On rare occasions, the serenity of Saturn's atmosphere is disrupted by a significant storm. This image of one such storm was captured by the HST on December 1, 1994. It is the white feature shaped like an arrowhead lying close to the planet's equator. The extent of the storm, from east to west, was as great as Earth's diameter - about 12,700 kilometers (7,900 miles). It had changed very little since its discovery in September 1994.
Saturn's prevailing winds have shaped a dark 'wedge', seen here eating
into the western (left side) of the bright cloud. The Voyager spacecraft
measured the wind speed as 1,000 miles per hour at the latitude where the
wedge has appeared. These strong winds blowing over the northern part of
the storm created a disturbance that generated faint white clouds further
east. To the north of the storm, where the winds decrease, cloud is being
swept westward. The white clouds are made of ammonia crystals. They form
when warmer gases from below push upwards through the cold cloud tops.
The HST observed a similar but larger storm on Saturn in September
1990. Very major storms such as the 1990 one seem to occur about once every
57 years, which is about twice the time it takes Saturn to orbit the Sun.
White clouds associated with minor storms are reported more frequently,
but the 1994 storm seen here is a relatively large and scarce event.
When this image was taken, Saturn was 1,446 million kilometers (904 miles) from Earth.
Camera: WFPC2 in WF mode
Technical Information: Exposures through three color filters
(blue, green and red) within 6 minutes combined to create a true color
image. The blue fringe on the right limb is an artifact of the processing
used to compensate for the rotation of the planet between exposures.
Credit: R. Beebe (New Mexico State University), D. Gilmore and
L. Bergeron (STScI), and NASA
