[Previous] | [Session 18] | [Next]
A. P. Ingersoll, L. Li (Division of Geological and Planetary Sciences, California Institute of Technology)
Using the sequence of 70-day continuum-band images from the Cassini Imaging Science System (ISS), we record over 500 compact oval spots and study their relation to the large-scale motions (Li et al., in press, Icarus, 2004). The ~100 spots whose vorticity could be measured - the large spots in most cases - were all anticyclonic. Cyclonic features (chaotic regions) were not included in our study because they did not have a compact oval shape. We distinguish probable convective storms from other spots because they appear suddenly, grow rapidly, and are much brighter than their surroundings. The distribution of lifetimes for spots that appeared and disappeared during the 70-day period follows a decaying exponential with time constant (mean lifetime) of 4.1 days for probable convective storms and 16.9 days for all other spots. Extrapolating the exponential beyond 70 days seriously underestimates the number of spots that existed for the entire 70-day period. This and other evidence (size, shape, distribution in latitude) suggests that these long-lived spots are from a separate population. The zonal wind profile obtained manually by tracking individual features (this study) agrees with that obtained automatically by correlating brightness variations in narrow latitude bands (Porco et al. 2003). The number of westward jets that violate the barotropic stability criterion is about the same as in Voyager times. The Great Red Spot (GRS) regularly absorbs westward-moving spots that originate in the South Equatorial Belt (SEB), where most of the probable convective storms originate. Although the probable convective storms do not directly transform themselves into westward-moving spots, their common origin in the SEB suggests that moist convection and the westward jet compose a system that has maintained the GRS over its long lifetime. The Cassini project and the NASA Planetary Atmospheres Program supported this research.
[Previous] | [Session 18] | [Next]
Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.