An old Earth joke, told from any locale, goes like this: If you don't like the weather, wait a minute. Even a shopkeeper in Albuquerque once said that to me, during a 5-day visit in which nary a cloud appeared. Still, this entrenched sentiment reflects our planet's frequently fickle weather and its dramatic extremes.
On Jupiter, however, things are much more stable. Bands of clouds last for years and circle the planet at more than 300 mph (483 kilometers per hour). Storms that would dwarf a terrestrial hurricane drone on for centuries.
Now a new study quantifies some long-held expectations about how Jupiter and other giant gas planets develop their long-lived cloud bands. To understand the reasons, it's useful to first learn why the National Weather Service has such a tough job.
Earth vs. Jupiter
On Earth, wind and storms are driven ultimately by the Sun's energy and the uneven heating it causes in the planet's atmosphere, in the oceans, and at the surface.
Warm air rises, cool air sinks. Multiple horizontal and vertical layers and air pockets develop, locally and regionally.
Temperature differences lead to large-scale pressure differences, and air rushes from high-pressure areas to low, creating wind.
All the while, solar energy dissipates rapidly in what's described as a very turbulent atmosphere, explains Boris Galperin, a University of South Florida scientist who worked on the new mathematical modeling. Small pockets of air give up heat due to friction with each other and by interaction with the surface.
Large cloud bands can't develop, because energy isn't allowed to build up. Instead, we get sun in the morning, heavy wind in the afternoon, rain by nightfall, and something else the next minute.
Jupiter's winds are solar powered, too (some internal heat
Jovian weather). Yet Jupiter is much farther away from the Sun, so it receives less solar energy -- something like 4 percent what Earth gets. One might think it couldn't develop such stable storm and cloud systems. Importantly, however, Jupiter has no surface, at least not anywhere near its outer reaches where visible clouds develop.
The new study finds that Jupiter's rotation characteristics, along with relatively limited atmospheric turbulence and no surface friction together play a larger role than solar heating in determining how the wind blows.
"The farther away from the Sun the planet is, the less turbulent is its atmosphere and the less intense the friction becomes, such that the smaller amount of energy is being dissipated," Galperin told SPACE.com. "In the thin atmospheres of giant planets, the physical processes are such that a large part of the energy of small parcels is transferred to larger parcels, and so on and so on until the energy is delivered to the largest flow structures, which are zonal jets."
These jets are the cloud bands of Jupiter, visible even in fairly small telescopes. Adjacent bands travel in opposite directions. Their color reflects slight differences in chemical makeup.
Other planets
Saturn and the other gas giants have zonal jets, too and the research applies to them as well.
"We found that the amount of energy that these jets can hold is determined by the angular velocity of the planetary rotation, the planet's radius and some friction parameter -- but not the intensity of solar heating," Galperin said.
Scientists at Ben-Gurion University of the Negev in Israel also participated in the study, which will be detailed Monday in the journal Physical Review Letters.
The research does not speak to the creation and maintenance of large, rotating storms on Jupiter, such as the Great Red Spot, whose physics are more complex than the zonal jets, Galperin said. This storm has existed for at least 300 years and spans some 15,400 miles (24,780 kilometers). Countless smaller storms -- still hundreds of miles wide -- are embedded in Jupiter's cloud bands and last for years.
While Jovain weather is enduring, it is by no means benign. Little is known about what goes on beneath the clouds, but other scientists say Jupiter has rain and probably outsized lightning bolts.
Finally, the new equations explain why Neptune has the strongest wind flows of any planet, even though its the furthest gas giant from the Sun. Neptune's constant gales have been clocked at 1,200 mph (1,930 kilometers per hour).
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