Sky & Telescope - September 2021 - 21

of waves traveling through the planet's gaseous interior.
As the moons orbit the pulsating planet, their orbital periods
by chance might resonate with the frequencies of some of
those pulsations. When that happens, the resonance amps up
the volume. " It's like the moon is pushing on Saturn just at
the natural frequency of the note it wants to play, so it makes
the planet play a very loud note, " Fuller says.
Once a moon's orbit hits the resonant frequency of the
oscillations inside the planet, it will suddenly find itself
migrating outward much faster, like a surfer catching a wave.
The moon rides that wave, its orbit and Saturn's interior
evolving in lockstep until the wave eventually breaks.
There are two related implications to come out of this
resonance locking. First, just because a moon is migrating out
quickly now doesn't mean it always has been - the dissipation
of tidal energy isn't constant over time.
Second, the friction that tides generate still depends on
distance - but in the opposite way than previously thought.
While classical theory would tell you that farther moons
should migrate outward more slowly, resonance locking
instead suggests that farther moons migrate outward faster.
In terms of an individual moon, that suggests its rate of
migration was slow to begin with, but it sped up once it
caught the resonant wave.
The expected difference in a moon's behavior depending
on its distance from Saturn became a test, one that resonance
locking passed with flying colors.
" One of the predictions of our paper was that Titan might
be migrating out much faster than people expected, " Fuller
says. And that's exactly what Lainey found. " In fact, [Lainey]
said he'd already seen it, but he didn't believe the results until
he looked at the predictions, " Fuller adds.
In 2020, Lainey and collaborators reported in Nature
Astronomy that Titan was receding from Saturn at a surprising
rate of 11 cm per year - 100 times faster than previously
thought. Another team, including Neveu, is now looking to
confirm that result independently.
The new understanding of tides caused the scientists
involved to re-evaluate the midsize moons' origins: While
their fast migration rates had pointed to young ages, now
they only mean the moons are migrating fast today - the
past is a different story. Likewise, theorists can no longer use
predicted moon-moon resonances to date the moons, unless
those simulations also take into account the effect of varying
tides on orbital matchups.
But does that mean all of the moons really are ancient? " I
don't think it provides a definitive answer either way, " Fuller
muses, " but it allows them to be older than people have previously
assumed. "
The Way Forward
While the new understanding of tides has completely
changed the study of the Saturnian system - and even rendered
some previous studies obsolete - it hasn't settled the
debate. If anything, there's only more to consider now.
25
Titan
20
15
Tidal dissipation varies
Constant tidal dissipation
10
Dione
Rhea
5
Enceladus
Tethys
Mimas
F ring
Saturn
1
2
3
Time in billions of years
100,000
10,000
1,000
Tidal dissipation (Q)
CHANGING TIDES This plot shows how the evolving " stickiness " of
gas within Saturn affects the orbits of its moons. Many of the midsize
moons are migrating outward fast enough right now that, if time were
rewound, those moons would fall within the rings not that long ago. But
that's only true if Saturn's tides always dissipated the same amount of
energy. If tidal dissipation evolves with time, then migration rates might
have been slower in the past, and the moons could be much older.
Young moons still have adamant advocates. Pointing to
the observations of Titan's migration, Ćuk says, " Every time
you add motion to the system, it makes old moons harder to
have. " Neveu also notes that changing tides could make collisions
between moons more likely. And even Fuller's calculations
show that some moons might yet be young.
Putting his idea into detailed numerical simulations will
be difficult, Fuller says, because no one really understands
Saturn's interior or how it has changed over time.
But there are ways to chip away at the problem. Several
groups still work with the scads of exquisite data that Cassini
returned during its years at Saturn. Fuller, for example, is
using those data to turn the rings into a kind of seismometer
to probe the planet's interior. Others continue to examine
(and re-examine) the moons' migration.
Meanwhile, theorists are trying to find ways to incorporate
the new physics into their simulations - it's an unexpected
wildcard that has changed the game. But while this beautifully
weird system may be keeping its cards close, that doesn't
mean we'll stop playing.
¢ News Editor MONICA YOUNG, award-winning science journalist,
is on a first-name basis with Saturn's eclectic moons.
sk yand tele scope .o r g * SEPTEMBER 2021 21
100
4
5
GREGG DINDERMAN / S&T; SOURCE: LAINEY ET AL. /
NATURE ASTRONOMY 2020
Average distance from Saturn (in Saturn radii)
https://skyandtelescope.org/
Sky & Telescope - September 2021

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