In 1979, Voyager I provided the first strong evidence for the existence of lightning on another planet. Two pictures taken while the spacecraft was in Jupiter's shadow reveal about three dozen luminous spots on the night side of the planet. After careful examination of these spots, we conclude that they are lightning flashes occurring somewhere within Jupiter's atmosphere. A search through the additional Voyager I and II images of Jupiter's night hemisphere failed to locate any additional lightning flashes. The lower limit for the planetary lightning rate on Jupiter is found to be 10⁻⁴ km⁻² yr⁻¹. It must be noted that the spacecraft could only detect lightning discharges at least 1000 times higher than typical terrestrial flashes. Furthermore, due to attenuation, any discharges occurring deep within the atmosphere could not have been imaged. Calculations suggest that the actual flash rate could be about 0.1 km⁻² yr⁻¹. Analysis of the lightning images reveals that the flashes group near 50°N latitude. High-resolution photographs of the lightning region made in daylight about 65 hours before the discovery images show long, light ribbon-like clouds. Almost every flash appears to be associated with one of these clouds. Calculations made with a Monte Carlo radiative transfer code that computes the luminosity distribution of the spot on the top of the ammonia cloud that is produced by a point source within the atmosphere indicate that the lightning production region is near the top of the lower cloud deck. The average optical energy radiated by a discharge is calculated to be about 10⁹ J. The total electrical energy is estimated to be about 3 x 10¹² J. Calculations made with a chemical equilibrium model show that lightning synthesis cannot account for the observed abundances of such disequilibrium species as CO, HCN, and C₂H₂.

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According to both spectroscopic measurements and interior models, Jupiter, Saturn, Uranus and Neptune possess gaseous envelopes that are enriched in heavy elements compared to the Sun. Straightforward application of the dominant theories of gas giant formation - core accretion and gravitational instability - fail to provide the observed enrichment, suggesting that the surplus heavy elements were somehow dumped onto the planets after the envelopes were already in existence. Previous work has shown that if giant planets rapidly reached their cur rent configuration and radii, they do not accrete the remaining planetesimals efficiently enough to explain their observed heavy-element surplus. We ex plore the likely scenario that the effective accretion cross-sections of the giants were enhanced by the presence of the massive circumplanetary disks out of which their regular satellite systems formed. Perhaps surprisingly, we find that a simple model with protosatellite disks around Jupiter and Saturn can meet known constraints without tuning any parameters. Fur thermore, we show that the heavy-element budgets in Jupiter and Saturn can be matched slightly better if Saturn’s envelope (and disk) are formed roughly 0.1 — 10 Myr after that of Jupiter. We also show that giant planets forming in an initially-compact con figuration can acquire the observed enrichments if they are surrounded by similar protosatellite disks. Protosatellite disks efficiently increase the capture cross-section, and thus the metallicity, of the giant planets. Detailed models of planet formation must therefore account for the presence of such disks during the early stages of solar system formation.