CASSINI CHRONICLES THE LIFE AND TIMES OF SATURN'S GIANT STORM
Ever since the birth of a colossal atmospheric disturbance that has ravaged the northern face of Saturn for nearly a year, the high-resolution cameras of NASA's Cassini spacecraft have been systematically recording the associated tumultuous changes in the planet's appearance. Today the mission's imaging team is releasing a series of images and movies that chronicle, month by month, the development and evolution of this monster tempest, the longest lasting ever observed on Saturn.
These brand new, full-color mosaics and animated movies begin with the storm's emergence as a tiny spot in a single image on Dec. 5, 2010, and follow its subsequent growth to a storm so large it completely encircled the planet by late January 2011. The disturbance, which extends north-south approximately 9,000 miles, or 15,000 kilometers, grew to be the largest observed on Saturn in the past 21 years, and the largest by far ever observed on the planet from an interplanetary spacecraft. Other instruments on Cassini have detected the storm's electrical activity and revealed it to be a convective thunderstorm. Its active convecting phase ended in late June, but the turbulent clouds it created linger in the atmosphere today.
The storm's 200-day active period also makes it the longest-lasting planet-encircling storm ever seen on Saturn. The previous record holder was an outburst sighted in 1903 which lingered for 150 days. The large disturbance imaged 21 years ago by NASA's Hubble Space Telescope was comparable in size to the current storm. That 1990 storm lasted for only 55 days.
"The Saturn storm is more like a volcano than a terrestrial weather system," said Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena, Calif. "The pressure builds up for many years before the storm erupts. The mystery is that's there's no rock to resist the pressure - to delay the eruption for so many years."
Cassini has taken hundreds of images of this storm as part of the imaging team's "Saturn Storm Watch" campaign. During this effort, Cassini takes quick looks at the storm in between other scheduled observations, of either Saturn or its rings and moons. The new images, together with other high quality images collected by Cassini since 2004, allow scientists to trace back the subtle changes on the planet that preceded the storm's formation and have revealed insights into the storm's development, its wind speeds, and the altitudes at which its changes occur.
The storm first appeared at approximately 35 degrees north latitude on Saturn and eventually wrapped itself around the entire planet to cover approximately 2 billion square miles (5 billion square kilometers). The biggest disturbance heretofore witnessed by Cassini on Saturn occurred in a latitude band in the southern hemisphere called "Storm Alley" because of the prevalence of thunderstorms in this region; it lasted several months from 2009 to 2010. That storm was actually a cluster of thunderstorms, each of which lasted up to five days or so and affected only the local weather. The new northern disturbance is a single thunderstorm that raged continuously for more than 200 days, and impacted almost one-fifth of the entire northern hemisphere.
"This new storm is a completely different kind of beast compared to anything we saw on Saturn previously with Cassini," said Kunio Sayanagi, a Cassini imaging team associate and planetary scientist at the University of California, Los Angeles. "The fact that such outbursts are episodic and keep happening on Saturn every 20 to 30 years or so is telling us something about deep inside the planet, but we are yet to figure out what it is."
Current plans to continue the mission through 2017 will provide opportunities for Cassini to witness further changes in the planet's atmosphere as the seasons progress to northern summer.
"It is the capability of being in orbit and able to turn a scrutinizing eye wherever it is needed that has allowed us to monitor this extraordinary phenomenon," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colo. "Seven years of chasing such opportunities have already made Cassini one of the most scientifically productive planetary missions ever flown."
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the U.S., England, France, and Germany. The imaging operations center and team leader (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.