Did you think space was silent?
Well it might be for audio or compression
waves.
But what if you were in orbit around Saturn,
listening for radio and plasma waves?
[Spooky Saturn screaming audio]
The Cassini spacecraft, which orbited around
Saturn for 15 years, had a Radio and Plasma
Wave Science Instrument onboard, that could
pick up what you are hearing now being emitted
by Saturn and its moons.
These radio waves have been compressed for
our benefit, converting roughly 30 minutes
worth of radio waves to a minute’s worth
of audio waves.
This wasn’t just done to create a creepy
audio file, but it had real scientific purpose,
which was to examine the source of these radio
waves.
In the case of this audio file, the source
was Saturn’s aurora.
You see, the Sun is always blasting out bits
of its mass in the form of electrically charged
particles, called the solar wind.
Planets with a magnetic field surrounding
them redirect this solar wind around the planet
to its poles, where the charged particles
impact the planet’s atmosphere, or more
specifically, its ionosphere.
This impact causes particles in the atmosphere
to become excited, or in other words, the
collision ionises the particles in the atmosphere,
causing them to emit electromagnetic radiation.
We can see this ionisation with our own eyes
in visible light, but aurorae also emit radio
waves, which we need special instruments to
detect, like the one that was on Cassini.
These radio waves bounce around within the
planet’s magnetic field, which is where
Cassini picked them up.
How do we know this?
Through another audio file.
[Bow shock audio]
What you are listening to here is the Cassini
spacecraft passing through the bow shock of
Saturn’s magnetosphere.
As I mentioned, Saturn has a large magnetic
field surrounding it, called the planet’s
magnetosphere, which deflects the solar wind
around it.
For the first section of the audio, Cassini
is outside of Saturn’s magnetosphere.
Outside the magnetosphere, radio sources are
quiet and there is calm in the audio.
Then, Cassini’s orbit takes it through the
magnetosphere’s boundary, called the bow
shock, which is the place where the magnetic
field and the solar wind collide.
There is a sudden increase in radio waves.
Past the bow shock, Cassini begins to detect
a lot more radio waves bouncing around Saturn’s
magnetosphere, noticeably different from the
outside of the magnetosphere.
But Cassini heard a lot more than just aurora.
Cassini often made some close approaches to
Saturn, listening closely to the planet itself.
Saturn has a thick and dynamic atmosphere,
with massive storms spanning the planet.
One such storm sounded like this.
[Lightning audio]
What you are listening to here are radio waves
being generated by lightning in one of Saturn’s
storms.
It may sound small and almost puny, but remember
that this is not the representation of audio
you’re hearing, these lightning strikes
are so powerful that Cassini could detect
their radio waves in space.
To give you some kind of scale, scientists
estimate that the lightning in Saturn’s
storms are roughly 1,000 times more powerful
than typical lightning on Earth.
To get this close to Saturn, Cassini had to
pass through its rings.
During flybys like this, Cassini listened.
What do you think could be the cause of this
audio?
[Ring flythrough audio]
Cassini was travelling extremely fast around
Saturn, as fast as 121,000kph.
As it orbited through the Janus-Epimetheus
ring, a very faint and dusty ring, it would
impact tiny particles, which would vaporise
into tiny clouds of plasma.
These tiny charged explosions could then be
picked up by Cassini’s Radio and Plasma
Wave Science Instrument, and these impacts
were then converted into the audio you are
now hearing.
The increased frequency of impacts showed
the densest parts of the ring, and as the
impacts subsided, it indicated that Cassini
was moving away from the ring again.
Cassini tried this again later in the mission,
except this time through a gap between Saturn
and the D ring.
[Second ring flythrough audio]
You’ll notice this time that there wasn’t
a noticeable increase in collisions.
At first, scientist’s thought that this
was because the gaps between the rings were
just really barren of particles.
However, a second orbit through the gap revealed
something different.
[Third ring flythrough audio]
This time, and every orbit after that, there
was a noticeable increase in particle collisions.
So, what happened the first time?
Well, as it turns out, there were particle
collisions, but none of the particles in that
part of the gap were big enough for the RPWS
instrument to detect.
Cassini had another useful instrument onboard,
however, called the Cosmic Dust Analyser,
which could detect particles up to only one
millionth of a millimetre in size, and this
instrument did detect an increase in particle
collisions during the flythrough.
After passing the ring during the second flythrough,
you’ll also notice these rising tones.
Scientists are unsure what causes these exactly,
but it is reminiscent of what Cassini detected
earlier in the mission.
[Unknown rising tones]
The other most interesting source of radio
and plasma waves around Saturn is actually
its moon Enceladus.
Two weeks before Cassini plummeted into Saturn’s
atmosphere, it captured this as it passed
by Enceladus.
[Enceladus audio]
These observations show that Enceladus and
Saturn have a plasma circuit between the two,
much like Io and Jupiter, just perhaps to
a lesser extent.
Although, the strength of this plasma circuit
surprised scientists greatly.
Cassini was also equipped with a magnetometer.
This is what it detected by Enceladus.
[Enceladus magnetic audio]
During the flyby of Enceladus, it was able
to detect a bending of the magnetic field,
indicative of a thin atmosphere, the source
of which is likely to be the jets and cryovolcanism
near the south pole of the tiny moon.
This geological activity draws parallels again
to Jupiter’s moon Io.
The magnetic fields of both planets interact
strongly with the ejected atmosphere of the
moons, creating a flux tube, or a current
of charged plasma particles.
There’s just one more place I want to have
a look at around the Saturn system, and that
is Titan.
Cassini’s first major objective upon reaching
the Saturn system was to launch the Huygens
probe towards Saturn’s biggest and perhaps
most interesting moon, Titan.
Although Huygens didn’t have its own RPWS
instrument, it did have some other instruments.
See if you can guess what this is a recording
of as it descended towards Titan’s surface.
[Huygens radar]
If you guessed radar echoes, well done!
Huygens used this is determine its altitude
above the surface.
And what about this?
[Huygens mic audio]
This is perhaps the most impressive of any
of the clips I have shown so far.
Why?
It doesn’t sound that impressive… until
you realise this is actual audio taken by
a microphone on the Huygens probe as it descended
through Titan’s atmosphere.
This is it leaving the vacuum of space and
diving into another world.
What you are hearing is exactly what you would
have heard if you had been sitting on Huygens
as it descended.
This gives this clip such an awe and perspective
like nothing I’ve heard before.
Somehow its quite incredible for me to think
about.
We often have visuals of other planets, but
I’m really not sure if there’s anything
else quite like this out there.
And there we have it.
The sounds of Saturn and a couple of its moons.
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