According to a new analysis of data collected by the Stellar Reference Unit instrument onboard NASA’s Juno spacecraft, an unexpected form of electrical discharge, ‘shallow lightning’ originates from Jovian clouds containing an ammonia-water solution.
Fig: This illustration uses data obtained by NASA’s Juno mission to depict high-altitude electrical storms on Jupiter. Juno’s Stellar Reference Unit camera detected unusual lightning flashes on Jupiter’s dark side during the spacecraft’s close flybys of the planet. Image credit: NASA / JPL-Caltech / SwRI / MSSS / Gerald Eichstädt.
Since NASA’s Voyager mission first saw Jovian lightning flashes in 1979, it has been thought that the planet’s lightning is similar to Earth’s, occurring only in thunderstorms where water exists in all its phases — ice, liquid, and gas.
At Jupiter this would place the storms around 45 to 65 km (28-40 miles) below the visible clouds, with temperatures that hover around 0 degrees Celsius (32 degrees Fahrenheit).
Voyager, and all other missions to the gas giant prior to Juno, saw lightning as bright spots on Jupiter’s cloud tops, suggesting that the flashes originated in deep water clouds.
But lightning flashes observed on Jupiter’s dark side by Juno’s Stellar Reference Unit tell a different story.
Ammonia is the key. While there is water and other chemical elements such as molecular hydrogen and helium in Jupiter’s clouds, ammonia is the antifreeze that keeps water in those upper atmospheric clouds from freezing entirely.
The collision of the falling droplets of mixed ammonia and water with suspended water-ice particles constitutes a way to separate charge and produce cloud electrification — resulting in lightning storms in the upper atmosphere.
The spatial resolution of some cameras allowed investigators to confirm 22 flashes with HWHM greater than 42 kilometres, and to estimate one with an HWHM of 37 to 45 kilometres. These flashes, with optical energies comparable to terrestrial ‘superbolts’—of (0.02–1.6) × 10^10 joules—have been interpreted as tracers of moist convection originating near the 5-bar level of Jupiter’s atmosphere (assuming photon scattering from points beneath the clouds).
Previous observations of lightning have been limited by camera sensitivity, distance from Jupiter and long exposures (about 680 milliseconds to 85 seconds), meaning that some measurements were probably superimposed flashes reported as one.
Authors reported optical observations of lightning flashes by the Juno spacecraft with energies of approximately 10^5–10^8 joules, flash durations as short as 5.4 milliseconds and inter-flash separations of tens of milliseconds, with typical terrestrial energies.
The flash rate is about 6.1 × 10^−2 flashes per square kilometre per year, more than an order of magnitude greater than hither to seen. Several flashes are of such small spatial extent that they must originate above the 2-bar level, where there is no liquid water. This implies that multiple mechanisms for generating lightning on Jupiter need to be considered for a full understanding of the planet’s atmospheric convection and composition.
References: (1) H.N. Becker et al. 2020. Small lightning flashes from shallow electrical storms on Jupiter. Nature 584, 55-58; doi: 10.1038/s41586-020-2532-1 link: https://www.nature.com/articles/s41586-020-2532-1