Figure 1. Artistic impression of a capsule landing by parachute in a sea on Titan
Juan Arvelo – Juan.Arvelo@jhuapl.edu
Ralph Lorenz – Ralph.Lorenz@jhuapl.edu
The Johns Hopkins University Applied Physics Laboratory
11100 Johns Hopkins Rd.
Laurel, MD 20723-6099
Popular version of paper 2aPA5
Presented on Tuesday morning, October 23, 2012
164th ASA Meeting, Kansas City, Missouri
Sound and light are polar opposites in many ways. Light travels fastest in vacuum while sound simply doesn't propagate without a gas, liquid or solid traveling medium. Sound tends to travel faster with increasing medium density while light tends to slow down. Similar contrasting behavior is observed with sound and light absorption.
However, the universe is mainly a vacuum sprinkled with billions and billions of widely spaced particles and particle clusters. Therefore, if light appears to reign supreme in outer space, what is sound good for in this vast universe?
Well. more than you may think. Planets, stars, moons, comets, asteroids and other celestial bodies are made of matter. For example, sound has been extensively used on planet Earth due to the presence of vast bodies of water and a thick atmosphere.
Saturn has an extraordinary moon called Titan. Titan is the only moon in our entire solar system that is known to have an atmosphere that is denser than that on Earth and liquid bodies resembling our oceans. However, Titan's atmosphere contains methane and its seas are mainly of liquid ethane. Additionally, while the coldest temperature recorded on Earth was of -129 oF in Antarctica, the nominal atmospheric temperature in Titan is known to be around -294 oF. Therefore, Titan is a harsh environment for human habitation.
Titan's habitat is also too harsh for most commercial-off-the-shelf (COTS) instrumentations. To further our understanding of Titan's unique environment, rugged measurement equipment (i.e., without moving parts to withstand such cold temperatures) must be developed.
Infrasound (sound below 20 Hz) may also be used to interrogate its cores with seismic pulses to infer their size and composition.
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Figure 1. Artistic impression of a capsule landing by parachute in a sea on Titan |
It is also important to know the depth of Titan's hydrocarbon seas. Therefore, a custom-made depth sounder (i.e., fish-finder) may be installed under a drifting probe to measure the time that it takes for an emitted pulse to bounce back from the sea floor. Converting the two-way travel time to depth requires knowledge of the speed of sound in Titan's seas. This speed was estimated to increase with depth (due mainly to increasing pressure) and to be generally much faster and less absorptive than in the Earth's oceans.
The ruggedized depth sounder was designed with a pair of piezoelectric (material that converts sound to electric signal and back) rings sandwiched between thick stainless steel and aluminum layers. Their size and thickness were designed for a transducer (sound projector and sensor) resonance frequency of around 20 kHz (upper limit of human hearing). The chosen piezo-ceramic material was once known to function at cryogenic (very low) temperatures, but to be confident in the performance of the assembled transducer, one needs to actually build it and test it in Titan's environmental conditions.
Visiting Titan or emulating its physical habitat for transducer sensitivity measurements is too difficult and expensive. Therefore, a cost-effective solution was formulated to predict the transducer performance at Titan from measurements conducted while enjoying the comforts of planet Earth.
The transducer has a dual role as the emitter of short pulses and as sensor to detect the seafloor-reflected signals. The transducer's voltage response describes how the electronic signal is converted into sound, while its receive sensitivity determines its efficiency in converting the received echo back to an electric signal. Both values are key independent measures of the transducer's overall performance.
Two medium physical properties that affect the transducer's performance are the temperature and acoustic impedance (medium density multiplied by the speed of sound). Titan's seas feature similar acoustic impedance as in fresh water. However, water solidifies at Titan's temperature. In contrast, liquid nitrogen has similar temperature to that in Titan, but about half the acoustic impedance.
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Figure 2. Frozen (yet operational) transducer mounted on a PVC pipe end cap (left) |
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Step (1) in our approach is to conduct the usual transducer calibration measurements in a fresh-water tank. These values are later adjusted by the measured received voltage ratio between two identical transducers mounted on opposite ends of a PVC pipe submerged in water (2) and in liquid nitrogen (3) where the pipe serves as an acoustic waveguide. A final 6 decibels (or twice the voltage) adjustment was also calculated to account for the difference in the acoustic impedance of fresh water and liquid nitrogen.
A number of prototype transducers were built and their sensitivity in water and liquid nitrogen was measured to conclude that this custom-made cryogenic apparatus would perform about 20 decibels (or 100 times) better in Titan than on Earth!
In conclusion, we successfully developed an efficient cryogenic depth sounder that is ready for implementation in future space exploration missions.