June 9, 2020 : A New Study Suggests That Titan Is Drifting Away From Saturn Much Faster Than Previously Believed

A new research work entitled « Resonance locking in giant planets indicated by the rapid orbital expansion of Titan », published on June 8, 2020 in Nature Astronomy and led by Valéry Lainey reveals that Saturn's largest moon Titan is drifting away from the Gas Giant faster than previously believed. The new finding represents a major clue for the evaluation of the potential age of the giant moon and leads to a new approach or theory regarding the influence of the gravitational interactions of the numerous moons of the Ringed Planet and Saturn. The progressive outward migration phenomenon of Titan is also observed in the Earth/Moon System in which the Moon is progressively drifting away from the Earth. However, Titan is drifting away much faster from Saturn than the Moon from the Earth. Thanks to lasers in particular, we have been in a position to determine that the Moon drifts 1.5 inches or 3.8 centimeters from the Blue Planet each year. Titan apparently evolves approximately 4 inches or 11 centimeters farther from Saturn each year compared to the previous year. Yet, the mean distance between Titan and Saturn is much higher than the mean distance between the Moon and the Earth.

Planetologists previously believed that they knew the rate at which the Opaque Moon is moving away from the Ringed Planet. However, the new study based on data acquired from the Cassini spacecraft during its long mission in the Saturn System reveals that they had clearly underestimated the extent of the progressive enlargement of Titan's orbit around Saturn. In fact, the group of researchers involved in the new research work has managed to determine that the giant moon is moving away from Saturn a hundred times faster than previously evaluated. The moons observed in the Solar System tend to migrate outward from their host planet due to phenomena related to the various gravitational interactions between the moons and the host planet and between the different moons. During the orbital dance of the moon around the Gas Giant, gravitational forces from the moon engender a temporary bulge in the Gas Giant. The rotation speed of Saturn or Jupiter is particularly fast so that the bulge in the Gas Giant engendered by the moon will constantly move. One can imagine that the gravity of the moon is likely to engender waves or atmospheric movements within the Gas Giant.

Saturn is likely to undergo relatively significant phenomena of bulging and distortion. The energy generated by those phenomena is transfered from the Gas Giant to the moon so that the orbit of the moon progressively increases. The new determination of the expansion rate of Titan's orbit has potential implications regarding the age of Titan, the other moons of the Gas Giant and its famous rings. Currently, the consensus regarding the history of Saturn is that it formed 4.6 billion years ago during the formation or development process of the Solar System. However, did the major moons of Saturn form in the same period ? Planetologists have an insufficient amount of clues to conclude that the more than 80 moons of Saturn formed at the same time during the formation process of the Gas Giant. At the present time, the mean orbital distance of Titan around Saturn is 1,221,870 km or about 759,235 miles. The orbit of Titan around Saturn and around the Sun is far from being perfectly circular which implies particularly complex interactions between Saturn, Titan, the other moons and the Sun. As a rule, the orbit of the moons or planets tends to be elliptical.

The new determination of the expansion rate of Titan's orbit, which is largely higher than previously thought, implies that the Orange Moon took shape much closer to Saturn than previously believed. The new finding also implies that the system of moons evolving around the Ringed Planet expanded faster than previously thought. The observation of the current dynamics of Saturn moons is likely to tell us a lot regarding their future evolution. Can instabilities take shape over time ? Valéry Lainey who performed the research work as a scientist at NASA's Jet Propulsion Laboratory in Southern California before integrating the Paris Observatory at PSL University pointed out : « 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. » The new determination upon the outward drift of the Opaque Moon can be regarded as a discovery since it confirms and fuels a new theory upon the dynamics of moons around a planet or Gas Giant. The new theory or approach explains and predicts how the planet gravitationally interacts with its moons. The evolution of the orbit of the natural satellites appears more complex to anticipate than we previously believed.

