Cassini watches storm on Saturn choke itself
By David Szondy
February 4, 2013
NASA’s Cassini probe witnessed a giant storm on Saturn that raced around the ringed planet, ran into its own tail, and consumed itself like the legendary Ouroboros – a snake that eats its tail. The storm, which at its peak was almost as wide as the Earth, lasted for 267 days and was marked by violent lightning bursts and an unusual rise in air temperature. This self-destructing storm is a phenomenon never seen before, and sheds new light on Saturn’s meteorology that could help with weather prediction on Earth.
Storms on the outer planets are quite common and have been known to science since the first was detected on Jupiter by radio astronomers in 1950. Despite decades of study, they’re still not very well understood, but what is certain is that they operate on a scale that is unimaginable by any terrestrial counterpart.
The Great Red Spot on Jupiter, for example, is a hurricane large enough to engulf up to three Earths and may have been raging for over 400 years and is still going strong. On Saturn, lightning has been observed by the unmanned Cassini orbiter that is 10,000 times as powerful as the greatest lightning strikes on our planet. In other words, a typical day on these giants would make Hurricane Katrina look like a mild zephyr.
The tail-eating storm on Saturn was first detected by Cassini on December 5, 2010 at around 33° north latitude and was tracked with the probe’s radio and plasma wave subsystem and imaging cameras. The storm quickly grew in intensity until a bright, turbulent head appeared with a powerful, clockwise-rotating vortex following behind at a slightly slower speed as the storm moved west.
Over the months that followed, the storm spread in a band around the planet until it was about 190,000 miles (300,000 km) in circumference with a dramatic display of thunder and lightning creating shear forces going in both directions. In addition, Cassini detected very large increases in air temperature in the storm as well as an unexpected increase in ethylene gas.
The storm continued its career until the head ran into its own tail and disrupted it. It soon sputtered out and thunder and lightning ceased by August 28, 2011. According to NASA scientists, why this happened remains a mystery.
This event is the first time scientists have observed a storm consume itself in this way anywhere in the Solar System. In some ways, the storm is similar to hurricanes and typhoons on Earth. Like hurricanes, the Saturnian storm is basically a heat engine that feeds on warm air and leaves behind a cold wake with the temperature differences giving the storm its enormous power.
The biggest difference is that storms on Earth can’t run into themselves because the land masses on our planet act as barriers that divert their courses. Since Saturn is just a big ball of gas, there aren't any such barriers and winds can roll right around the planet. This is not only one reason for the colored bands seen on Saturn, but it also makes it very interesting for meteorologists because it provides them with a simple model to better understand the more complex weather on Earth.
However, one challenge is that there are many mysteries about the gas giants. Saturn’s storms are different from those of Jupiter. Jovian storms can last for centuries while Saturn's are much more short-lived. “Short-lived,” however, means that they last for months. Jovian storms also have relatively calm centers whereas those of Saturn are extremely violent, with lots of thunder and lightning – giant storms known as “great white spots” erupt once every Saturnian year (about 30 Earth years). This particular one showed up ten Earth years early.
"This thunder-and-lightning storm on Saturn was a beast," said Kunio Sayanagi, a Cassini imaging team associate at Hampton University, Virginia. "The storm maintained its intensity for an unusually long time. The storm head itself thrashed for 201 days, and its updraft erupted with an intensity that would have sucked out the entire volume of Earth's atmosphere in 150 days. And it also created the largest vortex ever observed in the troposphere of Saturn, extending up to 7,500 miles (12,000 km) across."
This storm was not only extremely violent, it was also the longest running giant storm and the second longest storm yet seen on Saturn. A 2009 storm in the southern hemisphere lasted 334 days, but was 100 times smaller than the 2011 storm, which left behind a wide band of turbulence that has never before been seen on Saturn. Infrared detectors on Cassini still show lingering effects in the upper atmosphere, but the weather-producing troposphere is now quiet.
The Cassini probe is a joint NASA/ESA mission. Weighing 5,560 lbs (2,523 kg), it carries a battery of instruments and is powered by three radiothermal generators – at such a distance from the sun, nuclear power is the only current option for a craft of that size. Cassini was launched October 15, 1997 from the Cape Canaveral Air Force Station, Florida, and is controlled from the Jet Propulsion Laboratory in Pasadena, California. It was flung toward Saturn after two flybys of Venus and then Jupiter before arriving in orbit about the ringed planet on July 1, 2004. The Cassini spacecraft also carried with it the ESA Huygens probe, which landed on Titan on January 14, 2005.
The findings of the Cassini team were published in the journal Icarus.
The video below discusses the Saturnian storm.