Moons can drift away from their planets. Our own Moon drifts about 3.8 centimeters (1.5 inches) from Earth each year, and we know Titan, the largest moon of Saturn, does too. We just didn't know how fast. In a new study, scientists have established that Titan is moving away from the ringed planet at 11 centimeters (4 inches) every year; a lot faster than expected.
As reported in Nature Astronomy, observations of Titan and five other moons suggest a necessary revision of the evolution of Saturn’s moon system. In particular, the research provides a new estimate of the so-called Tidal Quality Factor, a measure of how a celestial body is affected by the gravitational pull of other objects in the system. It turns out that for Saturn this is 100 times smaller than thought, meaning Titan is drifting 100 times faster than thought.
These new insights are not only important to understand the future of the Saturnian system, they can help us understand its past, too. The drift rate can be used to estimate the age of moons and where they formed. In the case of Saturn, it suggests that the whole system must have expanded much more quickly than previously believed.
"This result brings an important new piece of the puzzle for the highly debated question of the age of the Saturn system and how its moons formed," lead author Dr Valery Lainey from the Paris Observatory at PSL University said in a statement.
Titan is located 1.2 million kilometers (759,000 miles) from Saturn and is one of 82 moons. It was thought that more distant moons migrated more slowly, being further from the planet, but a few years ago co-author Dr Jim Fuller showed that it is perfectly possible for outer and inner moons to drift at the same rate. Planetary properties such as particular wobbles as well as certain orbital patterns can push these outer moons out faster.
"The new measurements imply that these kinds of planet-moon interactions can be more prominent than prior expectations and that they can apply to many systems, such as other planetary moon systems, exoplanets – those outside our Solar System – and even binary star systems, where stars orbit each other," explained Dr Fuller.
The data is based on Cassini's observations of the system. The international mission spent 13 years studying the Saturn system and the data collected is invaluable. Researchers used the motion of Titan against distant stars as well as radio data sent by the probe when around Titan.
"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," added co-author Dr Paolo Tortora from the University of Bologna.
The Cassini mission was a cooperative project of NASA, ESA (the European Space Agency), and the Italian Space Agency. The craft was destroyed by plunging into Saturn in September 2017 when its mission was complete.