Transcript of Bruce Betts's Audio Explanations
Sound in Space
Sound is basically pressure waves that are moving through some
medium. So the sound that we usually hear is going through the
air. And it's transmitted as what are called "longitudinal
waves." So they push back and forth kind of like a slinky if
you push it back and forth. And what that does is in our ears
it vibrates the eardrum. So you get no sound if you're in the
vacuum of space, but you get sound if you're either in air or
in a solid or liquid. How loud something is at a given
distance, or what frequencies of the sound—whether high
frequencies or low frequencies—get damped out or go away
faster will depend on the medium you're in.
Huygens' Descent
So basically what you are hearing is the sound of wind noise
transitioning to a lack of wind noise. The Huygens probe had
an acoustic sensor, basically a microphone, to try to detect
thunder if you actually had lightning. The acoustic sensor
would sample two-second intervals in certain frequencies and
create this kind of average sound that changed as you
descended through the atmosphere. And so this is a transition
where you hear something real in this data, despite the fact
that it's sounding like Crrrrrrrrrrrrr. It actually
goes from that to something very silent, which does represent
when the lander landed a billion miles away, by far the
farthest we've ever transmitted something resembling sound in
our solar system.
Carrier Signal
When you're sending data from a spacecraft you will have a
so-called carrier signal. It's basically broadcasting a
tone—if you think of it in audio terms—
broadcasting as strong as it can get signal at one particular
frequency. And that they were able to pick up from Earth, it
turns out. And then Cassini used it to lock onto Huygens.
You can do one critical piece of science with the carrier
signal alone and that is measure the Doppler shift of that
signal—the old oncoming train concept, where you hear
Eeeeeeeeeeoooooooow. So the frequencies change if
something's going away from you or coming towards you, and it
depends on velocity. So they were able—even from
Earth—this just amazed me, using large antennas on
Earth, to analyze the carrier signal and how the frequency
shifted, and from that get an idea of the velocity of the
Huygens probe as it went through the Titan atmosphere and how
it changed with altitude, for example, and with shifting
winds. It's just amazing at those distances.
Radar Altimeter
What you're hearing is the Huygens' radar altimeter data that
has been converted to sound. Basically, as the spacecraft came
down, they bounced radar off the surface to figure out how far
down it was before they were going to touch down. And so what
you're hearing is, they've coded this as related to altitude,
and you're hearing it as it approaches the surface.
Radio Ear
Huygens did not have the capability to transmit data back to
Earth. And it relied on the Cassini spacecraft flying
overhead. Instead of having to carry something big enough to
transmit the billion miles, it sent things to Cassini to send
things back the billion miles. So this is some very creative
interpretation of how strong the signal was coming from the
Huygens lander to the Cassini spacecraft.
And so it's combining the different factors of signal strength
and frequency into sound to give some idea of the probe going
through the atmosphere. But it's something really, really
unrelated to sound in its initial physics.
"Titanized" Sound
Titan is the only moon in the solar system that has a big,
thick atmosphere. And it actually has a pressure on the
surface that's comparable to Earth. It's about one and a half
times the surface pressure on Earth. But it is also mostly
nitrogen, just like the Earth's atmosphere is mostly nitrogen.
But it is down at temperatures that are something like 100
Kelvin, so, like -300°F—so extremely
cold. Having that cold temperature actually ends up leaving
you with sound traveling sounding very similar to what the
surface of Mars sounds like.
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