Equatorial Clouds and Haze before, during, and after Jupiter's Global Upheaval

HST/WFPC2 images of Jupiter from 2000, before the current upheaval, showed thick and nearly complete cloud coverage in the equatorial zone. Images from 2006/2007 show a decrease in cloud cover; most remaining clouds are associated with plumes extending from the northern and southern boundaries of the equatorial zone. The decreased cloudiness corresponds to a 20-30% decrease in the 953-nm continuum I/F within ± 5° of the equator, between 2000 and 2007. A preliminary comparison with 892-nm methane band images from both epochs suggests that the tropospheric haze, located just above the upper cloud sheet and just below the tropopause, is unchanged. The fine aerosols composing this haze may be composed of condensed hydrazine (a product of ammonia photolysis at the altitude of the haze layer) mixed with smaller amounts of hydrocarbon and other photochemical products drifting down from the stratosphere (Atreya et al., Icarus 31, 1977; Atreya et al., Plan. Sp. Sci. 53, 2005). Cloud particles from deeper in the troposphere may also be lofted into the haze region (West et al., Icarus 65, 1986), where particles with radii < 1 {μ}m precipitate on a timescale of about a year (Rossow, Icarus 36, 1978). Due to the large energies needed to penetrate into the stably stratified haze region, particle compositions may include NH4HS and H2O as well as NH3. If the clearing of equatorial clouds associated with the upheaval persists for several years, it will provide an opportunity to compare the contributions of the two haze formation mechanisms. The observed reduction in equatorial cloud cover implies a drop in the upward transport of fine particles, which would lead to a reduction of tropospheric haze within 1-3 years, as the small particles gradually fall out of the upper troposphere. Hydrazine haze should be in a steady state balance between photochemical production and loss through particle growth, precipitation, and evaporation. The reduced equatorial cloud opacity will lead to warming in the haze layer, with changes apparent in about 5 years according to estimates of the radiative timescale (Conrath et al., Icarus 83, 1990). Monitoring Jupiter's haze and cloud opacity over the next few years may therefore constrain the origin of the upper tropospheric haze, with rapid changes implying a significant source from deeper tropospheric condensation clouds.