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

So, Mars is the better place to set monasteries and the like.


Recording sounds on other planets would yeld little science data, but it would have an overwhelming impact on people here on Earth, perhaps more than images. For instance when a dust devil passed right on Spirit, there was very probably some sound to record.

A little known fact on Earth is that sounds can travel hundred of kilometres, thanks to a change of the propagation speed into the stratosphere, which acts a bit like a waveguide. It is difficult to extrapolate to mars, but eventually such a phenomenon could occur here, or on any other planet. From here the interest to have a sound recording, which would catch wind, meteorites, earthquakes, etc. (In infrasound, the noise of large meteorites can travel to thousand kilometres, on earth).

Another property is that the timbre of sounds depends on the chemical composition of air. More precisely, of the number of atoms the molecules have. When people speak in helium (1 atom), their voice gets a "donald duck" timbre, compared to their normal timbre in air (2 atoms). In C02 (3 atoms), there would be a reverse effect (but don't try to speak in CO2!!) giving eventually a special Mars sound. On Venus also, but here sounds could eventually travel very far, due to high density, low attenuation, and eventually reflections into the upper atmosphere. Any probe on Venus should have a microphone, and eventually several probes with each a microphone could allow to spot noisy events (thunder, volcanoes...)

That's a good idea! Ksanfomality's GROZA experiment on Venera-13 had a seismometer and microphone and a radio wave detector. But I believe the microphone was just to listen to wind noise, to estimate velocity. And it only returned samples of average sound amplitude. But it is a good combiation of detectors, because a volcanic eruption should produce seismic, sonic and maybe atmospheric-electric discharges.

They really need a "Venera-Glob" like mission, a network of seismometers to "CAT-scan" the planet like we do the Earth. Maybe drop a big bunker-buster bomb or a nuke then, to get some controlled readings.

I always imagined that sounds on Venus would be a little like when you go underwater in a swimming pool. I thought about doing a simulation, modeling how the terrain reflects sounds. And I think the pressure gradient of the atmosphere would give you a ground-hugging effect, so not only is the high-pressure atmosphere conducting sound, but you would get a 1/r instead of 1/r**2 effect.

The effect of the type of gas depends on quantum mechanical effects. Helium is strange because it is made up of single He atoms, not diatomic molecules like O2 or N2. This means there are no vibrational or rotational energy states. CO2 is a little slower than air, but nothing as dramatic as the helium effect (where the speed of sound is three times faster).

sound spreading in 1/R instead of 1/R2 is what happens on Earth, with loud sounds, once they are powerful enough to reach the stratosphere and being reflected.

IIRC, during a Martian year, there are sizable variations in air pressure as CO2 freezes out at the poles, so at times, sound would be even more constrained.