Concepts and Approaches for Mars Exploration 6083.pdf

AN ACOUSTIC SENSOR FOR THE NETLANDER MISSION G.T. Delory1, J.G. Luhmann1, F.S. Mozer1, D.W. Curtis1, L. D. Friedman2, J.J. Berthelier3 and P. Lognonne4, 1Space Sciences Laboratory, University of Cali- fornia, Berkeley, CA 94720, 2The Planetary Society, 65 North Catalina Avenue, Pasadena, CA 91106, 3Institut de Physique du Globe de Paris, 4 Avenue de Neptune 94100 Saint Maur des Fosses, France. 4Institut de Physique du Globe de Paris, BP89 4, pl. Jussieu 75252 Paris cedex 05.

educational and public outreach capabilities of the Netlander instrument package. Science Background and Objectives: The basic properties of acoustic signals on Mars have been in- vestigated recently by several groups, who were in part motivated by the presence of Mars Microphone on MPL. Given the recent MPL failure combined with a complete lack of direct acoustic measurements on pre- vious missions these results depend heavily upon basic atmospheric conditions measured by the Viking landers and the Mars Pathfinder mission. On Mars the typical surface pressure is 5-10 millibars, the average tem- perature ~210 K, the atmospheric mass density is ~1.4% the terrestrial value, and the sound speed is roughly 230 m/sec compared to 360 m/sec on Earth. Under these conditions Sparrow [1] showed that the sound intensities are only reduced by 20 dB, or a factor of 10 in amplitude, compared to the same source in the terrestrial environment. Frequency shifting effects may also be present on Mars, with the resonant Helmholtz frequency ~0.66 of the value in Earth’s atmosphere. Beyond these simple affects, more detailed calculations by Williams [2] has shown that attenuation of higher frequency sounds is much greater on Mars than on Introduction: We describe a microphone sensor Earth. By considering molecular, thermal, and viscous currently under development by the University of Cali- relaxation effects in the Martian atmosphere, a sound fornia at Berkeley Space Sciences Laboratory for use wave with a 50 dB source intensity may propagate only on the CNES sponsored Netlander mission, based on a few meters before being reduced to less than 0 dB in the original Mars Microphone constructed for the Mars power, below the detection threshold for the average Polar Lander program. The Netlander mission, sched- human ear. Thus distant sound sources may appear to uled to land four identical probes on the surface of emit spectra weighted towards the lower frequencies, Mars in 2005, represents a unique opportunity to sam- with the higher frequencies becoming detectable as the ple from multiple locations the acoustic signature of source moves closer. the meteorological environment as well as lander gen- An acoustic sensor on a planetary lander is likely to erated sounds which may be of both engineering and record a mixture of natural and artificial sounds. Natu- scientific significance. The microphone will be part of rally occurring sounds will almost certainly result from an integrated suite of sensors, including a seismometer, weather driven processes, caused by the wind, sand- infrasound detector, and an electric field sensor, form- storms, or dust devils, and the interaction of these phe- ing a comprehensive measurement system for the nomena with the lander structure. A microphone on the acoustic and electric properties of the atmosphere dur- Russian Venera Grozo 2 instrument was able to meas- ing storm activity as well as quiet periods. Currently ure the wind speeds on the surface of Venus by using selected for a phase B design study, the Netlander mi- calibration data from wind tunnel tests as well as sup- crophone development effort will focus on a smaller, porting measurements from other instruments on the lighter weight version of the instrument flown on MPL. mission [3]. In the case of the more violent events on This instrument is sponsored by The Planetary Society, Mars such as dust devils and sandstorms, we can use and is expected to add a significant contribution to the terrestrial analogs as a guide to their acoustic signature. Terrestrial tornados emit noise in the kHz range within Concepts and Approaches for Mars Exploration 6083.pdf

MICROPHONE on NETLANDER: G.T. Delory et al.

