Storms on Saturn
Saturn's storms range from thunderstorms the size of Earth to the Great White Spots, giant outbreaks that erupt roughly once per Saturnian year, about every 30 years, and can stretch all the way around the planet. The most recent began in December 2010 and was followed by Cassini for over half a year. Saturn also has huge hurricane-like vortices at both poles and lightning far stronger than typical lightning on Earth.
Contents
Why Saturn has violent storms
Like thunderstorms on Earth, Saturn's storms are powered by moist convection. Deep in the atmosphere, water vapor rises, condenses into clouds and releases latent heat, which makes the rising air even more buoyant. Saturn also has a strong internal heat source, emitting about 2.5 times the energy it receives from the Sun, which keeps the deep atmosphere churning. See interior.
What makes Saturn special is how the energy builds up. On Saturn, water vapor is heavier than the surrounding hydrogen-helium air. Moist air therefore tends to stay put below the water clouds, and convection can be suppressed for years. A model published in 2015 by Cheng Li and Andrew Ingersoll proposed that the upper atmosphere slowly cools by radiating to space until, after decades, it becomes unstable enough for a massive plume to break through. This would explain why Saturn's giant storms are rare, enormous and roughly periodic. See atmosphere.
The Great White Spots
The largest storms appear as brilliant white clouds because they carry fresh ammonia ice up from below. Observers have recorded six major outbreaks:
| Year | Latitude | Notes |
|---|---|---|
| 1876 | Equatorial | Observed by Asaph Hall, who used it to time Saturn's rotation |
| 1903 | Northern mid-latitudes | Observed by E. E. Barnard |
| 1933 | Equatorial | Discovered by the English actor and amateur astronomer Will Hay |
| 1960 | Northern mid-latitudes | Spotted by amateurs and followed by professional observers |
| 1990 | Equatorial | Discovered by amateur Stuart Wilber, imaged by Hubble |
| 2010–2011 | About 35° N | Followed in detail by Cassini and amateur astronomers |
Most outbreaks occurred around northern summer, roughly 30 years apart, which led to the idea of a seasonal cycle. The 2010 storm broke the pattern by appearing about a decade early, during northern spring, a sign that the timing depends on internal processes and not only on sunlight.
The 2010–2011 Great Springtime Storm
On 5 December 2010, Cassini's radio instrument detected a burst of lightning signals, and its cameras saw a small bright spot at about 35° north. Amateur astronomers picked it up within days. The storm head grew explosively, spawning a turbulent tail that spread eastward, and by late January 2011 the disturbance had wrapped around the entire planet, a band of clouds roughly 300,000 km long.
Cassini recorded lightning at rates of up to around ten flashes per second. Its infrared instruments discovered that the storm had heated a region of the stratosphere, above the clouds, by more than 80 °C, creating "beacons" rich in ethylene, a compound otherwise barely detectable on Saturn. Spectra also revealed water ice, evidence that the plume had dredged up material from the deep water-cloud layer.
The storm lasted about 200 days. In June 2011 its head collided with the tail it had generated as it circled the planet, and convection shut down within weeks. The atmospheric aftermath, including the warm stratospheric vortex, remained detectable for years. The event was the most thoroughly observed planetary storm ever recorded beyond Earth. See Cassini.
Polar vortices
Both poles of Saturn host giant cyclones. In 2006, Cassini imaged the south polar vortex: a warm storm about 8,000 km across, with a clear eye surrounded by towering walls of cloud and winds of around 550 km/h. Its eyewall clouds rise tens of kilometres above the surrounding cloud tops, two to five times taller than those of terrestrial thunderstorms. Unlike an Earth hurricane, it does not wander across an ocean; it stays locked at the pole.
The north polar cyclone, seen in detail once sunlight reached it in 2012, has an eye about 2,000 km wide, with winds of roughly 150 m/s. It sits at the center of the famous six-sided jet stream, described on the hexagon page.
Storm Alley and lightning
Between 2004 and 2009, Cassini found that thunderstorms on Saturn clustered around a band at about 35° south, nicknamed Storm Alley. One long-lived storm there, observed in 2004, became known as the Dragon Storm for its shape in infrared images. These storms were the source of Saturn Electrostatic Discharges, bursts of radio noise produced by lightning, which Voyager 1 had first detected in 1980 without being able to pinpoint their origin.
Spotting the flashes themselves was harder, because Saturn's dayside is bright and its night side was often lit by sunlight reflected from the rings. In 2009, Cassini finally captured visible lightning on the night side. Estimates suggest individual flashes are far more energetic than typical terrestrial lightning, consistent with very deep, powerful updrafts. As the seasons changed, storm activity migrated to the northern hemisphere, and the 2010 outbreak began in the same latitude band, mirrored in the north. See seasons.
Watching Saturn's storms
Large storms are within reach of amateur telescopes, and amateurs have discovered or co-discovered several Great White Spots. Today, Hubble's long-term planetary monitoring and the James Webb Space Telescope continue to watch for new outbreaks. The Hubble and Webb page covers recent observations, and photographing Saturn explains how amateurs image such features.
FAQ
How often do giant storms happen on Saturn?
Great White Spots have appeared roughly once per Saturnian year, about every 30 years: in 1876, 1903, 1933, 1960, 1990 and 2010. The pattern is not strict, as the 2010 storm came early.
How big was Saturn's 2010 storm?
Its head was larger than Earth, and within about eight weeks its tail had wrapped around the whole planet, a distance of around 300,000 km. It lasted about 200 days.
Is there lightning on Saturn?
Yes. Lightning was first detected through its radio signals by Voyager 1 in 1980, and Cassini photographed the flashes in 2009. Saturn's lightning appears to be much more energetic than ordinary lightning on Earth.
Does Saturn have hurricanes?
It has hurricane-like cyclones at both poles. The south polar vortex is about 8,000 km across with a well-defined eye, and the north polar cyclone at the center of the hexagon has an eye about 2,000 km wide.
Sources
- Sánchez-Lavega, A. et al. (2011), "Deep winds beneath Saturn's upper clouds from a seasonal long-lived planetary-scale storm", Nature 475
- Fischer, G. et al. (2011), "A giant thunderstorm on Saturn", Nature 475
- Li, C., Ingersoll, A. P. (2015), "Moist convection in hydrogen atmospheres and the frequency of Saturn's giant storms", Nature Geoscience 8
- NASA Science: Cassini mission