For the last 50 years, researchers have mobilized the same formulas to evaluate the potential speed or rate at which the moons drift away from the host planet in their orbital dance over a long period of time. Planetologists are in a position to evaluate the potential age of any moon on the basis of the speed at which it drifts away from the host planet. Prior to the new outcome, the prevailing theories assumed that the moons which evolve farther from the host planet are migrating outward from the host planet at a slower pace due to the reduced force of gravity exerted by the host planet at a higher orbital distance between the host planet and the natural satellite. Therefore, on the basis of the previous theories or on the basis of the formulas of the classical theories, a moon like Tethys which evolves closer to Saturn than Titan was supposed to drift away at a faster pace than Titan because the celestial body undergoes a higher level of gravitational force from the Gas Giant. The mass of the moons as well as the mass of the planet play a key role in the evolution of the dynamics of each moon. It turns out that the previous formulas or theories have their limits to predict the orbital evolution of the moons.

A study performed by Jim Fuller, a theoretical astrophysicist who works now at Caltech, had revealed, four years ago, a prediction that contradicts the prediction related to classical theories upon the orbital evolution of the moons in the long run. The prediction of the researcher is characterized by a configuration in which the outer moons will tend to migrate outward at a similar rate to inner moons due to the fact that the natural satellites become locked in a different type of orbital pattern that is related to the particular oscillation of the host planet. That system allows a relatively uniform outward movement of the various moons. Jim Fuller pointed out : « The new measurements imply that these kind 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. » The dynamics of the complex system of moons around Saturn is likely to tell us a lot regarding the dynamics or the evolution of the system of moons of Jupiter, the system of moons of Uranus, the system of moons of Neptune or the system of moons of the Dwarf Planet Pluto.

The determination of a relatively high rate of outward migration of Saturn's largest moon was made possible thanks to the mapping of stars in the background of images obtained from the Cassini orbiter in conjunction with the tracking of the position of the Opaque Moon. The team of researchers confirmed its findings by comparing them with an independent dataset in the form of radio science data captured by the Cassini probe during its long mission in the Saturn System from 2004 to 2017. During ten close flybys of the Orange Moon between 2006 and 2016, the Cassini orbiter sent radio waves to the Blue Planet and researchers analyzed how the signal's frequency was modified by their interactions with their environment to evaluate how the orbit of Saturn's largest moon evolved. Paolo Tortora, a collaborator of the study who is from the University of Bologna in Italy, who is a member of the Cassini Radio Science team and who worked on the study with the support of the Italian Space Agency pointed out : « 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. »

We often say that the Saturn System represents, to a certain extent, a mini Solar System since its moons evolve around Saturn on, roughly, the same plane. A parallel can be drawn between Titan around Saturn and Jupiter around the Sun. Titan is the most massive body evolving around Saturn whereas Jupiter is the most massive body evolving around the Sun. A parallel could also be drawn between the rings of Saturn and the Asteroid Belt between Mars and Jupiter. Gravitational interactions between Titan and the rings of Saturn and between the Gas Giant and its rings have already been identified. The structure of the system of moons around Saturn implies that those moons must have formed at roughly the same time in the history of Saturn or in the history of the Solar System. The images obtained from the Cassini spacecraft during its mission in the Saturn System lead some researchers to suggest that tiny moons may regularly develop within the rings of the Gas Giant. From the rings of Saturn to Titan, one can observe that the size or the mass of each major moon is higher than the size or the mass of the major moon that is closer to Saturn. Thus, that's the case for Mimas, Enceladus, Tethys, Dione, Rhea and Titan. Will those moons leave the Saturn System one day ?

The image above revealing Titan, Saturn and its system of rings represents a mosaic of 6 images obtained on May 6, 2012 with the Wide-Angle Camera of the Cassini spacecraft. The natural color view was generated on the basis of data taken at a distance of about 483,000 miles or 778,000 kilometers from Titan. Titan currently evolves more than three times farther from Saturn than the Moon from the Earth. Image credit: NASA/JPL-Caltech/Space Science Institute.

- To get further information on that news, go to: https://www.nasa.gov/feature/jpl/saturns-moon-titan-drifting-away-faster-than-previously-thought and https://www.nature.com/articles/s41550-020-1120-5.



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