By world standards Saturn [19459008 ]’s moon Titan is a strange place. Larger than the planet Mercury, Titan is surrounded by a thick atmosphere (the only satellite in the Solar System that has an atmosphere) and is covered by rivers and seas of liquid hydrocarbons such as methane and ethane. Beneath these is a thick crust of water ice, under which there may be an ocean of liquid water that is likely to host life.
Decades of measurements and calculations reveal that Titan’s orbit around Saturn is widening, meaning the moon is moving further away from the planet. Moreover, this is happening 100 times faster than expected. The research shows that Titan was born much closer to Saturn and migrated to its current location, 1.2 million kilometers in 4.5 billion years.
“Most previous studies had predicted that satellites like Titan or Jupiter’s moon Callisto formed at a similar orbit distance to what we see now,” says Jim Fuller, a theoretical astrophysicist at Caltech and co-author of the newly published article. “This means that the Saturn satellite system, and possibly its rings, formed and evolved more dynamically than previously thought.
The Moon is also Moving away from Earth, but we will not lose the Moon
To understand the basics of orbital migration, we can look at the Moon. As Earth’s moon rotates, it exerts a small gravity on the planet. This is what causes the tides: the rhythmic pulls of the Moon cause the world’s oceans to swell from side to side. The frictional processes inside the Earth convert some of this energy into heat and disrupt the Earth’s gravitational field, thus pulling the Moon forward in its orbit. This causes the Moon to gain energy and to move away from the Earth at a speed of approximately 3.8 centimeters per year. But this process is really slow; We will not “lose” the Moon until both the Earth and the Moon are swallowed up by the Sun in roughly six billion years.
Titan also exerts a similar gravity to Saturn, but the frictional processes inside Saturn are generally thought to be weaker than those on Earth, due to their gaseous composition. Standard theories predict that because of its distance from Saturn, Titan should move away at a rate of 0.1 centimeters a year at most. But the new results contradict this prediction.
In the study detailed in Nature Astronomy , two research teams used different techniques to measure the trajectory of Titan over a period of 10 years. A technique called astrometry revealed precise measurements of Titan’s position relative to the background stars in photos taken by the Cassini spacecraft. Another technique, called radiometry, measured Cassini’s velocity as it was altered by Titan’s gravitational effect.
Valéry Lainey, the lead author of the study, states that using two completely independent data sets and two different analysis methods, they obtained fully compatible results. Lainey worked with the astrometry team.
New values are 100 times more than thought
The results are also in line with the theory put forward by Fuller in 2016, which predicted that the Titan’s migration velocity would be much faster than standard tide theories predicted. Fuller’s theory says that Titan must squeeze Saturn with gravity at a certain frequency, and this will cause the planet to oscillate strongly, much like swinging your legs on a swing with the right timing will constantly move you higher. This tidal forcing is called resonance locking. Fuller suggests that the high amplitude of Saturn’s oscillation would consume a lot of energy and eventually cause Titan to move away from the planet at a higher rate than previously thought. Indeed, observations showed that Titan is moving away from Saturn at a rate of 11 centimeters a year. This is 100 times faster than previous theories predicted.
“The resonance locking theory can be applied to many astrophysical systems,” says Fuller. “Now I’m doing some theoretical work to see if the same physics can be in binary star systems, or exoplanet systems,” he adds.