a few hundred meters and the same may be true of very low cost, power, and telemetry bandwidth con- Martian dust devils [4]. The Mars Pathfinder pano- straints. Consisting of a small circuit board 2.5 cm ramic camera imaged at least five dust devils that square, it had a mass of 50g, used less than 100 mW of moved with velocities between 0.5-5 m/sec which car- power, and yielded 2.5 to 10 second long snapshots of ried sufficient quantities of dust to reduce the lander sound sampled at rates of up to 20 kHz requiring ~24 solar cell efficiency by as much as 1% [5]. Thus future kilobytes of data. The microphone used was an electret landers may be likely to encounter these violent wind type with a frequency response between 100 Hz and 10 vortices at sufficient proximities for useful acoustical kHz and a lower sensitivity of ~3 dB. A sound proces- measurements to be taken. In addition to producing a sor chip consolidated A/D sampling, digital filtering, simple rush of atmosphere past the microphone, sand- sound compression, and data I/O on one IC. Non- storms and dust devils may be electrically active due to volatile memory stored several 10-second sound clips triboelectric charging effects, especially if the Martian in-between power cycles; the device was rad tolerant to dust carried in these storms has a wide range of particle 10 kRADs and could operate between temperatures of sizes [6,7,8]. Electrical charging due to relative dust –80 to +50 degrees C. motion can result in a static glow discharge, which may The next generation microphone for use on Net- emit sounds. Discrete electrical discharges are a regular lander faces different yet equally challenging design result within volcanic dust plumes [9] and may also be considerations. The severe mass limitations for Net- present in sandstorms and dust devils on Mars, in lander instruments require a lighter microphone sys- which case the Mars Microphone could record Martian tem, no more than 25g in mass. Unlike the sunlit MPL thunder. landing site in the high southern latitudes, a Netlander- Artificial sounds generated by the lander will also based sensor will likely have to withstand a wider tem- be of interest for both scientific and engineering con- perature range of –120 to +50 C. Both of these design siderations. The microphone can be used to record the constraints can be mitigated through the use of hybrid deployments of instruments after landing, as well as to circuits, producing a smaller, more temperature tolerant troubleshoot and verify critical events during entry, device. Additional mass savings may be accomplished descent, and landing that may generate noise. The through consolidating some of the microphone sam- tones of camera or other electric motors during the pling electronics with similar circuits in the electric mission can be recorded and compared to identical field experiment. It is also desired to increase the data tests under Terrestrial conditions; subsequent FFTs of rate of the device, which is easily accommodated by this data may verify the predicted high frequency at- the availability of higher density memory chips since tenuation and frequency shifting effects for sounds the MPL design effort, resulting in ~10 times the sound generated in the atmosphere of Mars. storage capability of the original Mars Microphone, Educational Outreach Objectives: The signifi- enabling up to several minutes of 5 kHz sampled sound cant public interest generated by the inclusion of Mars per day or more, depending upon the telemetry alloca- Microphone on the MPL mission underscored the po- tion given to the instrument. tential for public outreach and education on missions References: [1] Sparrow, V. W., (1999), J. that endeavor to bring the public closer in every way Acoustic. Soc. Am., 106, 2264. [2] Williams, J-P., possible to the experience of planetary exploration. (1999) The acoustic environment of the Martian sur- The MPL microphone generated radio and essay con- face, submitted to JGR., Dec 1999. [3] Ksanfomaliti, tests among students and adults, as well as significant L.V. et al. (1983), Kosmicheskie Issledovaniya, Vol press coverage on radio, television, and the internet 21., No. 2, pp. 218-224. [4] Arnold, R.T. et al., (1976) worldwide. In the case of a future microphone on Net- J. Acoustic. Soc. Am., 60, 584. [5] Metzger et al., lander, it is planned to leverage this now well- (1999) JGR. [6] Eden, H.F., and B. Vonnegut, (1973), established public enthusiasm for an extra-terrestrial Science 180, 962. [7] Kamra, A.K., (1972) JGR., 77, microphone in order to create a significant amount of 5856. [8] Eden, H.F., (1977) Electrical Processes in educational outreach for the mission. The Planetary Atmospheres, ed. by H. Dolezalek, R. Reiter, Springer Society, with significant experience in the process of Verlag. [9] Farrell, W.M. et al., (1999) JGR, 104, involving and exciting the public in planetary explora- 3795. tion, will accomplish this through its worldwide mem- bership activities as well as through partnerships with organizations such as Disney and educational web content providers. Technology Development: The original micro- phone developed for the MPL mission was driven by