Does the wind on Saturn blow to the east or to the west? After centuries of observation, scientists can finally answer: Yes. Saturn's powerful jets move in both directions, researchers have discovered, a finding that hints at the mass of the planet's mysterious core.
To gauge wind direction on Earth, all you have to do is plant a flag in the ground and watch which way it points. But try doing that on a planet whose rocky "ground" lies beneath several tens of thousands of kilometers of gas. With Jupiter, the largest gas giant, scientists got around the problem by treating the planet's tilted magnetic field as the ground; all researchers had to do was track cloud motions relative to it.
But Saturn's magnetic field isn't tilted, which means that it rotates without giving a clue to how fast it or the planet is spinning. Lacking a rock-solid reference point, planetary scientists Peter Read of the University of Oxford in the United Kingdom, Timothy Dowling of the University of Louisville in Kentucky, and Gerald Schubert of the University of California, Los Angeles, turned to Saturn's dynamic meteorology.
They knew from previous studies that they could depend on a steady relationship between wind speed and a property called potential vorticity, a measure of spin in the atmosphere that is independent of the planet's rotation rate. Combining this with what they knew about cloud movement and temperature on the planet, the researchers were able to calculate the length of a saturnian day: 10 hours, 34 minutes, and 24 seconds. That's the most robust calculation in more than 3 decades of spacecraft-based effort, says planetary meteorologist Adam Showman of the University of Arizona, Tucson. It's also the shortest estimate so far.
A shorter day and faster rotation for Saturn makes for a dramatic reconfiguration of the planet's atmosphere, at least to a scientist's eye. A slower planetary rotation had made it appear that all of Saturn's jets blew eastward--fastest at the equator and slower with increasing latitude--and were separated by narrow zones of no wind. But the new faster rotation makes for a broad eastward jet at the equator and jets alternately blowing eastward and westward going toward the poles, just as on Jupiter, the team reports today in Nature.
The findings should help researchers understand how the churning interior of Saturn works, says planetary physicist David Stevenson of the California Institute of Technology in Pasadena. It even points to a larger rocky core for Saturn, something that might make it easier to understand how the nascent planet could have gravitationally grabbed enough gas to become a giant.