don't go

Titan is drifting away from Saturn 100 times faster than predicted

The moon and its host planet don't have that much time left together.

NASA/JPL-Caltech/Space Science Institute

Out of the 150 known moons in the Solar System, Titan is pretty special. Saturn's largest moon is the only other place besides Earth known to have rivers, lakes, and seas -- though they are made of flammable greenhouse gases.

We'd hate to see Titan go.

Unfortunately for us, this chaotic lunar world is drifting away from its host planet at a much faster rate than astronomers had originally believed, around four inches each year. That's 100 times faster than previous predictions.

Using data from NASA's Cassini mission, a team of scientists made the surprising discovery which suggested that Titan started off much closer to Saturn and that the whole system of the planet's 80 moons expanded much quicker than previously believed.

The results were detailed in a study published this week in the journal Nature Astronomy.

A view of Titan, hidden behind Saturn's rings.NASA

"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, a scientist at NASA's Jet Propulsion Laboratory in Southern California and lead author of the study, said in a statement.

Why do moons migrate? All moons drift away from the planets they orbit. Even our own Moon that shines down on us every night is slowly getting further away from Earth, drifting 1.5 inches each year. However, this slight movement will take millions of years before it starts affecting us.

Moons form in many different ways, either at the same time as its host planet, through a collision or it was floating through space as a rock when it was captured by the gravity of a planet. Moons are kept in their orbit through this gravitational tug between the planets and their natural satellites. However, over time, this tug nudges the moon further and further away from its host planet.

As it stands today, Saturn has 82 moons. Previous theories have suggested that planets with this enormous number of natural satellites will push out their outer moons, such as Titan, at a much slower rate than the moons in the innermost orbits since they are the furthest away from the planet's gravitational pull.

However, this new finding suggests that outermost moons can in fact be pushed further away at a similar rate to the innermost moons.

In order to measure Titan's rate of migration, the scientists behind the new study mapped stars in the background of images obtained by Cassini, and tracked Titan's position in these images.

They then compared the data with radio waves that the spacecraft had sent to Earth between 2006 and 2016, tracking how the signal's frequency changed over time by their interactions with their surroundings in order to map out the evolution of Titan's orbit.

Cassini launched on October 15, 1997, and entered Saturn’s orbit on June 30, 2004. During its 20-year mission, the spacecraft conducted nearly 300 orbits of Saturn, and took more than 453,000 images and sent back 635 gigabytes of data.

Abstract: Saturn is orbited by dozens of moons, and the intricate dynamics of this complex system provide clues about its formation and evolution. Tidal friction within Saturn causes its moons to migrate outwards, driving them into orbital resonances that pump their eccentricities or inclinations, which in turn leads to tidal heating of the moons. However, in giant planets, the dissipative processes that determine the tidal migration timescale remain poorly understood. Standard theories suggest an orbital expansion rate inversely proportional to the power 11/2 in distance1, implying negligible migration for outer moons such as Saturn’s largest moon, Titan. Here, 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, corresponding to a tidal quality factor of Saturn of Q≃ 100, which is more than a hundred times smaller than most expectations. Our results for Titan and five other moons agree with the predictions of a resonance-locking tidal theory2, sustained by excitation of inertial waves inside the planet. The associated tidal expansion is only weakly sensitive to orbital distance, motivating a revision of the evolutionary history of Saturn’s moon system. In particular, it suggests that Titan formed much closer to Saturn and has migrated outward to its current position.
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