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You may think you know Saturn. Its iconic rings are the largest in the Solar System, and its hazy
yellow surface is both enigmatic and instantly recognisable. It is the 2nd largest planet, the
least dense, and the 6th from the Sun. And yet, Saturn is a planet that is baffling scientists.
There is a vast mystery lurking beneath its obscuring atmosphere, uncovered by
the CASSINI probe; one that the scientific community still does not have consensus about.
Saturn behaves in ways that our conventional models claim is impossible, from its temperature,
to its magnetosphere, to even the very length of its days. What is going on with
Saturn? And what are our best attempts at explaining these baffling phenomena?
I’m Alex McColgan and you’re watching Astrum. And today, over 5 years after the CASSINI mission
ended, we are going to explore the strange questions CASSINI’s data raised about Saturn.
The CASSINI probe arrived at the gas giant on July 1, 2004. It was not the first probe to
arrive at Saturn – 3 others had already done flybys. However, it was the most thorough,
as unlike its predecessors, CASSINI stayed around the planet for 13 years before ultimately running
out of fuel and being deliberately crashed into Saturn’s atmosphere. In that time, it collected
reams of data – taking 453,048 images, along with countless readings from its various spectrometers,
magnetometers and other equipment. In that time, it captured information about Saturn’s rings,
its many moons, and its atmosphere. It built up a thorough understanding of Saturn’s magnetic field,
and even took the best images so far of Saturn’s mysterious hexagon storm on its north pole.
The edges of this 29,000km wide storm could each fit the Earth comfortably inside,
and as best we are able to tell does not shift its longitude, remaining fixed in its location,
travelling with the rotation of the planet; unlike the rest of Saturn’s swirling clouds, pushed along
by its up to 1800km/h winds. This storm in and of itself is a strange one, defying our common sense
and scientists’ best explanations as to how such a storm could be formed (although there are some lab
experiments that have created close approximations to hexagons on much smaller scales).
But as the mountains of CASSINI data started getting analysed in the years since the end of its
mission, more mysteries have started coming in. It started with a fairly innocuous question:
How long is a day on Saturn? Although we’ve watched Saturn in our sky for thousands of years,
scientists did not yet have a definitive answer. After all, it wasn’t as simple
as looking up at the planet and seeing how long it took for its rocky core to rotate.
Saturn’s thick cloud cover obscures any landmarks that might exist on any hard surface below,
and those clouds move at different speeds to the core, due to the powerful wind.
The rings themselves orbit Saturn at different speeds, so they don’t settle
this question – you can’t look at how fast they orbit and hope it’s the same.
After all, most moons don’t do this, so rings are no different.
Rather cleverly, scientists do have techniques for figuring out the rotation of a gas giant
by looking at its magnetic field, and it was hoped that this method might be used on Saturn.
Planets with magnetic fields produce them via a dynamo effect. As the planet rotates, liquid metal
in the core spins and shifts, and this colossal motion of molecules create massive currents of
energy, which in turn produce magnetic fields. One feature of this is that, thanks to this rotation,
the pole of this magnetic field is always off from the axis of rotation for the planet itself.
If the two lined up, the magnetic field wouldn’t survive for long.
So, all you have to do is detect the magnetic fields around a planet – which CASSINI could
do – and watch as the magnetic pole shifts in a circle around the true axis of the planet.
Once the magnetic pole has completed one rotation, you have your day. Only,
Saturn apparently does not play by the rules, as its magnetic pole is almost perfectly aligned with
its axis of rotation, to an order of accuracy of less than 0.1 degree. And in spite of dynamo
theory claiming that this should be impossible, Saturn’s magnetic field is alive and well.
Its magnetic moment is 580 times more powerful than Earth’s, and is extremely influential
on the Saturn system as a whole. Scientists do not know how this magnetic field occurs,
as it is impossible under dynamo theory, but no other method for producing such a field is proven.
Either a planetary dynamo IS at play in Saturn’s core, but some other effect is occurring in the
atmosphere to warp the magnetic field lines to be perfectly aligned with the axis of rotation, or we
are observing a completely new method of producing a magnetic field on Saturn. So, it was back to
the drawing board for solving a Saturn day. Fortunately, radiation proved to be of more help.
Saturn is an electrically live system. Charged ions move between the layers within
its atmosphere, and even between its rings and its ionosphere. Scientists noticed that Saturn
emitted radio waves as a result of all of the magnetic fields at play, and these radio waves
rose and fell in intensity. In fact, it seemed to mirror what you might expect to see as a
result of the planet rotating – higher levels of radiation would appear every 11 hours or so.
As a result, scientists concluded with some certainty that this was Saturn’s day-length.
Their initial estimate based on Voyager data was 10 hours, 39 minutes and 23
seconds. It was hoped that CASSINI would be able to improve the accuracy of this figure.
Initially, CASSINI was able to do so. But then it got really weird, because over the
course of the 13 years of CASSINI’s study of Saturn, this number started to shift.
