Jupiter’s whirlwinds: Turning the other way
Computer simulations explain the location of Jupiter’s impressive atmospheric whirlwinds – and why their direction of rotation is opposite to storms on Earth.
The numerous whirlwinds covering Jupiter are caused by upward gas flows originating deep within the giant planet. This is the conclusion reached by scientists at the University of Alberta (Canada) and the Max Planck Institute for Solar Research (MPS) in Germany after extensive computer simulations. The ascending flows are deflected in higher-lying, stable gas layers and swirled by the Coriolis force. For the first time, the new model succeeds to simulate that Jupiter-whirlwinds occur predominantly in wide bands north and south of the equator. There, the Great Red Spot can be found, a giant anticyclone in the planet’s atmosphere that has been stable for centuries. The model also explains why Jupiter’s storms rotate in the opposite direction from those on Earth. The researchers report their results today in the journal Nature Geoscience.
The atmosphere of gas giant Jupiter is a turbulent place. Broad east- and westbound jet streams drive clouds of frozen ammonia grains around the planet at speeds of 550 kilometers per hour. Other regions are dominated by huge, long-lived whirlwinds. The largest of these is the Great Red Spot, a giant anticyclone, which measures up to two times Earth's diameter and has existed for at least 350 years. Until now, how exactly these weather phenomena originate, could not be explained comprehensively.
Jupiter’s whirlwinds rotate opposite to the rotation of the planet, i.e. clockwise on the northern and anti-clockwise on the southern hemisphere. On Earth hurricanes rotate in the opposite sense. How Jupiter's storms are formed and why they are so different from those on Earth has long been controversial. "Our high-resolution computer simulation now shows that an interaction between the movements in the deep interior of the planet and an outer stable layer is crucial," sums up Johannes Wicht from the MPS.
Jupiter consists essentially of hydrogen and helium. Due to the high pressure of the overlying masses, this mixture becomes metallic and thus electrically conductive at about 90 percent of the planet’s radius. Further outside, the gas exists in its non-metallic "normal state". Measurements suggest that the outermost part of that layer, home to the observable weather events, is stably stratified.
The new simulation performed by the Canadian and German researchers for the first time consider this stable layer in an elaborate computer model. "We simulate only the topmost 7,000 kilometers of the non-metallic layer, because the magnetic field significantly slows down the dynamics in deeper regions. The outer 5 percent of this layer corresponding to the outer 350 kilometers are stably stratified”, says MPS-scientist Thomas Gastine.
Driven by the heat further inside the core of the giant planet, gas rises upwards in packages - similar to water boiling in a pot. However, the overlying stable air layers provide a kind of barrier. "Only when the buoyancy of the gas package is strong enough, it can penetrate into this layer and spreads out horizontally. Under the influence of planetary rotation, the horizontal movement is swirled, just as is observed for hurricanes on Earth”, says Wicht. When the gas has cooled off enough, it sinks again into the depths of the atmosphere. "The interplay of buoyancy, horizontal motion, rotational motion, and subsidence gives rise to a characteristic signature which corresponds well to actual observations of the planet”, Wicht says. This includes a colder anticyclonic core with a typical diameter and a cyclonic ring which arises where the gas sinks back down again.
"Cyclones on Earth form in a similar way", says Wicht. There, too, the Coriolis force from the planet’s rotation swirls air masses rising upwards. However, the cyclones on Earth rotate in the opposite direction from those on Jupiter. The reason: On Jupiter, the vortices are formed when rising gas strives apart in the upper atmosphere. On Earth, however, they start at the bottom, where air converges and then rises.
"Simulating the conditions in Jupiter’s atmosphere is tricky since many properties of this region are not well known," explains Gastine. The researchers rely on data of NASA’s Galileo mission. a small probe released from the space craft penetrated more than 100 kilometers below the cloud layer until it was destroyed at a pressure of 24 bar.
As a result, the new calculations offer a very realistic picture of the uppermost layers of Jupiter’s atmosphere: the currents from the inside don’t produce the anticyclones randomly, but preferably in the vicinity of the poles as well as in certain bands above and below the equator. the size of these features diminishes with increasing distance from the equator. This is consistent with observations. "The pattern is determined by the dynamics within the planet, and in particular by the interaction of rising gas packages with the eastward and westward jet streams, which the computer model also reproduces realistically," says Wicht.
"However, we were not able to capture the actual lifetime of the anticyclones correctly," he adds. While typical Jupiter-anticyclones last for up to a few years, the model storms dissolve after only days. This is most likely due to the unrealistic value for the viscosity of Jupiter gases that the researchers assumed for their calculations. It was deliberately chosen too high in order to limit the required computing time.
But even with a more realistic viscosity and unlimited computing power, the amazing stability of the Great Red Spot could not be achieved. "We are just beginning to understand Jupiter’s weather phenomena", Wicht explains. "In addition to its size and durability, the Red Spot has other special features such as its characteristic color. Additional processes seem to be involved here that we don’t yet comprehend.”