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 MWR 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. This new vision
of the mechanisms at play, which are both deep and latitude-dependent,
has consequences for our understanding of Jupiter’s deep interior and of
giant-planet atmospheres in general.