NASA spacecraft offers detailed views of Saturn’s Great White Spot
Saturn’s Great White Spot, a recurring storm on that planet that has intrigued scientists since it was first observed in 1876, is a windy, towering cloud of ammonia and water spewing out super jolts of thunder and lightning. Now astronomers and NASA’s Cassini spacecraft, which has been orbiting Saturn since 2004, have captured the most detailed views to date of the phenomenon.
The luminous storm, which may be the gaseous planet’s main mechanism for dissipating heat, occurs about once every Saturnian year, the equivalent of about 30 Earth years. The storms, however, do not follow a precise schedule. The latest round, the most intense on record, was first noticed by ground-based professional and amateur astronomers as a bright speck on Saturn’s northern hemisphere on Dec. 5, about nine years before schedule. The previous storm occurred in 1990.
Their observations coincided with Cassini’s detection of a deluge of radio waves emitted by Saturn. These radio waves are a signature of lightning and can be used as a measure of its strength.
During the days that followed, that small blemish, moving westward about 65 mph, grew to a size nearly equal to the diameter of the Earth. Two months later, the behemoth had blanketed the entire planet, spanning more than 180,000 miles.
“It turned into a very spectacular storm, with so many [lightning flashes] we couldn’t resolve individual ones,” said Donald Gurnett, a physicist at the University of Iowa and a contributing author of one of two reports published Wednesday in the journal Nature.
This massive eruption of lightning is caused when heat and water vapor rise from deep within Saturn’s atmosphere up to its troposphere, the region of the atmosphere where weather occurs. When that water vapor cools and condenses, it releases heat and, under the right conditions, produces lightning.
The lightning on Saturn originates deep inside the planet’s atmosphere, where vapors are at higher pressure. That makes the lightning very intense.
The images and measurements gave scientists new insight into the shape of the current Great White Spot. As water vapor and ammonia were pushed to the troposphere by vertical currents, some of the materials were dragged by eastern winds, creating the storm’s characteristic “head” and straggling “tail,” both of which are visible from Earth.
First author Agustin Sanchez-Lavega, a planetary scientist at the University of the Basque Country in Bilbao, Spain, and his colleagues were able to estimate that the storm’s head, where most of the lightning was concentrated, extended about 160 miles below the cloud tops. Because the sun doesn’t shine there, this suggests that the planet’s internal heat helps the storms to form, the scientists wrote.
Great White Spots are 10 times larger than normal storms on Saturn and are about 10,000 times stronger than those on Earth. They occur seasonally due to changes in how much sunlight reaches Saturn. Scientists still don’t fully understand the interplay between solar energy and Saturn’s internal stores in generating these storms.
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