Thu. Dec 3rd, 2020

Space news: Saturn’s moon migrating from planet 100 times faster than previously predicted

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Titan’s orbit around Saturn is expanding, meaning the moon is drifting away from the planet – at a rate approximately 100 times faster than anticipated. The research indicates Titan was formed much closer to Saturn and migrated out to its current distance of 746,000 miles (1.2 million km) over 4.5 billion years.

Jim Fuller, assistant professor of theoretical astrophysics at the California Institute of Technology (Caltech) and co-author on the new paper, said: “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.

This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed

Professor Jim Fuller

“This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed.”

To better understand the basics of orbital migration, astronomers can use Earth’s Moon for comparison.

Our Moon exerts a small gravitational pull on the planet as it orbits, with this rhythmic tug responsible for tides.


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Frictional processes inside the Earth convert some of this energy into heat, distorting the Earth’s gravitational field so that it pulls the Moon forward in its orbit.

This allows the Moon to gain energy and gradually drift farther away from the planet, at an annual rate of about 3.8cm.

This process is, however, extremely gradual – Earth will not “lose” the Moon until both bodies are engulfed by the Sun in approximately six billion years.

Titan exerts a similar pull on Saturn, but the frictional processes inside Saturn are usually thought to be weaker than those within Earth because of Saturn’s gaseous composition.

Standard theories predict that, because of its distance from ringed planet Saturn, Titan should be migrating away at a slower rate of at most 0.1cm per annum.

However, the newly published results contradict this prediction.

Researchers each used a different technique to measure Titan’s orbit over a period of 10 years.

One technique, called astrometry, produced precise measurements of Titan’s position relative to background stars in images taken by the Cassini spacecraft.

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The other technique, radiometry, measured Cassini’s velocity as it was affected by the gravitational influence of Titan.

Valéry Lainey of Paris Observatory, who worked with the astrometry team, said: “By using two completely independent data sets – astrometric and radiometric – and two different methods of analysis, we obtained results that are in full agreement.”

The results are also in agreement with a theory proposed in 2016 by Professor Fuller, who predicted Titan’s migration rate would be much faster than standard tidal theories estimated.

His theory notes Titan is expected to gravitationally squeeze Saturn with a particular frequency that makes the planet oscillate strongly, similar to how using legs on a swing can drive you higher.

This process of tidal forcing is called resonance locking. The astronomer proposed the high amplitude of Saturn’s oscillation would dissipate a lot of energy, which in turn would cause Titan to migrate outward away from the planet at a faster rate than expected.

The observations both found Titan is migrating away from Saturn at an annual rate of 11cm, more than 100 times faster than anticipated.

Professor Fuller added: “The resonance locking theory can apply to many astrophysical systems.

“I’m now doing some theoretical work to see if the same physics can happen in binary star systems, or exoplanet systems.”

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