Saturn's moon Titan is drifting away 100 times faster from the planet than previously believed, say scientists

The revised rate of its drift of 11 centimeters per year suggests that the moon started out much closer to Saturn, which would mean the whole system expanded more quickly than previously believed


                            Saturn's moon Titan is drifting away 100 times faster from the planet than previously believed, say scientists
(NASA/JPL-Caltech/Space Science Institute)

Scientists thought they knew the rate at which the giant moon Titan is moving away from Saturn, but they have now made a surprising discovery. Using data from NASA's Cassini spacecraft, researchers found that Titan is drifting a 100 times faster than previously understood, about 11 centimeters (4 inches) per year. According to standard theories, Titan should be moving away from Saturn at a rate of 0.1 centimeter per year.

Saturn formed 4.6 billion years ago in the early days of the solar system. The new analysis suggests that Titan formed much closer to the planet during its birth, and has migrated outward to its current position, which is a distance of 1.2 million kilometers (759,000 miles) from Saturn. "The revised rate of its drift suggests that the moon started out much closer to Saturn, which would mean the whole system expanded more quickly than previously believed," said the study published in Nature Astronomy.

According to experts, the findings may help address key questions regarding the formation of the planet's rings and its system of over 80 moons. "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," said Valery Lainey, lead author of the study, in a statement. He conducted the research as a scientist at NASA's Jet Propulsion Laboratory in Southern California before joining the Paris Observatory at PSL University.

Saturn’s largest moon Titan is the second-largest moon in the solar system and is larger than the planet Mercury. Among our solar system’s more than 150 known moons, Titan is the only one with a substantial atmosphere. According to scientists, of all the places in the solar system, Titan is the only place apart from Earth known to have liquids in the form of rivers, lakes and seas on its surface. Titan orbits Saturn, which orbits the Sun at a distance of about 1.4 billion kilometers (886 million miles), about 10 times farther from the Sun than Earth's orbit.

Using data from NASA's Cassini spacecraft, researchers found Titan is migrating away from Saturn at a rate of 4 inches or 11 centimeters per year. (Getty Images)

After 20 years in space, 13 of those years exploring Saturn, Cassini exhausted its fuel supply and plunged into Saturn's atmosphere on September 15, 2017. For the analysis, scientists mapped stars in the background of Cassini images and tracked Titan's position. To confirm their findings, they compared them with an independent dataset: radio science data collected by Cassini.

During 10 close flybys between 2006 and 2016, the spacecraft sent radio waves to Earth. Scientists studied how the signal's frequency was changed by their interactions with their surroundings to estimate how Titan's orbit evolved. "We use two independent measurements obtained with the Cassini spacecraft to measure Titan's orbital expansion rate. We find that Titan rapidly migrates away from Saturn on a timescale of roughly ten billion years," says the study.

The Earth's moon drifts 3.8 centimeters (1.5 inches) from it each year. Other moons are doing the same with their host planets. As a moon orbits, its gravity pulls on the planet, causing a temporary bulge in the planet as it passes. Over time, the energy created by the bulging and subsiding transfers from the planet to the moon, nudging it farther out.

The findings on Titan's rate of drift also provide important confirmation of a new theory that explains and predicts how planets affect their moons' orbits. For the last 50 years, scientists have applied the same formula to estimate how fast a moon drifts from its planet, a rate that can also be used to determine a moon's age. Those formulas and the classical theories on which they are based were applied to moons large and small all over the solar system. The theories assumed that in systems such as Saturn's, with dozens of moons, the outer moons like Titan migrated outward more slowly than moons closer in because they are farther from their host planet's gravity. A study upended these theories four years ago. 

Theoretical astrophysicist Jim Fuller predicted that outer moons can migrate outward at a similar rate to inner moons because they "become locked in a different kind of orbit pattern that links to the particular wobble of a planet and slings them outward." The new results are in agreement with Fuller's theory, which predicted a much faster migration of Titan.

"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," said Fuller, who is a co-author of the current study.

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