Sky and Telescope - May 2012 - (Page 22)

Stormy Weather NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE (2) reﬂectivity at visible wavelengths and in its size, about six times that of the most frequent bright spots detected from Earth-based telescopes under good seeing conditions. Amateur images were of fundamental importance to capturing the initial stages of the disturbance’s evolution. They enabled my planetary science colleagues and I to determine the spot’s growing size and brightness, which in turn allowed us to constrain the storm’s early dynamics and vertical cloud structure. Except for the ISS’s initial December 5th image of the spot, no Cassini picture was obtained until December 22nd. During this critical period, amateurs around the world took images every planet rotation (about 10 hours, 39.37 minutes). This major eﬀort provided the most complete database of highquality images of a GWS. You can see these contributions at the Planetary Virtual Observatory and Laboratory’s website at www.pvol.ehu.es/pvol. The Spot Grows a Tail THE SPOT EMERGES The new Great White Spot was discovered in amateur images taken on December 8 and 9, 2010. But analysis of this Cassini image taken on December 5th revealed a small white spot at the site of the eruption. Jean-Jacques Poupeau in France showed that the spot had grown to a width of 8,000 km while simultaneously increasing in brightness at red, green, and blue wavelengths. Two days later, images from multiple observers around the world showed that the spot was expanding eastward, developing a tail. All of these lines of evidence — the large size, high brightness, and zonal expansion — were unambiguous signatures of a sixth GWS event. In a matter of a week its head (the GWS proper, i.e. the “source” of the disturbance) grew to 10,000 km in diameter — almost the width of Earth. The feature’s brightness made it easily visible through small amateur telescopes against Saturn’s usual pale-yellow clouds and featureless disk. The GWS was magniﬁcent in its high The head remained alive for several months. But highspeed winds quickly drove the disturbance eastward, forming a tail. Similar to previous storms, the tail of the 2010 GWS fully encircled the planet in about 55 days, producing a band of patchy bright clouds ringing Saturn. The tail’s pattern resulted from the complex turbulent motions and eddies generated in the head’s wake as it interacted with high-altitude winds. The shapes of the storm’s head and tail were sculpted by the shear of the permanent zonal winds, which consist of parallel bands that alternately blow east and west. The 2010 GWS head, centered at 40° north, moved westward, dragged by easterly winds of 30 meters per second (67 mph). But at 35° north, the wind blows eastward and the speed increases to 50 m/s (112 mph). At 44° north, the winds blow eastward at only 20 m/s (45 mph). Thus the tail’s structure depended on the outbreak latitude and how the winds changed in speed north and south of the outbreak point. In the 1990 equatorial event, for example, the cloud pattern moved eastward and westward from the head, forming a tail on each side of the original GWS. S. Gomizadegh 12/8 T. Kumamori 12/9 A. Wesley 12/10 M. Delcroix 12/11 C. Go 12/13 T. Akutsu 12/13 OUTBREAK These amateur images from December 2010 were critical in showing the eruption and early expansion of the Great White Spot. 22 May 2012 sky & telescope http://www.pvol.ehu.es/pvol

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