Your Neighbors Would Love You on Mars

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, R.I., Amanda Hanford (ald227@psu.edu) and Lyle Long of Pennsylvania State University presented detailed computer calculations that simulate how sound travels through the Martian atmosphere, which is much thinner than Earth's (exerting only 0.7 percent of the pressure of our atmosphere on the surface) and has a very different composition (containing 95.3 percent carbon dioxide, compared to about 0.33 percent 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 nanometer 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.

Paper 2aPA3 at the American Acoustical Society meeting
Also see Computer Simulations of the Propagation of Sound on Mars, a lay-language paper on the ASA meeting's Web site
Contact Amanda Hanford, Pennsylvania State University
ald227@psu.edu