The Jupiter/Saturn cases (top and middle) develop eastward wind at the equator (shown in red), with multiple weaker banded flows at high latitudes, similar to Jupiter and Saturn (though with speeds that are too slow on Saturn). The Uranus/Neptune case (bottom) has westward wind at the equator and eastward winds at high latitude, similar to Uranus and Neptune. Credit: Adam Showman/University of Arizona
Planetary scientists have long puzzled over why fast-moving rivers of air called jet streams flow eastward at the equator of Jupiter and Saturn, but go westward on Uranus and Neptune. Now a new simulation has begun unraveling that mystery by showing how turbulent thunderstorms create the jet streams.
Whether a jet stream flows east or west seems to depend on the amount of water vapor in a planet's atmosphere — but researchers confess that the "how" still eludes them.
"Under these conditions, the eastward equator flow prefers low water vapor abundance," said Yuan Lian, an atmospheric dynamics researcher at the University of Arizona in Tucson. "The westward equator flow prefers high water vapor abundance. However, we still don't know exactly how this happens."
The equatorial jet stream goes westward on Earth, but all the other jet streams on our planet go eastward, including the one that frequently dips down from the Arctic to bring winter storms across North America.
Rivers of air
Jet streams feed on swirling eddies that can form the basis of thunderstorms on giant planets. Eddies don't necessarily all merge together to form a jet stream — some can simply spin off their angular momentum into the jet to sustain howling wind speeds.
Some jet streams have clocked in at 400 mph (644 km/h) on Jupiter, and almost 900 mph (1,448 km/h) on Saturn and Neptune. Wind speeds on Venus can hit almost 230 mph (370 km/h).
"You have a little vortex that gets stretched out and sheared apart by the wind," said Adam Showman, a planetary scientist at the University of Arizona. "As it's shearing apart, it gives the jet stream a little push."
Eddies and vortexes themselves form from rising water vapor. The vapor condenses in the cooler upper latitudes of planet atmospheres and releases energy in the form of heat, which disturbs the surrounding atmosphere.
Simulating the flow
Showman and Lian estimated that Uranus and Neptune contain 10 times as much water vapor as Jupiter and Saturn. They plugged the data into their simulation runs and found that they came up with jet streams with directions matching those observed on each planet.
"We took our best guess with our best models for each of the planets," Showman told SPACE.com. "We did a bunch of simulations varying the water. Even if we don't think the planet has that amount, it allows us to understand role of water in that simulation."
The simulations also came up with the 20 jet streams each for Jupiter and Saturn, as well as three jet streams each for Uranus and Neptune. Likewise, they produced simulated storms similar to thunderstorms previously spotted on Jupiter and Saturn.
Yet the question remains as to why jet streams at the equator go either east or west.
Without knowing the details, researchers can only speculate on water vapor condensation creating a topsy-turvy atmosphere. A more unstable atmosphere may result in jet streams that happen to form in an eastward- or westward-running direction.
"When you have this occurring in a complicated 3-D circulation, it can develop latitudinal temperature differences," Showman noted. "More water vapor means more temperature differences that change the stability of the atmosphere."
However, a better understanding will have to wait for improved simulations. Lian pointed out that the simulated jet streams did not quite reach the high speeds of real jet streams. The current model also ignored some processes such as precipitation, evaporation and cloud formation.
"We want to include as many factors as we can," Lian said. "That way, we can probably produce jet speeds similar to observations."
The findings were detailed at the 40th annual meeting of Division of Planetary Sciences of the American Astronomy Society in Ithaca, New York.