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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