Anvil clouds tower more than 30 miles high, casting a pall over a hazy sky. Amid the gathering gloom, 100 mph winds whip clouds across the sky, while lightning punctuates the tumult repeatedly. Meanwhile, clouds from yet another giant storm dump several inches of rain daily over an area more than 600 miles on one side.
Given that severity, and thunderheads three times as high as we see in North America, this storm is obviously not on Earth, although the storms have similarities to terrestrial weather systems. This is Jupiter.
Astronomers from Cornell University, the California Institute of Technology and the NASA Galileo Imaging Team at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., have discovered that some thunderstorms on Jupiter closely resemble clusters of thunderstorms, called mesoscale convective complexes (MCCs), found on Earth. Contrary to previous belief, these MCCs develop from the intense heat emanating from Jupiter's core rather than from the sun. And these MCC's drive the planet's weather system. The findings appear in the latest issue (Feb. 10) of the journal Nature.
"There is a lot of activity we see on Jupiter that we see on Earth," says Peter J. Gierasch, Cornell professor of astronomy and a lead author on a letter to Nature detailing the team's findings. "We see jet streams, large cyclonic elements, large anti-cyclonic elements and many elements of unpredictability and turbulence."
Of all the tempest-tossed storms in the solar system, the astronomers chose to examine an area west of the giant planet's great red spot, in a region known as the south equatorial belt. The scientists studied images taken by the NASA Galileo spacecraft when it orbited Jupiter on
May 4, 1999. The images were part of a planned effort to search for and study local convection.
What is remarkable about the Jovian MCCs, says Gierasch, is that they have the same physics as thunderstorm clusters on Earth, but the heat source to generate these events is completely different. Generally, thunderstorms on Earth are small cells of cumulonimbus clouds, singular in nature, and caused by summertime heat from the sun. An MCC, or cluster of many cells of thunderstorms of the type that commonly strikes the midwestern United States, also is formed by intense summertime heat.
The difference between the formation of an MCC and that of a hurricane or cyclone is where the system gets its fuel. Hurricanes and cyclones on Earth are fueled by the warm ocean. MCCs develop because of atmospheric instability. Where it is warm near the Earth's surface in the summer and cooler aloft, condensation rises and forms many cells of intense thunder clouds over a vast area. These summertime giants can last for hours, even days, and dump unusually large amounts of rain. On Jupiter, they can also last from about 12 hours to several Earth days, producing huge quantities of rain.
Jupiter, the largest planet in the solar system, is about 480 million miles from the sun, compared with Earth's distance of 93 million miles. The giant planet generally is thought to have been developed some 5 billion years ago, and it is perpetually shrouded by swirling clouds of water and ammonia ice. But because of the heat reservoir of highly compressed hydrogen in the planet's center, this gaseous giant emits nearly 70 percent more heat than it absorbs from the sun. This is what leads these astronomers to suggest that the source of the stormy turbulence on Jupiter seems to be the planet itself.
Gierasch explains that, interestingly, the physical attributes of Jupiter's vast thunderstorms are the same as those on Earth, except Earth's storms develop because of the sun's heat and Jupiter's storms develop from its own internal heat source. Jupiter's core still retains heat from the planet's original formation by collapse and compression. "It is in the process of cooling, and it will likely continue to cool for at least another five billion years," Gierasch says.
An almost-continuous cycle drives the Jovian weather, he explains. The storms develop, drop precipitation, the precipitation evaporates prior to reaching Jupiter's core heat-source and the condensation rises again. Galileo's instruments are not able to detect lightning on the planet's sunlit side. But once the storm crosses into the dark side, astronomers are able to see the lightning and confirm the existence of MCCs.
One part of these Jovian storm systems that dwarfs anything on Earth, says Gierasch, are the lightning bolts. According to research published by his colleagues, Andrew. P Ingersoll of the California Institute of Technology (Caltech), Pasadena, Calif., and Blaine Little of ITRES Research, Calgary, Canada, in the planetary science journal Icarus (December 1999), the lighting bolts are as many as several times the size of the largest terrestrial bolts. With lightning strikes much larger than those found on Earth, "I wouldn't want to fly through that storm," says Gierasch.
Other authors on the Nature letter, "Observation of moist convection in Jupiter's atmosphere," in addition to Gierasch and Ingersoll, are Shawn P. Ewald of Caltech; Donald Banfield, Paul Helfenstein and Amy Simon-Miller of Cornell; Ashwin Vasavada of the University of California, Los Angeles; and Herbert H. Breneman and David A. Senske of JPL.