Page 20 - Jul2009

Fig. 2. Picture from the surface of Titan, taken by ESA’s Huygens probe. The light- toned rock below and left of center is only about 15 centimeters across and lies 85 centimeters away (Credit: ESA, NASA, Descent Imager/Spectral Radiometer Team, Jet Propulsion Laboratory).
perature (-178oC) means that, despite its small size, Saturn’s moon Titan has a thick atmosphere. At ground level, the atmospheric pressure on Titan is 1.5 bar, and the sound speed is only 62% that of Earth52 (Fig. 2). It is assumed, for the pur-
pose of this exercise, that the organ contains only flue pipes, so that the note of a given organ pipe scales linearly with sound speed. Under this assumption, Bach’s Toccata and Fugue in D minor (293.66 Hz) played on Titan will automat- ically be transposed down to the key of ~F# minor (185 Hz). The atmospheres of Mars (Fig. 3) and Venus (Fig. 4) are both dominated by CO2 and N2. However, their surface tempera- tures are extremely different, leading to ground-level sound speeds that are, respectively, 70% and 120% of the sound speed on Earth.52 Thus Mars’ thin and cold (-46 oC) atmos- phere transposes Bach’s Toccata down to ~G# minor (207.65 Hz), while Venus’ dense and hot (457 oC) atmosphere trans- poses it up to ~F minor (349.23 Hz)—nearly an octave above Titan’s rendition at F# (185 Hz).55
The acoustic absorption, on the other hand, affects the propagation of sound in a different manner on the four worlds.52 Thus Titan’s nitrogen-based atmosphere is less lossy than Earth’s, so that the music can carry to similar distances (although, as on Earth, variations due to season and latitude, atmospheric stratification and any wind could become impor- tant, especially at very long distance propagation e.g., of infra- sound). The CO2 on Mars and Venus absorbs the sound far more than does Earth’s air, such that on Mars the music at full volume is barely audible merely 10 meters from the organ (sug- gesting that the Mars Polar Lander and Phoenix microphones, had they been activated, would have had very limited range).
Figure 5 shows the transmission loss (TL) in dB as a function of frequency calculated based on geometrical spreading and acoustic absorption. Geometrical losses are assumed to be spherical for all these worlds and independent of frequency: at 10, 20, 50 and 100 m they contribute, respec- tively, 20, 26, 34 and 40 dB of the transmission loss (i.e., the bulk of the TL for Titan). Additional losses are contributed by atmospheric absorption—on Titan these losses are smaller than on Earth, while the carbon dioxide on Venus, and par- ticularly Mars, produces very high absorption of sound. The assumed atmospheric pressures (p), temperatures (T) and composition for each world are as follows, allowing the atmospheric sound speed (c) to be calculated—Earth (77% N2, 21% O2, 1% H2O; p = 1 bar, T = 22oC, c = 340 m/s); Titan (95% N2, 5% CH4; p =1.5 bar, T= -178oC, c = 210 m/s); Venus (96% CO2, 3.5% N2, trace SO2; p= 90 bar, T = 457oC, c = 410 m/s); Mars (95% CO2, 2.7% N2, 1.6% Ar, 0.13% O2 ; p = 0.007 bar, T = -46oC, c = 240 m/s), noting that the actual values (e.g., of temperature) can vary significantly with time and lat- itude.
While the attenuation of musical sounds with distance is similar for most instruments, the effects of the extraterrestri-
Fig. 3. A 360o panorama image of Mars, taken by NASA's Mars Exploration Rover Spirit from halfway up Husband Hill, the summit of which can be seen about 200 meters southward and about 45 meters higher (Credit: Jet Propulsion Laboratory).
Sounds in Space 19