151st ASA Meeting, Providence, RI
[ Lay Language Paper Index | Press Room ] Computer Simulations of the Propagation of Sound on Mars
Amanda D Hanford - ald227@psu.edu
Popular version of paper 2aPA3
With a plethora of spacecraft, rovers, and landers currently investigating the red planet, Mars has become a hotbed of scientific exploration. In fact, it has fascinated people for centuries. With the help of NASA's Mars exploration initiative, scientists are now gaining new insight into the geology, evolution, climate, and the possibility of life on Mars. The U.S. also has long-term plans for men and women to visit Mars. Currently, NASA's Mars Reconnaissance Orbiter (MRO) is positioning itself in the Martian atmosphere where later this year it will begin its primary science phase, when it is hoped that it will produce large amounts of data with state-of-the-art instruments. One of the objectives of this mission, among other things, is to help provide some answers to fundamental questions, such as: is there now or has there ever been life on Mars? Another interesting question is: if there were life on Mars, how would it sound ? In 1999, a low-cost microphone was constructed for the ill-fated Mars Polar Lander mission, in the hope that it would transmit sound samples back to Earth. While the Mars Polar Lander lost contact with Earth shortly after its descent into the Martian atmosphere and never recovered, there is hope that a future mission will include more acoustic experiments. So the question becomes, what will we hear, and how can we predict how sound behaves on the red planet? To know how sound propagated on Mars, we need to know a little bit about the atmosphere in which the sound propagates, since acoustics is a fluid dynamic process. The molecular composition of the Martian atmosphere differs greatly from that on Earth with 95.3% of the atmosphere being carbon dioxide, as opposed to trace amounts of carbon dioxide on Earth. The other constituents of the Martian atmosphere include 2.7% Nitrogen, and 1.6% Argon, with trace amounts of Water vapor and Oxygen. As well as having a different molecular composition, Mars' atmosphere is also much thinner than Earth's. Earth's atmosphere is about 3 km thick, while Mars' is very thin and transparent. The pressure of the surface of Mars is only about 0.7% (about 700 Pa) of that on Earth at sea level (about 101,000 Pa), but it also changes significantly due to the seasons. In winter some of the atmosphere's Carbon Dioxide freezes which then snows on the polar caps. The surface pressure also varies greatly throughout the planet, which is enough to support large winds which can create planet-wide dust storms that can last many months. Also, the temperature on Mars in cooler than Earth, ranging from 200 K to 300 K. Despite the dramatic differences between the atmospheres of Earth and Mars, there are similarities between these environments that should make studying the acoustics of Mars worthwhile. In fact, the physical properties that govern sound propagation on Mars are very similar to those on Earth. In particular, the mechanisms for the absorption of sound by the atmosphere are very similar and include: losses associated with the transfer of acoustic energy into heat, and losses associated with the redistribution of the internal energy of the molecules. However, the difference in molecular composition between Earth and Mars as well as the lower atmospheric pressure on Mars are enough to result in much larger values for the absorption coefficient on Mars than on Earth. For example, the sound from a lawnmower on Earth can travel a few miles before it gets absorbed into the atmosphere. This same lawnmower on Mars would produce a sound that could only travel a few hundred ft. This difference produces an interesting engineering question about how to efficiently PRODUCE sound on Mars. Since sound doesn't travel very far before it gets absorbed, how could we make loudspeakers that are loud enough so that sound travels longer than a few hundred ft? Without having sound data collected from Martian microphones, we must rely on our knowledge of the physical properties of the Martian atmosphere and sound production, as well as some computer simulations to help predict how sound behaves on Mars. The direct simulation Monte Carlo (DSMC) method is a simulation tool that we have used for modeling sound propagation in the Martian atmosphere. DSMC describes the dynamics of a gas through direct physical modeling of particle motions and collisions. DSMC is based on the kinetic theory of gas dynamics, where representative particles are followed as they move and collide with other particles. The movement of particles is determined by their velocities, while the collisions are determined statistically, but are required to satisfy mass, momentum, and energy conservation. Because of the particle nature of the method, DSMC is essentially able to capture all physical properties of interest to the sound propagation on Mars, as well as the details of sound absorption. Our results show that the absorption of sound on Mars is 100 times greater than it is on Earth due to differences in molecular composition and lower atmospheric pressure. Also, despite the fact that most of the atmosphere is Carbon Dioxide, the small amounts of Water vapor, Nitrogen, Oxygen and Argon are enough to change the absorption coefficient slightly. While this effect is small, it is still noticeable. Below is a movie clip of a DSMC simulated sound wave propagating in the Martian atmosphere. Notice as the wave travels, that the amplitude decays as it propagates. This rate of decay is much larger on Mars than on Earth. DSMC is a powerful computational tool that allows us to investigate the physics associated with sound propagation on Mars and beyond. Now we can determine how sound behaves where no person has ever been, yet... References: H.E. Bass, J. Chambers, "Absorption of sound in the Martian atmosphere," J. Acoust. America, 109 (6) 3069-3071 (2001). H.E. Bass, L.C. Sutherland, J. Piercy, and L. Evans, "Absorption of sound in the atmosphere," in Physical Acoustics, Vol. XVII edited by W. P. Mason, (Academic Press, New York, 1984), p. 145-232. Bird, G. A., Molecular Gas Dynamics and the Direct Simulation of Gas Flows. Clarendon Press, Oxford, 1994. A. Danforth, L. Long, "Nonlinear Acoustic Simualations Using the Direct Simulation Monte Carlo," J. Acoust. Soc. Am., 116, 1948 (2004). A. Danforth-Hanford, O'Connor, P., Long, L., Anderson, J., "Molecular Relaxation Simulations in Nonlinear Acoustics using Direct Simulation Monte Carlo" in 17th International Symposium on Nonlinear Acoustics, State College, PA, (2005).
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