Did Saturn Lose One of Its Moons?

Saturn and its satellite, Titan, captured by the Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute.

Several aspects of Saturn and its famous ring system have defied explanation. First, Saturn’s obliquity (the angle between its rotational and orbital axis) has a relatively large value of 26.7 degrees. The high obliquity of Saturn cannot be explained through its formation from a large impact or protoplanetary disk accretion. Second, Saturn’s ring system has been dated to be 100 million years old based on imaging. Plausible mechanisms to explain this young age are lacking. The rate at which Saturn’s spin (rotational) axis rotates in the vertical plane, also known as precession frequency, is similar to that of Neptune, a result consistent with the idea that Saturn’s obliquity results from resonance with Neptune.

There are two scenarios in which the precession frequency of Saturn could be in resonance with Neptune: the precession frequency of Neptune’s orbit changing during Saturn’s formation or the rapid movement of Saturn’s largest satellite, Titan. To compare these two hypotheses, Jack Wisdom and colleagues from the Massachusetts Institute of Technology used Cassini data to investigate the moment of inertia of Saturn, a quantity indicating how strongly a planet will resist changes in its rotation. Previous attempts to determine this quantity were deemed inconclusive due to high uncertainties related to resonance.

To understand Saturn’s moment of inertia using external gravity field data from Cassini, Wisdom and colleagues constructed interior models and numerical simulations of the Saturn satellite system. These models included parameters such as normalized angular momentum, the product of moment of inertia and rotation rate, and physical properties of Saturn. Results show that Saturn’s moment of inertia is outside the range required for resonance, leading the researchers to conclude that Saturn once had an additional, unstable satellite, which they named Chrysalis. Through interactions with Titan, Chrysalis caused Saturn’s obliquity to increase via Neptune resonance. The eventual destabilization of Chrysalis 100 million years ago placed debris from its breakup proximally to Saturn, enabling ring formation. READ MORE