Variation in Ammonia Abundances in Hot Spots on Jupiter
Abstract:We used NIRSPEC on the Keck telescope in 2014 to spectrally resolve line profiles of CH3D, NH3, PH3, and H2O in 5-micron Hot Spots on Jupiter. The profile of the CH3D lines is very broad in both NEB and SEB Hot Spots due to collisions with up to 8 bars of H2, where unit optical depth occurs due to collision-induced H2 opacity. The extreme width of these CH3D features implies that Hot Spots do not have significant cloud opacity for P > 2 bars. We retrieved NH3, PH3, and gaseous H2O inside a Hot Spot at 8.8°N in the NEB and also in an SEB Hot Spot at 17°S. Both Hot Spots have similar radiances, indicating that both regions have very thin upper-level clouds. They both have a strong depletion in H2O for P< 4 bars, suggesting that subsidence is responsible for the depletion of volatiles in Hot Spots. The NEB Hot Spot is depleted in NH3 with respect to adjacent latitudes. Interestingly, the SEB Hot Spot does not have a different NH3 abundance than neighboring regions. The SEB, including the Hot Spot, exhibits strong NH3 absorption in the 2 to 4-bar region.
The SEB went through a “fade” or whitening in 2010, and a subsequent “revival” or darkening between 2010 and 2014, implying a cycle of condensation and sublimation of NH3 ice. This cycle may play a role in the differences we observed between the NEB and SEB Hot Spots. The dynamical origin of SEB Hot Spots is much less studied than that of NEB Hot Spots, so our observations of NH3 humidity may provide a key piece of the puzzle. We plan to acquire 5-micron spectra simultaneously with spatially-resolved microwave observations, such as with the Juno Microwave Radiometer or the Very Large Array. Microwave opacity is dominated by gaseous NH3 rather than cloud opacity. We predict that SEB Hot Spots where the 5-micron spectrum shows strong NH3 absorption (like those we observed in 2014) would not stand out in microwave data, breaking the 1:1 correspondence seen between microwave and 5-micron intensity by Sault et al. (2004, Icarus 168 336-343.)