Saturn’s moon, Titan, is unique in that it is the only moon in our solar system that has an atmosphere. That, in turn, has led scientists to ponder whether there could also be lightning on Titan. To date, no such phenomena has been detected in data collected by various space missions.
However, tracking the acoustic signature of thunder on Titan might be a good way to detect it, according to Andi Petculesu of the University of Louisiana at Lafayette, who described his recent work in this area at the May meeting of the Acoustical Society of America in Seattle. If there is thunder and lightning in Titan’s atmosphere, he believes his latest computer models will help detect thunder (and hence lightning) directly, and enable scientists to track such storms when they occur.
You may recall from childhood science class that lightning is a particularly spectacular form of static electricity, with very high voltage and high intensity. Lightning occurs because clouds become negatively charged as the water droplets inside rub up against each other during the natural process of evaporation and condensation, when moisture accumulates in the clouds. This charge seeks out something with a positive charge — the ground, ideally — and the lightning is the “spark” closing the gap between the two.
Titan’s atmosphere forms clouds, just like on Earth, except because of the high atmospheric pressure — nearly double that of Earth — the “rain” isn’t composed of water, but of liquid methane. So in theory, the same charge separation should occur from time to time, producing lightning.
Lightning has already been observed in Saturn‘s atmosphere (see photos). Earlier analysis “indicates that Titan’s troposphere is very conducive to lightning events,” Petculescu told Discovery News, based both on observation and computer models of cloud electrification and charge accumulation.
Alas, no such high-intensity electrical discharges have been directly observed by the electromagnetic sensors on board various spacecraft, including Europe’s Huygens spacecraft, the Voyager missions launched in the 1970s, and NASA’s Cassini spacecraft.
But lightning also produces an acoustical shock wave (sound) in the form of booming thunder, and Petculescu proposes that adding acoustical sensors to the equipment of such missions could detect this, providing solid evidence of lighting on Titan. He has spent the last few years developing sound sensors to do just that, thereby enabling scientists to track lightning in the moon’s atmosphere.
The goal this time round is to assess the properties of Titan thunder as a means to corroborate the presence of lightning and to quantify lighting discharges on Titan in future missions.
If lightning does occur on Titan, it might be a very rare event. Earlier this year, Cassini scientist Don Gurnett and Georg Fischer — from the University of Iowa and the Space Research Institute in Graz, Austria, respectively — published a paper in Geophysical Research Letters reporting they had found no evidence of lightning on Titan. Their conclusions were based on their analysis of additional radio data collected by the most recent 72 “flybys” of NASA’s Cassini spacecraft.
Petculescu acknowledges there has been no direct electromagnetic evidence of lightning on Titan, adding that radio emissions are a good indirect tool used to look for lightning. Thunder, however, is the direct acoustic signature, ranging from the infrasound region of the spectrum, up to hundreds and possibly thousands of Hertz (Hz).
A sound wave’s frequency measures how many crests, or compressions, occur within one second; 1 Hz is equivalent to 1 vibration per second. Human hearing can detect sound waves whose frequencies fall between about 20 Hz to 20,000 Hz. Petculescu’s computer models indicate that Titanian thunder should have frequencies ranging from 100 Hz down to less than 20 Hz, a region known as infrasound, just below human hearing.
The lack of detection thus far might also be a question of seasonal variation; a year on Titan is equivalent to 28 years on Earth. So when the Huygens spacecraft passed by Titan, the moon “was in a relatively calm season,” says Petculescu. The most recent data from Cassini indicate we might be entering Titan’s stormy season, based on the unexpected appearance of methane-ethane lakes on the planet — in regions that prior missions found to be dry.
This is relevant because there is far more charge separation occurring during stormy meteorological seasons, making it more likely that lightning (electrical discharges) would occur. “If a sensor (e.g. a microphone) were placed inside Titan’s atmosphere, it would be able to record thunder signatures and invert them to quantify the discharge that produced them,” Petculescu explains. “Lower-frequency waveforms can travel with little loss over hundreds of miles on Titan, due to its very small sound absorption rates.”
Why do scientists care about a little thunder and lightning on one of Saturn’s moons? Well, it will help them better understand the intricacies of planetary atmospheres, for one thing. But another implication of thunder and lightning on Titan is that lightning provides just the sort of powerful electrical charge that can spark the critical chemical reactions needed to turn the soup of organic molecules floating around Titan into hydrogen cyanide and cyanogen.
Technically these are toxic compounds, but they are the precursors to compounds essential for carbon-based life, including amino acids. So finding thunder and lightning on Titan would be a coup for astrobiology as well.
Image credits: NASA, David Hardy.