Saturn's Moon Titan Is Drifting Away From Planet 100 Times Faster Than Thought

Saturn's largest moon, Titan, is floating away from its host planet much faster than previously thought, research suggests.

Scientists already knew that Titan's orbit was expanding very gradually over time, just like our own moon moves a tiny bit further away from the Earth every year. But a study published in the journal Nature concludes that Titan is drifting away from Saturn at a rate of 4 inches per year—around 100 times faster than previous estimates.

The findings could shed light on the long-debated issue of when Saturn's rings and system of more than 80 moons formed.

Researchers say the latest results suggest that Titan started out much closer to its host planet before migrating to its current distance of 759,000 miles over the course of 4.6 billion years. This indicates that the entire Saturn system expanded faster than previous models have suggested.

"Most prior work had predicted that moons like Titan or Jupiter's moon Callisto were formed at an orbital distance similar to where we see them now," Jim Fuller, assistant professor of theoretical astrophysics at Caltech and co-author of the paper, said in a statement. "This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed."

When a moon orbits a planet, it exerts a small gravitational pull on the host world, creating a temporary bulge as it passes. The periodic appearance and subsidence of these bulges on the host planet generates energy, which, over long periods of time, is gradually transferred to the moon, very slowly pushing it further and further away.

On Earth, we can observe this bulging effect in the ocean tides that are influenced by the gravity of the moon, which is drifting away from our planet at a rate of around 1.5 inches per year.

For half a century, scientists have mostly relied on the same standard theories to estimate how fast a moon migrates away from its host planet. This calculation can also be used to determine the age of the moon.

According to these theories, in multi-moon systems such as Saturn's, the outer moons, like Titan, should drift away from the host planet at a slower rate than the inner moons, because they orbit at a larger distance, so are further from its gravitational influence.

However, in 2016, Fuller published a study that laid out an alternative to these standard theories. The paper suggested that outer moons may migrate outwards at a similar rate to inner moons because they can get locked in a particular kind of orbital interaction with their host planet that can push them away at a faster rate.

Saturn, Titan
Natural color view of Titan and Saturn from NASA's Cassini spacecraft. NASA/JPL-Caltech/Space Science Institute

The standard theories suggested that Titan should be moving away from Saturn at a rate of around 0.04 inches per year. But the latest findings appear to confirm the predictions in Fuller's 2016 paper.

Researchers used images captured by NASA's now-defunct Cassini spacecraft between 2006 and 2016 to track Titan's position relative to background stars in order to measure its orbit. To confirm these findings, they compared the results to independent radio wave data collected by Cassini, which also allowed them to estimate how Titan's orbit was changing.

"By using two completely different datasets, we obtained results that are in full agreement, and also in agreement with Jim Fuller's theory, which predicted a much faster migration of Titan," Paolo Tortora, a co-author of the study from the University of Bologna, Italy, said in a statement.

The Cassini probe studied the Saturnian system for more than 13 years, revolutionizing our understanding of the planet, its complex rings and its numerous moons. In 2017, operators brought the mission to an end by deliberately plunging the spacecraft, which was rapidly running out of fuel, into the gas giant. This was, in part, to ensure it would never crash into one of Saturn's moons, such as Titan or Enceladus, contaminating pristine environments that scientists think could host the right conditions for life.

Titan is the second-largest natural satellite in the solar system, being just 2 percent smaller than Jupiter's moon Ganymede. In fact, Titan—with a radius of around 1,600 miles—is even bigger than the planet Mercury, and is nearly 50 percent wider than our own moon.

Titan is also the only moon in the solar system to have a thick atmosphere, which is mostly made up of nitrogen. It's surface is covered in rivers and seas of liquid hydrocarbons, such as methane and ethane. Scientists think that below the moon's thick crust of water ice lies an underground ocean of liquid water, which could host life..