Titan is Saturn's largest moon. The clouds in Titan’s atmosphere produce liquid methane rain. One lingering question is whether those rain clouds also produce lightning. One of lightning’s direct and unambiguous signatures, thunder, "may help corroborate the existence of electrical discharges on Titan, in tandem with the usual electromagnetic sensors," says physicist Andi Petculescu of the University of Louisiana.

In two new studies, Petculescu and his undergraduate physics students Peter Achi and Christopher Hill discuss computer simulations of the physical mechanisms through which a Titanian lightning discharge generates a shock wave of thunder. Among other effects, the models predict that Titanian thunder would have frequencies that range from a high of roughly 100 Hz down to inaudible frequencies below 20 Hz, known as infrasound.

The detection of thunder, Petculescu says, “will help corroborate and quantify lightning on Titan beyond the shadow of a doubt, which will be a very important step in inferring Titan’s atmospheric electrochemistry.” In addition, he adds, the discovery of lightning could inform hypotheses suggesting that the precursors to life, complex pre-biotic and biotic molecules, can emerge out of chemical reactions "induced by the strong deposition of charge in a 'primordial pond.'"


Modeling thunder propagation and detectability on Titan
Peter Achi and Andi Petculescu

This research is part of a study investigating the characteristics of thunderon Saturn’s largest moon, Titan. In tandem with electromagnetic signatures,thunder can corroborate and quantify lightning discharges. A physical modelfor the propagation of thunder on Titan, based on the most recent data collectedby the Cassini–Huygens mission, is being developed. The model approximatesa tortuous 20 km cloudtoground lightning channel by an anglewise randomwalk of small discharge segments, each generating a strong cylindrical shockwave, which acquires an Nwave shape after it travels through the relaxationradius, into the acoustic regime. These acoustic waves are then propagatedto the farfield detector where they are added linearly to form longrangethunder. The detectability of thunder signatures by a sensor in Titan’slower atmosphere depends on the moon’s atmospheric structure. In orderto constrain the fraction of acoustic energy reaching a detector in Titan’stroposphere, the model accounts for the upwardrefracting sound speed profileup to the inversion point at ∼45 km and also for ground effects. The soundspeed and attenuation are computed along the length of the lightning channel(20 km) using altitudedependent pressure, density, and temperature measurementsby Cassini–Huygens, as well as thermophysical parameters (specific heats,viscosity, thermal conductivity, and diffusivity) extracted from NIST’sChemistry WebBook.

Gas-dynamic modeling of strong shock wave generation from lightning in Titan’s troposphere
Christopher S. Hill and Andi Petculescu

In an effort to predict the characteristics of thunder on Titan, a modelis being developed for the formation of the initial strong shock wave by ashort cylindrical lightning discharge “segment.” (Such small dischargesegments are later used to synthesize a tortuous cloudtoground 20 Kmlonglightning channel in Titan’s troposphere.) The shock wave is obtainednumerically as a solution of the coupled gasdynamic equations of state andconservation of momentum, mass, and energy. The relevant acoustic quantitiesare the pressure, density, particle velocity, and specific internal energyof the shock wave immediately following the discharge. Different scenariosfor the initial deposition of energy from the discharge into the shock waveare investigated. The altitudedependent ambient conditions in Titan’slower atmosphere—temperature, pressure, and density—are extractedfrom CassiniHuygens data, while specific heatsand transport coefficients of the main constituents of Titan’s troposphere(N2, CH4) are obtainedfrom the NIST Chemistry WebBook and interpolated at each altitude. The CH<emphtype="inferior">4 molar fraction, measured by the mass spectrometeronboard Huygens, varies from 0.0492 at 5 m to 0.0162at 35 km (the latter marking the lower end of Titan’s tropopause). Theseparameters are used as inputs to the model for a complete thermodynamic characterizationalong the length of the lightning channel. [The work was funded by the LouisianaSpace Consortium (LaSpace) and NASA.]

These papers will be featured at the 161st meeting of the Acoustical Society of America (ASA) held May 23-27, 2011, at the Sheraton Seattle Hotel in Seattle, Wash. During the meeting, the world's foremost experts in acoustics will present research spanning a diverse array of disciplines, including medicine, music, psychology, engineering, speech communication, noise control, and marine biology.

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