Page 22 - January 2007

denser atmospheres of Titan and Venus make their acoustic impedances one and two orders of magnitude higher than Earth’s, respectively. This makes piezoelectric materials good candidates for acoustic sensing applications on both Titan and Venus. However, for measurements in the extremely hot envi- ronment of Venus (730 K), one must consider that past the Curie temperature, a piezoelectric material undergoes a phase transition and loses its spontaneous polarization and thereby its piezoelectric characteristics. Lithium niobate and bismuth titanate have very high Curie points (> 1000 K), which makes them viable choices for transduction in Venus atmosphere all the while keeping in mind that the sensor must be able to with- stand the extremely high pressure.
Vertical atmospheric profiles of attenuation and sound speed
The descent phase of a planetary science mission is cen-
tral to collecting data on the planet’s atmosphere. This is
when a descending probe can obtain first-hand information
on the physical properties of the atmosphere that surrounds
it. The development of active acoustic sensors designed to
extract information as the probe descends into the atmos-
phere requires a good prediction of sound propagation char-
acteristics as a function of altitude. Figure 3 shows the verti-
cal atmospheric profiles for acoustic attenuation (a) and
sound speed (b) at 15 kHz—the active sensing frequency
used on board Huygens. The profiles are based on pressure,
temperature, and density profiles obtained from general cir-
culation models for Titan,17 Mars,18 and Earth,19 and from
radio occultation measurements taken above 35 km for
20 o
Venus. For Mars, Titan, and Earth the profiles at 45 latitude
are shown. For Venus, data was only available at 67o latitude and above an altitude of 35 km.
For illustrative purposes, let us consider the descent of a spacecraft (say the Huygens probe) through Titan’s atmos- phere. Starting at an altitude of 120 km, an acoustic sensor would record a smoothly decreasing attenuation with three apparent regimes: a slight decrease down to about 50 km, a
more pronounced rate of decrease to about 20 km, and finally, a slow rate of decrease with altitude down to Titan’s surface. Simultaneous sound speed measurements would show a marked decrease in sound speed as altitude decreases followed by a plateau and a distinct increase down to the surface.
Over the altitude range considered, the vertical profile of the acoustic attenuation for Mars and Earth is also smooth, monotonically decreasing with altitude as the atmospheric pressure and temperature increase. The attenuation profile for Venus is quite different. It decreases as the probe descends through the higher atmospheric layers, but then plateaus from 70 km down to 50 km before increasing again from 50 km to 35 km. Sound speed profiles, on the other hand, show consid- erable structure, mirroring the layering of Earth’s and Mars’ atmospheres. For example, the vertical sound speed profiles reveal inversion layers for Earth (~15 to 35 km), Titan (~30 to 60 km), and Mars (~25 km), brought about largely by varia- tions in temperature. The sound speed for Venus increases as a probe descends in the atmosphere, to regions of higher tem- perature. The small-scale sound speed fluctuations occurring above 60 km are likely caused by internal gravity waves.
Summary
The characteristics of acoustic wave propagation in a planet’s atmosphere mirror the dynamics, structure, and composition of the respective environment. Despite their potential, acoustic sensing techniques have been overlooked in planetary science missions. Recently, the Cassini-Huygens mission marked a successful re-emergence of acoustic sens- ing, bringing to the forefront its potential ability to map out the ambient “soundscape” of planetary atmospheres and to monitor the conditions of descent and landing environments. Acoustic sensors can be passive in the form of microphones to record ambient sounds, or active such as would be imple- mented in wind velocity sensing (sonic anemometry) or measurements of sound speeds and turbulence fields. Acoustic sensors need not work alone: they can be combined with electromagnetic sensors to provide more information.
Fig. 3. Vertical atmospheric profiles for the acoustic attenuation coefficient (a) and speed of sound (b), calculated at 45o latitude for Titan, Mars, and Earth, and at 67 o lat- itude for Venus. The attenuation profiles are relatively smooth, with the exception of Venus where α dips noticeably over an altitude range of ~ 40–80 km corresponding roughly to the small-scale sound speed fluctuations that could be generated by internal gravity waves. The large sound speed swings, brought about largely by variations in temperature, mirror the structures of the four atmospheres.
20 Acoustics Today, January 2007