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ON MARS, NO ONE COULD HEAR A LAWN MOWER'S SOUND farther than a
couple of hundred feet, compared to the several miles it can travel
on Earth, according to a new computer simulation of sound
propagation on our next-door planetary neighbor. In general, what do
things sound like on Mars?

At this week's meeting of the Acoustical Society of America in Providence,
Amanda Hanford (ald227@psu.edu) and Lyle Long of Penn State presented
detailed computer calculations that simulate how sound travels through the
Martian atmosphere, which is much thinner than Earth's (exerting only 0.7%
of the pressure of our atmosphere on the surface) and has a very different
composition (containing 95.3% carbon dioxide, compared to about
0.33% on our planet).

The loss of 1999's Mars Polar Lander, which was to record sounds directly on
the planet, has compelled researchers to find other means to study how sound
travels there.

To determine the behavior of sound on Mars, the researchers analyzed
how gas molecules move and collide in its atmosphere. The
researchers took into account the gas molecules' mean free path, the
average distance a molecule travels before it collides with a
neighbor (6 microns, compared to 50 nm on Earth). They also
considered the different ways in which gas molecules could exchange
energy when colliding with each other. In their computational
approach, known as direct simulation Monte Carlo, collisions
occurred randomly, though at a statistically accurate rate.
Accounting for the different combinations of molecule species that
could collide along with the many different ways in which they could
lose or gain energy required a huge amount of computation---over 60
hours---even for simulating a small patch of atmosphere for every
sound frequency they considered, using a 32 processor "Beowulf"
computer cluster that was one of the most powerful computers in the
world. With their approach, the researchers could determine all
physical properties of interest in the propagation of sound on Mars.

The researchers' results show that the absorption of sound on Mars
is 100 times greater than it is on Earth, because of the differences
in molecular composition and lower atmospheric pressure. Owing to
computational considerations (they could only analyze collisions
over a relatively small region of space), the researchers only
simulated the propagation of lower-wavelength sounds (with
frequencies in the ultrasound regime) but extrapolated the results
down to audible frequencies.

Meeting paper 2aPA3; more information at:

http://www.acoustics.org/press/151st/Hanford.html

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PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

Number 780 June 9, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi http://www.aip.org/pnu