Sky and Telescope - May 2012 - (Page 25)

Storm remnant zone
NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE / ADDITIONAL PROCESSING BY S&T: SEAN WALKER

THE STORM SUBSIDES This Cassini image taken on January 17, 2012, shows that the storm and tail have subsided to a level where there remains very few traces of their former glory.

all formed from the same elements in the solar nebula. In order to reproduce the structure and motions of the GWS clouds, the winds we measured at the upper-ammonia-cloud level must extend in depth without diminished speed to at least the water clouds, below the level where solar radiation penetrates. The development of the GWS produced small changes in the structure of the wind system at the latitude of the storm. But the westward jet at 40° north remained essentially unchanged by the action of the GWS. All of these characteristics of Saturn’s winds suggest that motions in the upper cloud layers are probably deeply rooted to the massive atmosphere that extends down to about half the planet’s radius (30,000 km). The internal heat emanating from Saturn’s depths, coupled to the planet’s rapid rotation, could be driving the high-speed winds we observe at the cloud tops. This is diﬀerent than on Earth, where winds are driven by solar radiation.

Remaining Mysteries
Despite the groundbreaking observations of the 2010 storm, there are many unsolved questions raised by the GWS phenomenon. For example, why are these giant storms so rare, with only one per Saturn year? Probably

diﬀerent contributing factors must occur simultaneously in the atmosphere to trigger a GWS. On the other hand, if these giant storms are seasonally forced, how does the small and slow seasonal temperature variation in the upper atmosphere propagate 250 km downward to the water cloud level? And why did the 2010 event take place about 10 years before the usual summertime period when other Great White Spots were observed? Another mystery is the GWS’s conﬁnement to three latitude bands in the northern hemisphere (three at the equator, two at mid-latitudes, and one at a sub-polar latitude). Perhaps this eﬀect is related to the shear of the zonal jets that change with latitude. But Saturn’s jets are symmetrical by hemisphere, and don’t seem to favor the north or south. It’s possible that GWS phenomena many decades ago went undetected in the southern hemisphere, where they were obscured by the rings and their shadows on the globe, or that they took place when Saturn was hidden behind the Sun from Earth’s perspective. If the GWS occurs at a rate of one per Saturn year, the next one should not erupt until the 2040s. Perhaps then new observations will solve these and other mysteries. But before the next outbreak, scientists will continue to analyze and interpret the plethora of data from the 2010 storm retrieved by Cassini and ground-based observatories. ✦ Agustín Sánchez-Lavega is Professor of Physics at the Universidad del País Vasco in Spain. He is author of the book An Introduction to Planetary Atmospheres (2011).
Sk yandTelescope.com May 2012 25

To see more images of Saturn’s superstorm, visit skypub.com/saturnstorm.

Table of Contents for the Digital Edition of Sky and Telescope - May 2012

Sky and Telescope - May 2012
Table of Contents
Spectrum
Letters
75, 50 & 25 Years Ago
News Notes
Saturn’s Raging Superstorm
Stars Above, Earth Below: Astronomy in National Parks
The Remarkable Case of Comet Lovejoy
In This Section
May’s Sky at a Glance
Binocular Highlight
Planetary Almanac
Northern Hemisphere’s Sky
Sun, Moon, and Planets
Celestial Calendar
Exploring the Solar System
Deep-Sky Wonders
Going Deep
S&T Test Report
New Product Showcase
Telescope Workshop
Planetary Imaging with Your DSLR Camera
Gallery
Focal Point

Sky and Telescope - May 2012