According to the results of a new study, Saturn may owe its sweeping ring system and distinctive tilt to the death of an ancient moon named Chrysalis.
The gas giant Saturn is one of the most spectacular sites in our solar system. However, despite having been the target of several ambitious missions in the past, significant questions remain as to how the iconic planet acquired its impressive ring system, and exactly why it orbits at an inclination of 26. 7 degrees from its orbital plane.
Astronomers believed for a time that Saturn’s flippant angle was the result of a gravitational interaction between the gas giant, its 83 moons, and the tug exerted on it by the planet Neptune. The relationship to the latter was established when astronomers realized that Saturn’s spinning top motion matched Neptune’s orbital cadence extremely well.
In other words, the two massive planets were thought to share a strong gravitational association – also known as a resonance.
Images from NASA’s Cassini spacecraft
Data collected by the Cassini spacecraft, which explored the Saturnian system between June 2004 and September 2017, shed further light on the relationship, revealing that the massive moon Titan was moving away from Saturn at a startling rate of 11cm per year.
This has led some scientists to suggest that the gravitational influence and outward motion of this natural satellite – which is significantly larger than Earth’s moon – was likely responsible for maintaining orbital resonance between Saturn and Neptune.
However, this theory was based on a major and largely undefined characteristic of Saturn – its “moment of inertia”. It is basically the term used to describe the distribution of mass in a celestial body.
The moment of inertia is an important factor for astronomers seeking to understand the orbital properties of a world, because the mass distribution and density within a planet can significantly affect its tilt. Therefore, if scientists do not have a good understanding of the moment of inertia, it becomes more difficult to accurately understand what the history of this planet is when, for example, using computer modelling.
Now a team of researchers have now used Cassini data from the last part of its mission – known as the “Grand Finale” – to refine Saturn’s moment of inertia and have found that it is outside the range needed to maintain an orbital resonance with Neptune.
During the “Grand Finale”, Cassini was tasked with undertaking a series of daring dives between the surface of Saturn’s clouds and its innermost rings. A total of 22 dives were made, during which the spacecraft collected data on the internal structure of Saturn and the distribution of mass in it.
The team used computer modeling to create a map of Saturn’s mass distribution that matches Cassini’s real-world gravitational measurements. While it was close, the team’s findings revealed that Titan’s pull is not enough to maintain the gravitational resonance between Saturn and Neptune. However, judging by Saturn’s current tilt, the team believes there was a resonance that lasted billions of years in the past, which was then broken.
To unravel the mystery, the team began running computer simulations that essentially rolled back Saturn’s planetary evolution clock in an effort to uncover gravitational instabilities that could explain the breakup with Neptune.
After running numerous simulations, the team came to the conclusion that Saturn once hosted at least one other major satellite the size of the moon Iapetus, which has a current diameter of 457 miles (736 km).
The death of a moon
According to the results of the study, published in the journal Science, the gravitational influence of this long dead moon would have maintained the fragile resonance between Saturn and Neptune. Over billions of years, this gravitational dance between the gas giants and the moons would have slowly pulled Saturn’s axis into an extreme tilt.
However, the relationship was not to last. The team estimates that around 160 million years ago, the moon collided with the gravitational influence of its siblings Titan and Iapetus, and was forced closer and closer to Saturn’s surface.
Eventually, Chrysalis would have been torn apart by the vast forces exerted on her. The great bulk of the moon’s mass would have disappeared below Saturn’s cloudy surface. However, a small amount would escape this fate and eventually settle around the equator.
With the moon destroyed, the finely balanced resonance was broken, leaving Saturn with its distinctive orbital inclination and the ingredients of the fantastic ring system we see today.
Anthony Wood is a freelance science writer for IGN
Image Credit: NASA/JPL/Institute of Space Sciences
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