The occurrence of giant storms on Saturn, every 20 to 30 years, may be caused by moisture in the planet’s atmosphere, which suppresses storm formation for long periods of time, according to a study published online in Nature Geoscience. The research suggests that after a number of years a massive storm is generated.
Six planet-encircling storms called Great White Spots have been observed on Saturn over the past 140 years, alternating between the equator and mid-latitudes. The most recent Great White Spot, the giant eye of which was similar in size to the Earth, erupted in 2010 and encircled the planet in six months.
Cheng Li and colleagues theorized that moisture in Saturn’s atmosphere suppresses storm-generating circulation for decades. This is because water molecules are relatively heavy compared with the hydrogen and helium that dominate Saturn’s atmosphere, and thus prevent warm air from rising and forming thunderstorms. The authors suggest that the resulting cooling of the upper atmosphere will eventually override the suppressed circulation, causing warm moist air to rapidly rise and trigger a giant thunderstorm.
Using numerical modelling, the authors showed that this mechanism matches spacecraft observations of the 2010 Great White Spot. They also propose that the absence of planet-encircling storms on Jupiter could be explained by the prediction that Jupiter’s atmosphere contains less water vapour than Saturn’s atmosphere.