Upper limit set for level of lightning activity on Titan

Abstract

LIGHTNING is known to occur in the atmospheres of Earth and Jupiter¹, and there is strong evidence of lightning on Saturn² and Uranus³. Based on its extensive atmosphere, the presence of aerosols and the deposition of significant amounts of solar energy at its surface, it has been calculated that Saturn's largest moon, Titan, may produce lightning with an energy dissipation rate somewhat less than that at Earth⁴. An opportunity to search for evidence of lightning at Titan occurred during the Voyager 1 encounter with Saturn on 12 November 1980, when the spacecraft passed within 4,394 km of Titan's cloud tops. Because optically thick cloud and haze layers prevented lighting detection at optical wavelengths, we have searched for lightning-radiated signals (spherics) at radio wavelengths using the planetary radioastronomy instrument⁵ aboard Voyager 1. Given the maximum ionosphere density^6,7 of ∼3 × 10³cm⁻³, lightning spherics should be detectable above an observing frequency of 500 kHz. Failing to find any evidence for lightning-associated spherics, we infer an upper limit to the total energy per flash in Titan lightning of ∼10⁶ J, or about a thousand times weaker than that typical of terrestrial lightning. The level of lightning activity on Titan has implications for the production of certain hydrocarbons in its atmosphere and for the design of instruments on spacecraft such as Cassini, which is scheduled to arrive at Saturn in 2002.