The Thunderstorm-related Clouds of Saturn: Composition, Structure, and Origin as Constrained from Cassini/VIMS Spectral Imagery

Details The Thunderstorm-related Clouds of Saturn: Composition, Structure, and Origin as Constrained from Cassini/VIMS Spectral Imagery The Thunderstorm-related Clouds of Saturn: Composition, Structure, and Origin as Constrained from Cassini/VIMS Spectral Imagery

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.2807b&link_type=abstract

American Astronomical Society, DPS meeting #41, #28.07

Astronomy and Astrophysics

Astronomy

Scientific paper

Thunderstorm activity on Saturn is associated with two types of spectroscopically-unique clouds. Type #1 features are comparatively localized (< 2000 km in extent), short-lived clouds that are bright in most pseudo-continua wavelengths from 0.8 to 4.1 micron but are also lower in 2.73-micron reflectivity, similar to Jupiter's spectroscopically-identifiable clouds (2002, Icarus 159, 74-94). As on Jupiter, these clouds may be markers of unusually powerful updrafts - perhaps associated with underlying water-based thunderstorms - that transport ammonia gas from depth to the cloud-producing ammonia condensation level. Indeed, electrostatic discharge signals of lightning have been detected at these locales by Cassini/RPWS (e.g., 2008, Space Sci. Rev., 137, 271- 285). In contrast, Type #2 clouds are larger ( > 4000 km in extent), have a circular shape, and are longer-lived, lasting for weeks rather than days. These clouds are dark throughout the near-infrared, with reflectivities 20% less than typical neighboring clouds at all 0.8-4.1-micron pseudo-continua. We propose that active thunderstorms originating in the 10-20 bar water-condensation region vertically transport dark materials at depth to the observable level near 1 bar. As well, this material may be produced by lightning within such convective storms. Investigation of a variety of lightning-generated materials formed from Saturnian gases reveals the most viable candidate spectroscopic material to be relatively small particulates of elemental carbon, formed by lightning-induced dissociation of methane. We postulate that these particles are subsequently upwelled from depth - perhaps embedded within and on the surface of spectrally-bright condensates such as ammonium hydrosulfide. Initial results for the single-scattering albedo (SSA) of dark clouds with cloudtops in the 0.9-1.5-bar range indicate a Mie-scattering SSA of 0.9501 - 0.9552 for 1-micron radius aerosols at 0.8 micron, considerably lower than for the putative ammonia clouds which have a SSA of 0.9858-0.9898 for the same conditions.

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