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

Abstract

The detection1,2 of impulsive low-frequency (10 to 80 kHz) radio signals, and separate very-low-frequency (∼100Hz) radio 'whistler' signals3-5 provided the first evidence for lightning in the atmosphere of Venus. Later, a small number of impulsive high-frequency (100kHz to 5.6MHz) radio signals, possibly due to lightning, were also detected6. The existence of lightning at Venus has, however, remained controversial7-13. Here we report the results of a search for high-frequency (0.125 to 16MHz) 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 instrument easily detected signals from terrestrial lightning during a later fly-by of Earth (at a global flash rate estimated to be 70s-1, which is consistent with the rate expected for terrestrial lightning), no similar signals were detected from Venus. If lightning exists in the venusian atmosphere, it is either extremely rare, or very different from terrestrial lightning.