Modeling Saturn's Giant Storms: Water, Ammonia, and the 30-Year Periodicity

Abstract:

A giant planet-encircling storm occurred on Saturn on Dec. 5^th, 2010 at planetographic latitude 37.7^oN. It produced intense lightning, created enormous cloud disturbances and wrapped around the planet in 6 months. Six such storms, called Great White Spots, have erupted since 1876. They have alternated between mid-latitudes and the equator at intervals ranging from 20 to 30 years. The reason for the intermittent explosion is hitherto unclear and there are no similar storms on brother Jupiter. Here we describe the water-loading-mechanism, which could suppress moist convection for decades due to the larger molecular weight of water in a hydrogen-helium atmosphere. We show that this mechanism requires the deep water vapor mixing ratio to be greater than 1.0%. Observations imply that Saturn’s atmosphere is more enriched in water than Jupiter, which could explain why Saturn has such storms and Jupiter does not. We further use a two-dimensional axisymmetric dynamic model and a top-cooling convective adjustment scheme to connect our theory to observation. We show that for a deep water mixing ratio of 1.1%, the ammonia vapor is depleted down to 6 bars, the tropospheric warming is about 6 K, and the interval between two consecutive storms at one place is about 70 years. These values are confirmed by both ground-based and spacecraft observations.