Presentation #115.04 in the session “Ice Giant Atmospheres”.
Observations at radio wavelengths have shown that ammonia is absent of the atmospheres of Uranus and Neptune, even down to pressures of tens of bars. Recently, the detection of H2S at relatively low pressures, from infrared measurements, provided a definitive confirmation of this finding: In the presence of NH3, sulfur should be sequestered into NH4SH, preventing the presence of H2S and of an H2S condensation cloud at low pressures. The situation can be explained with the same approach as to explain the relative depletion of ammonia in Jupiter. Within violent storms, ammonia vapor can combine with water ice crystals to melt them, form water-ammonia hailstones ("mushballs") that lead to an efficient transport of both species into the deeper atmosphere. I will show that while in Jupiter, equilibrium chemistry predicts the mushball seed region to be confined to a narrow parameter space, it is much more extended in Uranus and Neptune, implying that the process should be much more efficient, thus explaining the relative lack of detectable ammonia in these planets. The extent of the ammonia (and potentially water) depletion region is however unknown. Being able to determine how deep these species are transported will require a dedicated mission to map the deep atmospheric structure and understand moist convection in planets with hydrogen atmospheres.