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.
Stability, stability
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.
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