By | September 16, 2022
Saturn's distinctive appearance may have its roots in the death of an ancient moon - IGN

Saturn may owe its sweeping ring system and distinct tilt to the death of an ancient moon named Chrysalis, according to the results of a new study.

The gas giant Saturn is one of the most spectacular sights in our solar system. But despite being the target of several ambitious missions in the past, significant questions remain about how the iconic planet got its impressive ring system and exactly why it orbits at a 26.7-degree tilt to its orbital plane.

Astronomers have long believed that Saturn’s 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 with the latter was drawn when astronomers realized that Saturn’s spinning top-like motion matched the orbital cadence of Neptune extremely well.

In other words, the two massive planets were thought to share a strong gravitational association – also known as a resonance.

Data collected by the Cassini spacecraft, which explored the Saturnian system between June 2004 and September 2017, further informed the relationship by revealing that the massive moon Titan was moving away from Saturn at a surprising rate of 11 cm per year.

This 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 probably responsible for maintaining the orbital resonance between Saturn and Neptune.

But this theory was based on a major and largely undefined property of Saturn – its “moment of inertia”. This is essentially the term used to describe the distribution of mass within a celestial body.

The moment of inertia is an important factor for astronomers who want to understand the orbital properties of a world, because the distribution and density of mass in a planet’s interior can have a large effect on its inclination. Therefore, if scientists do not have a good understanding of the moment of inertia, it becomes more difficult to accurately figure out the history of that planet when using, for example, computer modeling.

Cassini’s legacy

Now a team of researchers have now use Cassini data from the last part of its mission – known as the “Grand Finale” – to refine Saturn’s moment of inertia, and discovered that it is outside the range needed to maintain an orbital resonance with Neptune.

During the “Grand Finale,” Cassini was tasked with conducting a series of daring dives between Saturn’s cloud surface and its innermost rings. A total of 22 dives were made, during which the spacecraft collected data on Saturn’s internal structure and the distribution of mass therein.

The team used computer modeling to create a map of the mass distribution of Saturn that matched Cassini’s real-world gravity measurements. Although close, the team’s results revealed that Titan’s pull is not sufficient to maintain the gravitational resonance between Saturn and Neptune. But judging by Saturn’s current tilt, the team believes there was a resonance that lasted billions of years before, which was then broken.

To unravel the mystery, the team began running computer simulations that essentially turned back the clock on Saturn’s planetary evolution in an attempt to detect gravitational instabilities that could explain the rupture with Neptune.

After running many simulations, the team concluded that Saturn once hosted at least one more large satellite, roughly the size of the moon Iapetus, which has a current day diameter of 736 km.

Death of the 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 the course of several billion years, this gravitational dance between the gas giants and the moons would have slowly pulled Saturn’s axis to an extreme tilt.

However, the relationship would not last. The team estimates that, about 160 million years ago, the moon went against the gravitational influence of its siblings Titan and Iapetus, and was forced ever closer to Saturn’s surface.

In the end, the Chrysalis would have been crushed by the immense forces exerted upon it. Most of the moon’s mass would have disappeared beneath Saturn’s cloud cover. A small amount, however, would escape that fate and eventually settle around the equator.

When the moon was destroyed, the finely balanced resonance was broken, leaving Saturn with its distinctive orbital tilt and the creation of the amazing ring system we see today.

Anthony Wood is a freelance science writer for IGN

Image credit: NASA/JPL/Space Science Institute