Planetary lightning: Earth, Jupiter, and Venus

Abstract

Terrestrial lightning is usually produced by convective clouds that contain supercooled water, ice, and precipitation. Although convection and a variety of mechanisms based on the principle of electrostatic induction can, in principle, electrify clouds, the available evidence suggests that interaction between riming ice particles and supercooled water, followed by a gravitational separation of charged precipitation elements, is the dominant mechanism for cloud electrification on earth. These mechanisms probably operate on Jupiter in the H2O cloud, and optical images of lightning and whistlers have been detected by Voyager 1. Jupiter has no true surface; therefore the Jovian lightning flashes are cloud discharges. Observations indicate that Jovian lightning emits, on average, 1010 J of optical energy per flash, whereas lightning on earth radiates only about 106 J per flash. There is much uncertainty in the average planetary lightning rate on Jupiter, but estimates range from 3 × 10−3 to 40 km−2 yr−1. Venera probes have reported transient LF and VLF radio emissions in the lower atmosphere of Venus, and an experiment aboard the Pioneer Venus Orbiter has registered electromagnetic energy propagating in the whistler mode. Optical searches for lightning on Venus have been inconclusive, but have set an upper limit to the planetary flashing rate of about 30 km−2 yr−1. The Venus observations have led to widespread speculation that there is lightning in the atmosphere, but application of known charging mechanisms to the Venus environment does not suggest that there should be strong electrification. Since the spacecraft observations do not conclusively show that lightning does occur on Venus, we suggest that alternative explanations for the experimental results be explored.