Space probes have snapped stunning photos of Saturn's largest moon Titan, but the sounds of the cloudy satellite ? and, in fact, of most alien planets ? still remain largely a mystery. Now, scientists are developing new sound sensors to detect thunder on Titan that could potentially be used to track lighting in the moon's atmosphere.
Computer models created from recent data from NASA's Cassini spacecraft, the European Huygens that landed on Titan, and the Voyager missions launched in the 1970s have suggested that lightning should be possible on Titan based on the characteristics of its atmospheric electricity.
Despite that encouragement, scientists have yet to find conclusive evidence of lightning on Titan, though the moon's parent planet Saturn has offered stunning lightning shows recorded by Cassini.
Where is Titan's lightning?
Current models show that the hydrocarbon clouds inside Titan's troposphere (the lowest portion of the atmosphere) can accumulate enough charge to develop electric fields that overcome the threshold for lightning in the nitrogen-methane mix that constitutes Titan's atmosphere, said study author Andi Petculescu, a professor of physics at the University of Louisiana at Lafayette.
According to these current models, the typical cloud-to-ground discharge length of Titanian lightning is estimated at 12.4 miles (20 km).
Through a series of simulations in the preliminary study, Petculescu found that Titan's thunder should peak at approximately 100 Hertz.
"The structure of Titan's troposphere is such that an observer on the surface would receive the acoustic rays generated by the first 200 to 300 meters (656 to 984 feet) of the discharge channel, the rest being refracted upward," Petculescu told SPACE.com. ?
Titan is special
Titan's environment is unique because it is the only moon in the solar system with a significant atmosphere.
The temperature on the icy moon is a chilly minus 288 degrees Fahrenheit (minus 178 degrees Celsius). Titan also ?has an atmospheric pressure that is almost double what is found on the Earth's surface.
"What sets it apart is its heavy hydrocarbon presence," Petculescu said. "Imagine methane rain, snow, rivers, seas, and volcanoes, all at minus 178 degrees Celsius!"
While Titan's atmosphere is composed primarily of nitrogen, approximately five to 10 percent is made up of hydrocarbons, of which methane is the dominant species, Petculescu said. Compared to other alien atmospheres, this five to 10 percent is actually considered a fairly high concentration of methane.
"Currently the heavy presence of methane is not well understood," Petculescu said. "One of the hypotheses is that it could be due to "methanogenic" bacteria living deep inside [Titan's] crust."
Sounds on Titan's surface
Petculescu's models of Titan's acoustic environment also suggest some interesting differences. Based on these models, sound waves in Titan's atmosphere travel slower than on Earth ? 656 feet per second (200 meters per second) compared to 1,115 feet per second (340 meters per second) on Earth.
Sound would also travel with less attenuation on Titan, Petculescu said. This means that less sound energy is loss through scattering and absorption, allowing acoustic frequencies to travel farther on Saturn's largest moon than on Earth.
"Thus, one could say that Titan is ? acoustically ? a very "inviting" environment," Petculescu said.
If lightning is detected on Titan, the discovery would mark an important milestone in the field of climate physics.
"So far, the only world where lightning is 100 percent confirmed is Earth," Petculescu said. "So, unequivocal discovery of lightning on Titan would be significant in itself."
According to his study, extraterrestrial lightning is confirmed on Jupiter and Saturn, probable on Venus, Uranus and Neptune, and thought to be probable on Titan.
What Titan's lightning would mean
The detection of lightning on Titan would also have important impacts on atmospheric studies, Petculescu said, since analysis of lightning would also include examinations of atmospheric chemistry, composition and dynamics.
Furthermore, lightning is thought to be one of the main sources for possible organic chemistry on Titan.
"Since Titan's environment is not unlike that of pre-biotic Earth, further studies of lightning can shed light on life emergence not only on early Earth but also, possibly, on Titan itself," Petculescu said.
The study hopes to expand upon existing acoustic studies, while also demonstrating the benefits of having acoustic sensors onboard planetary science missions.
Acoustic sensors could provide a more direct way to detect lightning, and Petculescu envisions these sensors working in tandem with electromagnetic sensors to "corroborate the occurrence of alien lightning beyond the shadow of a doubt."
Next, the researcher hopes to further develop this study in order to synthesize realistic planetary "soundscapes" that incorporates data from planetary science missions and theoretical models, perhaps even one day presenting these findings in surround-sound.
"This will benefit educational programs, mass media, planetaria, museums, and even Hollywood when it picks up on the benefits of adding acoustic realism to documentary films and, why not, action movie," Petculescu said.
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