It drifted about 1 percent over the course of a year – sometimes rising, and sometimes falling.
This almost seemed to imply that the planet itself was altering the speed of its rotation;
sometimes speeding up, and sometimes slowing down.
To be clear, a planet as massive as Saturn should not be doing that. As such, scientists
concluded that this wasn’t happening, and some effect must be once again muddying the waters
around Saturn. Some aspect of the atmosphere was causing shifts in the fields, making the
radiation fluctuate over time, hiding the true rotation of the planet beneath a cloaking shimmer.
Another key consequence of the planet’s magnetic field is the auroras that shine at Saturn’s poles.
But while on Earth our auroras are the results of the solar wind interacting with our atmosphere,
it is thought that this does not account for the auroras on Saturn.
CASSINI detected that at least some of them occurred regardless of what the
solar wind was doing at the time, meaning that Saturn’s auroras are non-solar originating.
Something strange must be going on inside Saturn. It was hotter than it ought to be, too. It
radiates out into space about twice as much as it receives from the Sun. And while some of this may
just be gravitational compression of the planet, the rest needed some other way to account for it.
Ultimately, all of this needed some unifying explanation. And fascinatingly,
the key to all of it may lie within Saturn’s gaseous atmosphere.
Saturn is a gas giant. It is almost entirely made of hydrogen and helium, with some other trace
elements mixed throughout. Not all of this is gas, as the great mass of Saturn means that the deeper
into its atmosphere you go, the more intense the heat and pressure that you are subjected
to. It is thought that this pressure forces the gaseous hydrogen to become liquid metal.
Some scientists theorise that this liquid hydrogen, or perhaps even hydrogen compressed
so densely that it becomes diamond, rains down into the depths of Saturn, and the friction this
generates explains some of Saturn’s unusual heat. But the rest comes from the auroras.
Although these are not fully understood, it is thought that they are the product of
electrically charged particles coming into Saturn from its rings and moons. Whatever their source,
these auroras may be creating enough heat to warm the upper atmosphere of the planet.
Temperatures so hot create other effects. Where warm fronts meet cold, powerful winds are
formed. It is theorised that these winds carry charged ions around Saturn’s upper atmosphere,
and these electrically charged winds are what is skewing the data from Saturn’s radiation,
causing its variance. Perhaps such motion of electrical currents might also explain
why Saturn’s magnetic fields are not where they are supposed to be. This could be caused by a
conductive layer within Saturn’s clouds moving at a different speed to the others, creating fields
of their own. At least, so goes one explanation. In all this, it should be clear that many of
these answers lack final proof. Scientists are trawling through the data given by CASSINI,
hoping for further insights. If these are not forthcoming, then ultimately it will
take another mission to Saturn to find the answers to these puzzling questions.
But what about the length of Saturn’s day? Thankfully, this is one mystery that does
have an answer. And it turned out the clue lay in the rings after all.
Rather than investigate the electrical and magnetic effect of Saturn on its rings,
scientists realised that Saturn would also tug at them through its gravity. This is fairly obvious,
but what was more insightful was that Saturn would not tug at them uniformly. Saturn is not
a perfectly round ball. Any variance in its shape result in variance in its gravitational field.
Saturn’s rings were a perfect canvas to look out for such variance. As Saturn pulled at
its rings with rising and falling force, at certain locations in the rings, we would be
able to detect ripples. Scientists started looking for these ripples, and located them.
By measuring the distance between them, it was finally possible to calculate the length
of Saturn’s day. They weren’t far off. It was 10 hours, 33 minutes and 38 seconds. And this time,
scientists are fairly certain that they’ve got it right. So at least this mystery was solved.
And yet Saturn’s other mysteries still persist. The next mission to Saturn will launch in 2027
with NASA’s Dragonfly, although this will focus more on exploring Titan, Saturn’s largest moon.
Titan is the most Earth-like object in the solar system as it is the only place thought
to have atmosphere filled with nitrogen, similar to ours (albeit with no oxygen),
and rain cycles and liquid lakes on its surface (although these are methane rather than water).
So it’s understandable why scientists want to give it a look. Still, it means
that it may be some time before Saturn’s deepest mysteries find themselves answered.
Saturn breathes. The winds that might prove to be the lifeblood of the system thrum and flow
across its atmosphere. Its delicate balance of magnetic fields and gravity coax at its rings,
keeping them perfectly aligned. Saturn’s rings are thought to be surprisingly young. Saturn may be
over 4 billion years old, but there is evidence that the rings may have only formed in the last
100 million years. And they are slowly draining back down into Saturn’s atmosphere, so in another
100 million years they may be gone. This is sad to me, but I also feel strangely fortunate. If
humanity hadn’t arrived when we did, we might have missed this beautiful cosmic sight. It might be
unscientific to say that, as a planet has no real choice in whether it gives us a gift or not. But
I’m nonetheless grateful to have been witness to the enigmatic and wondrous world that is Saturn.
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