It has been a turbulent 200 days on Saturn and NASA’s Cassini spacecraft has captured it all on camera revealing brand new images and animations that chronicle the development and evolution of the greatest and longest lasting storm to hit the planet in more than a century.
“The storm is created when the internal heat from the planet is stored up as water vapour at the base of the clouds,” says Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena, California. “For some reason that we don’t understand, this heat is released in giant episodes, which occur every 20-30 years. The heat is released when the water vapour condenses as in a terrestrial thunderstorm but much more energetically. We know this because we detect copious amounts of radio emissions from lightning in the storm – 1,000 times as intense as terrestrial lightning.”
Monthly observations returned by Cassini show the storm beginning to make its presence known on 5th December 2010 through the disturbance of a tiny spot, soon noticed by amateur astronomers, that grew into a storm so large that it entirely encircled the planet by late January 2011, as illustrated a few snapshots later.
“This is only the sixth planet-encircling storm in modern times,” says Ingersoll. “The first was in 1876. The latitudes were 8, 36, 5, 58, 12 and 34 degrees and all in the northern hemisphere. On the other hand there is a 1 in 32 chance that they would all be in one hemisphere, so it could be a coincidence.”
Originally, the storm appeared at around 35 degrees north latitude on the gas giant before ringing the planet, covering approximately 2 billion square miles. Extending in a north-south direction for approximately 9,000 miles, the melee of angry rumbles and gusty winds is not only the largest and longest storm to ever be observed on the planet by an interplanetary spacecraft, but has topped the records as the greatest storm to have ever been witnessed on the usually bland visible surface, beating previous record holder, the 1903 outburst which clung to the atmosphere for 150 days.
“The change [in Saturn’s appearance] is definitely drastic! It affected one fifth of the northern hemisphere of Saturn,” says Kunio Sayanagi, a Cassini imaging team associate and planetary scientist at the University of California, Los Angeles. “From the storm clouds expansion rate [when it was growing, the storm was expanding at about 100 square miles per second!], we calculated how much mass must have been moving. We figured out that it was moving so much mass that it would displace an equivalent of the entire atmosphere of Earth in 150 days – so it was truly a huge event.”
Taking hundreds of images as part of the Cassini team’s Saturn Storm Watch campaign, the spacecraft briefly turns its attention to the raging storm between instructed observations of either the gas giant’s rings or moons that hug its sides. Combining the new images together with high quality images collected by Cassini since 2004, experts have been able to look back to when subtle changes began to appear on the surface that preceded the storm’s formation, revealing information about the disturbance’s development, wind speeds and the altitude at which the changes occur.
“All our images are taken with Cassini’s Imaging Science Subsystem (ISS) instrument, which covers the ultraviolet, visible and a little bit of near-infrared part of the spectrum,” says Sayanagi.
“The ISS gets the highest spatial resolution images among the Cassini instruments. Other instruments have been used to observe the storm as well, namely the Composite InfraRed Spectrometer [CIRS – covers infrared and gives very high spectral resolution], the Visible and Infrared Mapping Spectrometer [VIMS – covers a very wide band between visible to thermal infrared at relatively high spectral resolution and medium spatial resolution and is useful for studying the vertical structure of the storm] and the Radio and the Plasma Wave Science instrument [RPWS – detected static noise coming from lightning in the storm].”
While the team may have captured the storm, questions have been left in the aftermath. Fortunately Cassini’s investigation of the ringed planet will continue until 2017, covering the change in seasons that come and go in its gaseous atmosphere. “This new storm is a completely different kind of beast compared to anything we have seen on Saturn previously with Cassini,” says 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 it is.”
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