to be — have remained largely a mystery.

It may be easy to think that the belted gas giant's rings have always been there, drifting around the planet since its formation billions of years ago. But a growing body of recent evidence suggests that they are far younger and perhaps only 100 million years old.

Just how Saturn's rings could be so young has puzzled planetary scientists for decades. But now researchers have a new hypothesis that could help explain this mystery, as well as several others, including Saturn's unusual orbital tilt, or obliquity, and its peculiar connection to the distant planet Neptune: the violent destruction of a former moon.

In the new study published in the Sept. 16 issue of Science, Jack Wisdom, professor of planetary sciences at MIT and lead author of the study, and colleagues propose that Saturn, which currently hosts more than 83 moons, once had at least one more, a satellite they name Chrysalis.

"As a butterfly emerges from a chrysalis, the rings of Saturn emerged from the primordial satellite Chrysalis," said Wisdom.

Falling Out of Step

diagram of how Saturn moon may have disintegrated to form rings
Graphic: K. Holoski/ Science

Given the similarities in the tilted axes of both Saturn and Neptune, researchers have put forward the idea that the two planets have been gravitationally linked in some way. However, according to Wisdom, how mass is distributed in Saturn's interior — otherwise known as its moment of inertia — was unknown and provided a major source of uncertainty in testing this hypothesis.

Using some of the last gravitational measurements taken by NASA's Cassini spacecraft during its "grand finale" of observations before a controlled crash into the planet, the researchers developed computational models of the interior of Saturn to determine the planet's moment of inertia.

As it turned out, Saturn's calculated moment of inertia placed the planet just outside of gravitational resonance with Neptune, findings that suggest that although the planets may have once been in sync, they were no longer. The findings left Wisdom and the other researchers hunting for ways to explain just how Saturn could have escaped Neptune's gravitational resonance.

"We investigated various possibilities, but the only idea that worked was that Saturn used to have an additional satellite that contributed to the precession of Saturn, but was recently lost," said Wisdom.

Chrysalis Lost

According to Wisdom, Chrysalis — which would have been about the size of the planet's third largest current moon, Iapetus — may have orbited Saturn quietly for several billion years. During this time, the Saturnine system gravitationally interacted with that of Neptune, which kept the two planets in orbital resonance, resulting in the planet's obliquity.

However, sometime between 200 and 100 million years ago, Chrysalis became destabilized and eventually drifted so close to Saturn it disintegrated. While much of the material from the moon descended into the planet, a small fraction flung outwards to circle the planet and became the rings we know today.

With Chrysalis gone and Saturn's moment of inertia changed, Neptune could no longer gravitationally influence Saturn's spin axis, leaving the planet with its current obliquity.

"A nice thing about our findings is that we explain both the tilt and the young rings, which were previously not thought to be related problems," said Wisdom.


[Credit for associated images: NASA, ESA, A. Simon (GSFC), M.H. Wong (University of California, Berkeley) and the OPAL Team]


Walter Beckwith

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