Voyager highlights Saturn's mind-boggling weather
Try to picture a hurricane the size of Earth. If you can, then you have some idea of the scale of the weather on Saturn. Looking at the stream of television images of Saturn being sent back this week by Voyager 2, it is all too easy to start thinking of this planet as something Earth-scaled and more easily comprehensible than it actually is.
But Saturn and its decorative rings are big. It is the second-largest planet in the solar system. Only Jupiter is larger. These two planets, with Uranus and Neptune, constitute a class of planets that scientists call "gas giants." Realizing just how large they are gives a renewed appreciation of the concept of bigness.
Saturn and Jupiter are so large, in fact, that they have moons the size of the inner planets with which we are more familiar. If Saturn were a hollow sphere, you would have to grind up 815 Earths to fill it. But, if you could put Saturn on a cosmic balance scale, it would only take 95 Earths to equal its mass. In fact, Saturn is the only planet in the solar system that is lighter (less dense) than water.
So light is the ringed planet that the scientists studying it here at the Jet Propulsion Laboratory believe its rocky core is only about the size of Earth. This, they believe, is surrounded by an ocean of liquid, metallic hydrogen more than 12,000 miles thick. On top of this is atmosphere: 18,000 miles of it. Earth's atmosphere, in contrast, is less than 10 miles thick, less than 0.2 percent of the planet's radius.
In other words, you might call Saturn and the other gas giants weather planets. Cyclones and anticyclones, high and low pressure systems, hurricanes, lightning, and thunder -- on a titanic scale -- are their essence.
The Voyager spacecraft, even with its sophisticated cameras, can only send back pictures of the topmost fringe of Saturn's massive atmosphere. ("The dirt on the skin of the orange," as Garry E. Hunt of University College in London puts it.) From this, Voyager's meteorologists and atmospheric scientists are employing all the logic, intuition, and basic knowledge of atmospheric processes they can muster to construct a consistent picture of the planet's atmosphere.
"Hopefully, studying the atmospheres of other planets where the conditions are so different will help us arrive at a theory for climate change," explains Dr. Verner E. Suomi of the University of Wisconsin.
Although still far from complete understanding of these exotic atmospheres, the scientists believe that they have seen features on both Saturn and Jupiter that are comparable to weather events on Earth. "It's amazing how much is Earth meteorology in disguise," Dr. Hunt notes, but adds that there are striking differences as well.
Many of these basic similarities come from the fact that "every atmosphere is a heat engine," explains Dr. Andrew Ingersoll of the California Institute of Technology. That is, on any planet weather patterns are the result of an atmosphere's innate tendency to continually try to carry heat from hotter to cooler places.
On Earth, the hottest area is at the equator while the coldest area is at the pole. Thus, the atmosphere is continually carrying heat energy from lower to higher latitude. On Jupiter and Saturn, however, the situation is slightly different. Saturn, and to a lesser extent Jupiter, generate more energy at their cores than they receive from the Sun. As a result, transferring heat outward from the center appears to be more important than moving it from equator to pole, Dr. Ingersoll explains.
Earth's weather patterns tend to fall into latitudinal bands: the Trades, the Horse Latitudes, the Doldrums, and so on. The same is true of Saturn and Jupiter, but the "belts" and "zones" are more numerous. "You have something 10 times as big and you seem to have 10 times the bands," says Dr. Ingersoll.
Not only are there more of these bands, but they are also more visible from space. This is because the clouds on Jupiter, and to a lesser extent Saturn, have been stained with an artist's palette of colors. The source of these colors remains a mystery. Most of the material at the cloud tops is methane and ammonia, which form white clouds. There are trace amounts of phosphorus that might account for the reds, which are plentiful. The blue coloration might come from deep holes of transparent air that produce a "blue sky" effect. But beyond this the experts cannot go. As a result, Voyager camera team leader Dr. Bradford Smith has coined "chromophores" -- a Greek word for color elements -- to describe them.
This coloring scheme leads to unusual effects. On Jupiter, with the exception of the Great Red Spot, low pressure systems, or "storms," are red, and high pressure systems, or "fair weather," look blue, explains Dr. Suomi.
The Red Spot is the prime example of another difference between Earth's weather and that on Jupiter and Saturn: Weather features on the giant planets can persist for decades, even centuries.
Earth's weather has been referred to as "the fuzz on the peach." Viewed three-dimensionally, a terrestrial hurricane looks like a flat pancake. Our weather is squeezed between "two hard and unyielding surfaces" -- Earth's face and the stratosphere, explains Dr. Ingersoll. As a result, storms die out rapidly because of friction.
On Saturn, however, friction does not appear to play a great role. So massive are the gas planet's storms that they "can coast for several years just on their inertia," Dr. Ingersoll calculates. He says that a closer Earth analogy to these storms -- which look like spots and ovals on the surface -- are the large ocean eddies that have been discovered and studied in the last few years. These can persist for many months.
The eddies on Saturn act in an unexpected way that has the meteorologists fascinated. Normally, eddies suck energy out of an air stream. But on Saturn, they appear to be putting energy in, pushing the jet streams to faster speeds than anyone thought possible. At the equator, winds have been clocked at an incredible 1,100 miles per hour, four times faster than those at Jupiter and almost three times faster than the speediest jet stream on Earth.
"The eddies appear to be the rotors that are driving the conveyor belts," Dr. Ingersoll explains. He believes that the heat coming from the interior is the force that drives these "rotors."
In one of the latest pictures of Saturn, the scientists have seen a familiar, sine-wave shape in one of the jet streams. This, they say, is a Rossby wave, caused by horizontal differences in air temperature. On Earth, these waves are the source of some of the severest depressions, or storms. As on Earth, depressions on Saturn appear to be taking energy out of the zonal air currents.
One important question that the experts hope to solve is the depth of the weather systems that appear on the surface. Some scientists are trying to prove that they have roots deep in the planet. Others believe that most of the atmosphere is fairly quiet and rotates in time with the core and that the complex patterns are only a few hundred miles deep.
If these weather patterns are deep, scientists expect them to show considerable north-south symmetry. If shallow, they should reflect seasonal, north-south differences.
Jupiter appeared extremely symmetrical, but Saturn is showing greater differences, say several of the experts. Thus, it appears increasingly likely that weather is "the fuzz on the peach" on the gas giants, as well as on Earth.