Cassini’s mission to Saturn may have ended years ago, but scientists analyzing the data sent back by the probe continue to draw new insights about the sixth planet from the sun and its system of moons. We now know that Titan, Saturn’s largest moon (and one of its most interesting) is moving away from the planet 100x faster than predicted.
Some of you may be aware that the Moon is slipping the bonds of Earth, migrating outwards by about 3.5cm per year. While this would technically lead to the Moon escaping our gravity well eventually, that’s unlikely to happen in reality: The sun is expected to engulf both the Moon and Earth before it can occur.
Historically, it was believed that the moons around Saturn had formed more-or-less in their current positions and migrated over time. Data collected by Cassini over ten years through both measuring Titan’s precise position and its velocity shows that the Moon is pulling away from Saturn at about 11cm per year — substantially more than the 3.5cm increase in the Earth-Moon gap.
“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,” Caltech’s Jim Fuller, assistant professor of theoretical astrophysics and co-author on the new paper, told Phys.org. “This implies that the Saturnian moon system, and potentially its rings, have formed and evolved more dynamically than previously believed.”
Quadruple Saturn Moon Transit: Hubble snapped this pic of four of Saturn’s moons (Titan, Mimas, Dione, and Enceladus) transiting the planetary disk in 2009.
One of the major finds of Cassini was evidence that Saturn’s ring system is similarly much younger than believed. Initially, it was thought that Saturn’s rings formed at the same time our solar system did. Cassini data shows that they may be much younger, having formed roughly 100 million years ago, when the dinosaurs walked the Earth. That finding has, in turn, produced a backlash from scientists who believe there’s good reason to question Cassini’s data, so the science is still ongoing — but the idea of younger rings at least thematically fits the idea of a more dynamic Saturnian system.
The same processes that are pushing the Moon away from Earth are responsible for the Titan / Saturn shift. Titan creates a tidal bulge on Saturn the same way the Moon creates tides on Earth. Because the Earth rotates, this bulge sits “ahead” of the Moon and pulls it forward. This also raises the Moon’s orbit, resulting in it moving away from us ever so slightly over time.
Because Saturn is a gas giant, the tidal forces within it had been thought to be much weaker than on Earth, with a net movement of ~0.1cm per year for Titan. This new data, published in Nature, contradicts that finding and shows a movement rate of ~11cm per year. Titan’s rate of movement may have changed over time (the Moon’s has), but this is 100 times faster than previously expected.
The reason for this discrepancy is a phenomenon called resonance locking. Essentially, it’s thought that the moons of Saturn and Jupiter may have orbited in tidal resonances that squeezed the planet’s at particularly effective points in their own oscillations, magnifying the amount of energy transferred to Titan and accelerating its departure speed. Think of this as being analogous to swinging your legs on a swing as a means of driving yourself higher. If you swing at the wrong point, you’ll slow yourself down. Swing at the right point, and you can continue to drive the swing upwards.
This concept has implications for non-planetary systems as well, including potentially explaining features of certain binary star orbits. It’s also a great example of how the probes we’ve sent to other planets continue to provide us with material insights into the worlds around us, long after they’ve plunged to their deaths.
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