Sunday, May 12, 2013

Saturn’s Great Storm of 2010-2011

And “while the world runs round and round,” I said,
“Reign thou apart, a quiet king,
Still as, while Saturn whirls, his steadfast shade
Sleeps on his luminous ring.”
- Alfred Tennyson, Poems, “The Palace of Art”, stanza 4 line 13-16.

Figure 1: In a splendid portrait captured by NASA’s Cassini spacecraft on 7 November 2004, Saturn’s lonely moon Mimas is seen against the cool, blue-streaked backdrop of Saturn’s northern hemisphere. Delicate shadows cast by the rings arc gracefully across the planet, fading into darkness on Saturn’s night side. The part of the atmosphere seen here appears darker and more bluish than the warm brown and gold hues seen in Cassini images of the southern hemisphere, due to preferential scattering of blue wavelengths by the cloud-free upper atmosphere. Credit: NASA/JPL-Caltech/Space Science Institute.

Large planet-encircling storms are known to occur on Saturn. Such storms are the largest known convective cumulus outburst in the Solar System. The last one on Saturn took place in 1990 and was observed by the Hubble Space Telescope. In December 2010, NASA’s Cassini spacecraft detected a developing storm head which grew into a planet-encircling storm that lasted well into 2011. Being in orbit around Saturn, Cassini provided an unprecedented opportunity for close-up observations of the giant Saturnian storm. “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, Colorado. This event became known as Saturn’s great storm of 2010-2011. The storm was so huge that amateur astronomers on Earth could easily spot it using simple telescopes.

Figure 2: The largest storm to ravage Saturn in decades started as a small spot seen in this image taken by the Imaging Science Subsystem (ISS) onboard NASA’s Cassini spacecraft on 5 December 2010. The storm is visible as a spot on the terminator between night and day in the northern hemisphere. Credit: NASA/JPL-Caltech/Space Science Institute.

On 5 December 2010, Cassini observed the developing storm by chance. Although the storm was only starting to develop and appeared as a single spot on Saturn, it already measured 1900 kilometres east-west and 1300 kilometres north-south, covering an area of approximately 1.5 million square kilometres. The developing storm was centred at latitude 32 degrees north and longitude 245 degrees west. On the same day, the Radio and Plasma Wave Science (RPWS) instrument onboard Cassini detected radio pulses emitted by lightning discharges in the storm. In addition, Cassini’s Composite Infrared Spectrometer (CIRS) measured stratospheric heating caused by the developing storm and also found a thermal perturbation east of the developing storm that is consistent with the formation of an anticyclonic vortex.

Figure 3: In this view of Saturn taken by NASA’s Cassini spacecraft on 24 December 2010, the storm has grown to a north- south extent of 10,000 kilometres. The main part of the storm has an east-west extent of 17,000 kilometres. Other images taken at the same time show the tail extending almost one-third of the way around the planet - a distance of 100,000 kilometres. Credit: NASA/JPL-Caltech/Space Science Institute.

Figure 4: Saturn and Earth shown to scale. Credit: NASA/JPL-Caltech/Space Science Institute.

On 11 January 2011, Cassini acquired a full-longitude mosaic of the storm. The storm’s structure basically consists of three parts. At the westernmost end of the storm complex, a band of bright clouds constitutes the storm head. East of the storm head lies a large anticyclonic vortex. By 11 January 2011, the anticyclonic vortex had grown to an enormous size, spanning an east-west diameter of 12,000 kilometres. Continuing east from the anticyclonic vortex, the storm complex swirled on as a long tail which swept around Saturn’s northern hemisphere. Clouds within the tail of the storm complex appear turbulent with no well-defined edges.

All features of the storm complex drifted westward at different rates. Since the storm head had the fastest westward drift rate, it gradually caught up with the turbulent tail of the storm complex, like a mythical serpent biting its own tail. By the end of January 2011, the storm had completely swept around the northern hemisphere of Saturn, within a band of latitudes spanning 15,000 kilometres north-south. At those latitudes, the circumference of Saturn is 300,000 kilometres. That gave the storm complex a total area of 4.5 billion square kilometres, or about nine times the total surface area of Earth.

Figure 5: This picture of Saturn was taken on 25 February 2011 by NASA’s Cassini spacecraft. Here, the storm had already formed a tail that wrapped around the planet. Credit: NASA/JPL-Caltech/Space Science Institute.

Figure 6: These two mosaics were taken 11 hours apart by NASA’s Cassini spacecraft on 26 February 2011 when the spacecraft was 2.4 million kilometres from Saturn. White and yellow colours at the storm head are towering anvils of thunderstorm clouds created by strong convection from deeper within the atmosphere. At the anticyclonic vortex, the red colour denotes deep clouds. The blue oval on the far right of the mosaic is a cold spot in the stratosphere. Note the slight westward drift of the storm complex over the span of 11 hours. Credit: NASA/JPL-Caltech/Space Science Institute.

Between 5 December 2010 and 14 June 2011, the average westward drift rates were 2.79 degrees per day for the storm head and 0.85 degrees per day for the anticyclonic vortex. Eventually, the slower westward drift rate of the anticyclonic vortex allowed it to fall 360 degrees of longitude behind the storm head. As a result, the storm head caught up with the anticyclonic vortex by mid June 2011. First contact between the storm head and anticyclonic vortex occurred on 15 June 2011. The collision between the storm head and anticyclonic vortex caused the storm head to disintegrate. As the towering anvils of convective clouds that made up the storm head dissipated, lightning discharge rates also declined considerably and became intermittent. The storm head disappeared by 19 June 2011 while the anticyclonic vortex persisted through the collision event and continued shrinking. How the collision event shut down the storm remains unknown.

Figure 7: A series of images from NASA’s Cassini spacecraft shows the development of the storm from its start in December 2010 through mid-2011. Credit: NASA/JPL-Caltech/Space Science Institute.

Saturn’s great storm of 2010-2011 left the entire region between the latitudes 25 to 40 degrees north in a highly disturbed state. The occurrence of such storms every few decades suggests that a large amount of convective available potential energy can accumulate within the planet over a long period of time and suddenly trigger a convective cumulus outburst of planet-encircling proportions. “Saturn is not like Earth and Jupiter, where storms are fairly frequent. Weather on Saturn appears to hum along placidly for years and then erupt violently. I’m excited we saw weather so spectacular on our watch,” said Andrew Ingersoll, a Cassini imaging team member at the California Institute of Technology in Pasadena, California.

K.M. Sayanagi et al., “Dynamics of Saturn’s great storm of 2010-2011 from Cassini ISS and RPWS”, Icarus 223 (2013) 460-478