HR: 1330h
AN: A12A-0072    [PDF]
TI: Ammonia Clouds of Jupiter and Saturn - Here Today, Gone Tomorrow?
AU: * Atreya, S K
EM: atreya@umich.edu
AF: University of Michigan, Department of Atmospheric, Oceanic, and Space Sciences, Ann Arbor, MI 48109-2143 United States
AU: Wong, A
EM: aswong@umich.edu
AF: University of Michigan, Department of Atmospheric, Oceanic, and Space Sciences, Ann Arbor, MI 48109-2143 United States
AB: Ground-based, satellite, and spacecraft observations show that, with the exception of some relatively clear regions (belts, hot spots, warm areas), clouds exist everywhere on Jupiter and Saturn. Thermochemical models (Atreya et al., 1999) combined with observational evidence (e.g. Galileo NIMS obiter imaging, Banfield, et al., 1998; Galileo probe data, Atreya et al., 1999, 2003a) suggest that the composition of the upper cloud layer of Jupiter and Saturn is ammonia. Yet, spectrally identifiable ammonia clouds (SIAC) are detected only in certain discrete regions of Jupiter (Baines et al., 2002; Wong, et al., 2003a) and nowhere on Saturn. Less than 1% of Jupiter's area is covered with SIACs according to Galileo NIMS observations (Baines et al., 2002). In this work, we argue that spectral signature of all but the freshly made SIACs is masked by dusting of the ammonia clouds by photochemically produced hydrocarbon haze falling from the stratosphere (Atreya et al., 2003b). We estimate the deposition rate of the coating haze layer to be at least 100 A/year on Jupiter, enough to turn the ammonia cloud into a spectrally grey cloud. The haze particles are composed mostly of condensed polycyclic aromatic hydrocarbons (Wong, et al., 2003b), with smaller contribution from polyyne polymers, nitrile polymers, solid hydrazine and meteoritic dust. Cloud properties including cloud opacity, cloud top height, and the particle size can also be a factor in the lack of detection of ammonia clouds. The lack of spectral identification of Saturn's ammonia clouds can also be explained similarly. References: {\em Atreya et al.}, Planet. Space Sci., 47, 1243, 1999. {\em Atreya et al.}, Planet. Space Sci., 51, 105, 2003a. {\em Atreya et al.}, Jupiter's ammonia clouds-Localized or ubiquitous? Planet. Space Sci., submitted, 2003b. {\em Baines et al.}, Icarus, 159, 74, 2002. {\em Banfield et al.}, Icarus, 135, 230, 1998. {\em Wong, M.H., et al.}, PSS, in press, 2003a. {\em Wong A.S., et al.}, GRL, 30, art. no. 1447, 2003b.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0343 Planetary atmospheres (5405, 5407, 5409, 5704, 5705, 5707)
DE: 6207 Comparative planetology
DE: 6220 Jupiter
DE: 6275 Saturn
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting