On Mars, No One
Can Hear You Scream

By Kim Krieger

ScienceNOW Daily News

12 June 2006

Sound dies quickly in the cold, thin air of Mars.
Researchers have modeled a sound wave traveling
through the Martian atmosphere and report that it
doesn't go far--even a lawn mower's roar dies after
a hundred meters or so. The model presents an
unusually detailed picture of how sound travels in
an alien atmosphere and hints at what it would take
to communicate on the Red Planet.

The shriek of a baby, an ambulance's siren, or a
violin sonata are all essentially the same thing:
waves of pressure traveling through the air. Sound
can also travel through water, or a solid like the
ground, but because molecules must bump into each
other to propagate the pressure wave, the denser the
medium the better. Hoofbeats or footsteps travel
farther through the ground than through the air, for
example, because the molecules in air have to travel
further to bump into one another than those in soil,
thus losing energy more quickly.

The Martian atmosphere is mostly carbon dioxide
and only 0.7% as dense as Earth's is, so sound
should fade more quickly. But the details of how
sound waves travel in the Martian atmosphere were
unclear and could be important to future Mars
missions.

Now, a computer model has given a
molecule-by-molecule map of how sound moves on Mars.
Graduate student Amanda Hanford and physicist Lyle
Long of Pennsylvania State University in State
College presented the model last week at a meeting
of the Acoustical Society of America meeting in
Providence, Rhode Island. The model is unusual in
its molecular approach; most acoustical models of
sound treat the medium it travels through as a
continuous block with average properties. Such
models are fine for dense atmospheres like Earth's,
but treating the air like a loose bunch of
freewheeling molecules is more realistic for Mars'
rarefied atmosphere, say the researchers.

Hanford and Long first set up a virtual "box"
filled with about 10 million carbon dioxide
molecules floating about randomly, at the same
density as the Martian atmosphere. A sound wave then
appeared on one side of the box, and the model
calculated its progress across to the other side,
computing nanosecond by nanosecond exactly how the
carbon dioxide molecules bumped and moved. The
results show that a noise that would travel several
kilometers on Earth would die after a few tens of
meters on Mars. Quieter sounds would travel far
shorter distances, making eavesdropping on a quiet
conversation nearly impossible.

Henry Bass, a physicist at the University of
Mississippi in Oxford, notes that if people ever go
to Mars and want to communicate audibly, they'll
need to design devices that can work with the lower
frequencies transmitted by the Martian atmosphere.