They are popularly known as “great white spots” – akin to Jupiter’s Great Red Spot. But unlike Jupiter’s spot, which is calm at the centre, the Saturn spots generate fierce lightning storms.
The most recent great white spot, which developed in December 2010, encircled the planet within six months.
But their origins – and the reason for their scarcity – have remained somewhat of a mystery to scientists.
A new study published in the journal Nature Geoscience offers a potential explanation.
Andrew Ingersoll and Cheng Li, planetary scientists at Caltech, produced models that mimicked the creation of the great white spots. They suggest that these intense storms are due to the weight of the water molecules in Saturn’s atmosphere.
The water molecules are heavy by comparison to the hydrogen and helium that make up most of the atmosphere.
When water molecules rain out of the upper atmosphere, cold air is left lighter as a result. This halts the convection process: the cold air is no longer heavy enough to sink – at least not right away.
Using observations of the 2010 great white spot by NASA’s Cassini spacecraft, Ingersoll and Li point out that Saturn may have enough water in its atmosphere to experience what is known as a density minimum.
Saturn’s atmosphere first loses density as water molecules rain out, but soon becomes more and more dense as it continues to cool.
Eventually this extreme cooling makes the atmosphere dense enough to sink, triggering atmospheric convection and resulting in a colossal thunderstorm.
“The upper atmosphere is so cold and so massive that it takes 20 to 30 years for this cooling to trigger another storm,” says Ingersoll.