Storms and the Depletion of Ammonia in Jupiter: II. Explaining the Juno Observations

Abstract

Observations of Jupiter's deep atmosphere by the Juno spacecraft have revealed several puzzling facts: The concentration of ammonia is variable down to pressures of tens of bars and is strongly dependent on latitude. While most latitudes exhibit a low abundance, the Equatorial Zone of Jupiter has an abundance of ammonia that is high and nearly uniform with depth. In parallel, the Equatorial Zone is peculiar for its absence of lightning, which is otherwise prevalent most everywhere else on the planet. We show that a model accounting for the presence of small-scale convection and water storms originating in Jupiter's deep atmosphere accounts for the observations. Where strong thunderstorms are observed on the planet, we estimate that the formation of ammonia-rich hail (“mushballs”) and subsequent downdrafts can deplete efficiency the upper atmosphere of its ammonia and transport it efficiently to the deeper levels. In the Equatorial Zone, the absence of thunderstorms shows that this process is not occurring, implying that small-scale convection can maintain a near-homogeneity of this region. A simple model satisfying mass and energy balance accounts for the main features of Juno's microwave radiometer observations and successfully reproduces the inverse correlation seen between ammonia abundance and the lightning rate as function of latitude. We predict that in regions where ammonia is depleted, water should also be depleted to great depths. The fact that condensates are not well mixed by convection until far deeper than their condensation level has consequences for our understanding of Jupiter's deep interior and of giant-planet atmospheres in general.

Plain Language Summary

Measurements by the Juno spacecraft have shown that much more ammonia is present in Jupiter's atmosphere near the equator than at higher latitudes. This was never predicted by theory. In a companion paper, we showed that ammonia can combine with water to form hail-like particles that we call “mushballs.” Here we show that mushball formation in storms can effectively dry out the atmosphere of its ammonia. Our idea is supported by lack of lightning activity at the equator compared to higher latitudes. Because lightning is generated in rainstorms, the lack of lightning at the equator suggests that the thunderstorms forming the mushballs responsible for the depletion of ammonia are not present there. In contrast, in other regions where lightning is present, we predict that not only ammonia but also water are depleted to great depths, more than a hundred kilometers below the cloud tops. The complexity of Jupiter's meteorology means we must expect similar complexity in observing the weather on other giant planets in and beyond our solar system.