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.
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.
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.
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.
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.
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.