Future missions to the Saturn moon Titan may look for lightning in a novel way — by searching for its rumbling companion, thunder.
Detecting thunder on the frigid, distant moon won't be as simple as pointing a microphone and reading the signal. Sound waves are shaped and altered by the material they travel through, and Titan's air is much different than Earth's.
To help identify any possible signals, two teams of scientists modeled what the sound wave for a thunderclap would look like where lightning touches down on Titan, and how it would shift after traveling through the moon's atmosphere.
Different atmospheres make (different) waves
Nitrogen dominates the frosty air of Titan, the only moon in the solar system known to have a significant atmosphere. Methane clouds cross the planet, periodically raining liquid hydrocarbons onto the surface.
Many scientists hope that such storms bring with them lightning. The electrical discharge creates pockets of high temperatures that could allow the creation of complicated molecules. These, in turn, could have implications for the possibility of life on the moon.
The characteristics that make Titan's atmosphere so different from Earth also make it, from an acoustical standpoint, better.
"Sound carries farther on Titan than on Earth, or even Mars or Venus," Andri Petculescu, who worked on both of the recent studies, told Space.com.
Because of this, a detector could hear thunder rumble from relatively far away. But it would have to be created specifically for Titan.
"You can't bring any microphone off the shelf, designed for Earth, and assume it would work on Titan without modifications," Petculescu said. "Future missions would have microphones tailored for Titan's conditions."
Researchers would also have to keep in mind that lightning isn't the only possible source of loud noises on Titan. Petculescu pointed out that a meteorite hitting the surface or detonating in the air would boom loudly, much like thunder. The need to identify the distinguishing features of thunder is critical.
The two studies are part of a larger project to determine how thunder sounds on Titan, and to develop an accurate soundscape for the moon. Scientific documentaries, planetarium shows and even science fiction movies could utilize such a tool to accurately replicate the sounds heard by an astronaut standing on the surface of Titan, or various other planetary environments.
Petculescu described the various noises that could conceivably be heard on Titan, from the cascading of a methane creek to the rumble of thunder in the distance.
Scientists presented the two related projects at the May meeting of the Acoustical Society of America in Seattle.
Much of what scientists know about Titan comes from NASA's Cassini satellite, which studies the moon as part of its path around Saturn. In 2005, the Huygens probe, released from Cassini, took 2 1/2 hours to pass through the atmosphere and touch down on the surface of Titan.
The instruments on board Huygens were designed to study the surrounding atmosphere. The data was relayed from the probe to Cassini and back to Earth.
Because scientists knew about the potential for clouds — and thus lightning — Huygens was built with the idea that it might, in fact, be struck on the way in.
Neither Huygens nor Cassini (which is still sending information back to Earth) has detected lightning. However, NASA is presently exploring the possibility of another trip to the moon. The Titan Mare Explorer (TiME) is one of three missions considered for launch in 2016. If selected, it will attempt to land in and float on one of the large methane seas on the surface of Titan.
Unfortunately, the current proposal for TiME does not plan to include the equipment necessary to search for thunder. Studying the storms around the seas, however, will help scientists to understand more about how the weather cycle affects the moon.
The selection process will continue over the upcoming months, as teams continue to demonstrate progress.
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