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Cassini has an eye for a Saturn storm
Posted: 21 November 2011

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NASA’s Cassini spacecraft got a front row seat to a monstrous storm raging on the surface of Saturn as it snapped high resolution images and rolled film to chronicle what has been recorded as the gas giant’s most chaotic 200 days in the past 21 years.

A false-colour mosaic from NASA’s Cassini spacecraft showing the tail of Saturn’s huge northern storm. Image: NASA/JPL-Caltech/Space Science Institute.

The brand new true and false colour mosiacs, which contain dozens of images stitched together along with animated movies, begin with the emergence of the storm as a tiny spot located approximately 35 degrees north in a single frame on 5 December 2010 and follow its growth into a turbulent system that became so large that it encircled the planet by late January 2011, extending approximately 2 billion square miles. “The Saturn storm is more like a volcano than a terrestrial weather system,” says Andrew Ingersoll, a Cassini imaging team member of the California Institute of Technology. “The pressure builds up for many years before the storm erupts. The mystery is that there’s no rock to resist the pressure [and] to delay the eruption for so many years.”

Keeping a close eye on Saturn since the very first twinges of atmospheric disturbance, Cassini has produced a month-by-month account of the storm as it developed and evolved into a violent tempest ravaging the northern face of the gas giant. “Small storms do seem to happen at all latitudes we have looked at, but not very many compared to Jupiter,” says Kunio Sayanagi, a Cassini imaging team associate and planetary scientist at the University of California, Los Angeles. “But the huge ones – the ones that are so big that they engulf one entire latitude zone – have been seen only in the northern hemisphere, and we do not understand the reason for this. It’s very safe to say that these planet-encircling storms are the biggest thunderstorms in our Solar System and seem to be unique on Saturn.” Extending around 9,000 miles, the electrical storm is the largest by far to ever be observed on the planet by an interplanetary spacecraft, stealing the ‘greatest-storm’ crown from the previous record holder; an outburst sighted in 1903 which lingered for 150 days. “Saturn is normally very bland looking, but the latitudes from 29 to 39 degrees are now very chaotic, with lots of coloured swirls and turbulent streamers,” adds Ingersoll.

This series of images from NASA’s Cassini spacecraft shows the development of the largest storm seen on the planet since 1990. These true-colour and composite near-true-colour views chronicle the storm from its start in late 2010 through mid-2011, showing how the distinct head of the storm quickly grew large but eventually became engulfed by the storm’s tail. Image: NASA/JPL-Caltech.

With the spacecraft taking hundreds of pictures of the ringed planet, the imaging team devised ‘Saturn’s Storm Watch’ which sees Cassini taking quick glimpses of the storm between scheduled observations of either Saturn or its rings or moons. The new images, combined with high quality snapshots provided by the spacecraft since 2004, have allowed scientists to pinpoint any subtle changes on the planet that preceded the storm’s formation, revealing insights into the disturbance’s development along with wind speeds and altitudes at which the changes occur. “It is powered by the heat that is slowly leaking out of Saturn – all planets form with ‘primordial energy’; the energy collected during the formation,” says Sayanagi. “Earth has it too and volcanoes are the living evidence that the planet is continuing to cool down and release the internal heat. Saturn is like that too, except that because it is a gas giant planet, it does not have a hard crust like Earth does. On a gas giant planet, we’d expect the slow heat release to occur continuously [as Jupiter seems to operate that way], but Saturn seems to release that heat in a huge burst every 20–30 years.”

Just as the storm has whipped up the constituents of Saturn’s atmosphere, it has also unearthed questions about the gas giant’s interior. “This new storm is a completely different kind of beast compared to anything we saw on Saturn previously with Cassini,” concludes Sayanagi. “The fact that such outbursts are episodic and keep happening on Saturn every 20 to 30 years or so is telling us about something deep inside the planet, but we are yet to figure out what that is.”

With current plans to extend the mission through to 2017, the team hope that their questions will be answered by Cassini as it scrutinizes Saturn’s atmosphere, returning any further changes as the ringed planet spins on its orbit to northern summer.

Cassini’s collection of images and movies of Saturn’s storm can be seen here, here and here!