Non-detection at Venus of high-frequency radio signals characteristic of terrestrial lightning

The detection(1,2) of impulsive low-frequency (10 to 80 kHz) radio signals, and separate very-low-frequency (similar to 100 Hz) radio 'whistler' signa ls(3-5) provided the first evidence for lightning in the atmosphere of Venu s. Later, a small number of impulsive high-frequency (100 kHz to 5.6 MHz) r adio signals, possibly due to lightning, were also detected(6). The existen ce of lightning at Venus has, however, remained controversial(7-13). Here w e report the results of a search for high-frequency (0.125 to 16 MHz) radio signals during two close fly-bys of Venus by the Cassini spacecraft. Such signals are characteristic of terrestrial lightning, and are commonly heard on AM (amplitude-modulated) radios during thunderstorms. Although the inst rument easily detected signals from terrestrial lightning during a later fl y-by of Earth (at a global flash rate estimated to be 70 s(-1), which is co nsistent with the rate expected for terrestrial lightning), no similar sign als were detected from Venus. If lightning exists in the venusian atmospher e, it is either extremely rare, or very different from terrestrial lightnin g.