Simulating Thunder on Titan:
A Tool To Corroborate Lightning And A
First Step Toward A Titanian Soundscape
Andi Petculescu - andi@louisiana.edu
Department of Physics, University of Louisiana at Lafayette
PO Box 44210, Lafayette, LA 70504
Popular version of paper 5aPAb7
Presented Friday morning, Apr 23, 2010
159th ASA Meeting, Baltimore, MD
The largest moon of Saturn, Titan, is a world rich in organic compounds, possibly similar to pre-biotic Earth. Even though the Cassini-Huygens mission has not relayed any conclusive direct evidence of lightning, recent models of Titans atmospheric electricity combined with data analysis have shown that Titanian lightning is very plausible. Almost nothing is presently known about the acoustic environment of Titan (or any other extraterrestrial world). Titan's significant atmosphere of 1.5 bar, with sound absorption rates considerably smaller than Earths, is very favorable to long-range sound propagation.
Lightning processes mirror an atmosphere's electrical properties. The ability to detect and quantify lightning plays an important role in understanding not only atmospheric chemistry and dynamics, but also the interaction between atmosphere and surface. Going further, extraterrestrial lightning may give clues to the emergence of organic molecules not only on early Earth but, potentially, on other planets as well. However, unequivocal proof for the existence of lightning on other planets in our solar system remains an open challenge. To date, extraterrestrial lightning is confirmed on Jupiter and Saturn, probable on Venus, Uranus, and Neptune, and likely to occur on Titan. (On Mars, the heavy presence of dust in the atmosphere may generate diffuse electrical discharges of friction-induced charges but not lightning channels.)
The case for confirming lightning on Titan is strengthened further by the possibility of gaining insight into i) pre/proto-biotic chemistry such as it might have occurred on Earth when its ambient conditions were not unlike present-day, and ii) the emergence of pre-biotic molecules on Titan itself. Studies have shown that the Titan's methane clouds may accumulate enough negative charge to create electric fields in excess of the breakdown fields and initiate cloud-to-ground flashes. Acoustic sensors designed specifically for Titan's environment can detect, analyze, and quantify thunder waveforms, which, when correlated with electromagnetic signatures, will corroborate the occurrence of lightning. The power spectrum of thunder can be inverted to obtain the discharge energy released as sound. In addition, acoustic sensor arrays can be designed to pinpoint the location of the lightning channel.
So far, planetary science missions have employed a variety of electromagnetic (EM) sensors to look for lightning, from optical to radio waves. Many EM techniques rely on indirect measurement to assess whether or not an observed event is lightning, with the exception of optical flash detection. What has been habitually omitted was the other direct signature of lightning thunder. Thunder waveforms are easily detectable by omnidirectional microphones designed specifically for Titans environment. Thunder spectra can be inverted to obtain the average energy released per unit length by the discharge channels. When correlated with EM signatures, thunder will confirm the occurrence of lightning, both during a probe's descent and/or once it has landed.
We have initiated a pilot study to investigate the characteristics of thunder on Titan, with an eye not only on the science of low-frequency sound generation by lightning discharges on Titan but also on developing future acoustic sensors to detect and quantify Titanian lightning processes.
The ultimate goal is to develop physical models for the source mechanisms, propagation, and detectability of thunder waveforms on Titan based on the most recent environmental data. The models can then be used as a benchmark to drive the development of microphones for detecting lightning on Titan. In the larger perspective, the methods and models that will be developed will constitute the framework for similar acoustic investigations of other planets such as Venus, Mars, or the gas giants.
In the current simulations, the acoustic arrivals from both a line discharge channel and a tortuous one are simulated, for a receiver located 800 meters away. The tortuous lightning channel, shown in Figure 1, was obtained from nearly 7000 three-meter segments randomly oriented over a height of 20 km, which is the predicted distance between Titans surface and the charged cloud base. The current literature predicts electric fields in Titans troposphere in excess of two million volts per meter, stronger than the satellites atmospheric breakdown field. This is one of the critical ingredients for lightning. Immediately after the lightning discharge is initiated, a strong shock wave is produced by the relaxing heated gas column, which becomes a sound wave thunder as it propagates away from the channel. In this preliminary study, we modeled the acoustic signature of the tortuous lightning by summing up the individual contributions of the 7000 segments and propagating it over a distance of 800 meters in Titans atmosphere. The simulation accounts for the dependence of ambient temperature, pressure, and density with altitude in the atmosphere of Titan. The thunder simulation results are shown in Figure 2. The top graph of Figure 2 shows the simulated waveform, while the bottom graph presents the frequency content of spectrum. Also shown in Figure 2 are the simulation results for a 20-km long linear discharge (see the red dashed curves).
Figure 1. The 20-km long tortuous lightning discharge, formed of approximately 7000 three-meter long segments, oriented at random. The vertical axis is in kilometers, the horizontal axis in meters.
Figure 2. The acoustic waveform (top graph) and its spectrum (bottom graph) produced by the tortuous discharge channel of Fig. 1 and propagated 800 meters in Titans atmosphere. The red and dashed lines represent the acoustic signature of a 20-km long linear discharge.
To hear what thunder, as would be received by an acoustic sensor on the surface of Titan, would sound like, we have converted the waveform shown in the top graph of Figure 2 into a sound file, which can be accessed below.
Acoustics is re-emerging as a tool to characterize alien atmospheres, as evidenced by the recent acoustic data acquired by Cassini-Huygens. The proposed project is dedicated specifically to the detailed modeling of thunder in the atmosphere of Titan. The appeal of studying thunder in extraterrestrial atmospheres lies in the interplay between fundamental scientific questions (such as shock-wave generation by discharge channels and sound propagation in alien atmospheres) and a broader understanding of the mechanisms of lightning, atmospheric electro-chemistry, and the emergence of complex organic molecules, on Titan and elsewhere.
For the immediate future, the plan is to create realistic simulations of Titans acoustic environment or soundscape, based on various scenarios. Such scenarios would involve, beside thunder, other Titan-specific sound-producing phenomena such as methane cascades, a space probes splash-down in a hydrocarbon lake, a flowing creek of liquid methane, an exploding bolide etc. Using pertinent approximations, such intriguing phenomena can be simulated with a fair amount of realism. For an enhanced educational impact, the next step is to synthesize Titans soundscape in surround sound. Thus we hope to create immersive acoustic environments that will enable the listener to perceive realistic simulations of the sounds of an alien world.