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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 density6,7 of ~3 x 103 cm-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.
Figure 1 shows the Voyager 1 trajectory past Titan with closest approach occurring at about 05:40 UT when the spacecraft was 2.72 RT (the radius of Titan, RT, is 2,575 km) from the centre of the satellite. To allow sufficient time to record possible rare events but also recognizing the need for highly sensitive measurements, we restricted the observations to an interval within ±50 min of closest approach. During this interval Voyager 1 was with 17J?T of Titan, corresponding to a range of 102 (17/2.72- l)2 in sensitivity. About 98% of Titan's surface was 'visible' to the spacecraft during this interval.
Figure 2 (top) illustrates the characteristic signatures of what are believed to be lightning spherics from Saturn2 as they appeared in a Voyager planetary radioastronomy instrument (PRA) spectrogram taken on 13 November 1980. The spacecraft was about four planetary radii from Saturn, above the...