What Drives the Screaming Jet Streams of Neptune?

The gas giants of our solar system protect their secrets. That's especially true of the two blue planets that are farthest out. While unmanned explorers like Cassini and Galileo have beamed back all sorts of new information about Jupiter and Saturn, astronomers got their only up-close look at Neptune and Uranus when the Voyager probes passed by in the 1980s, making these distant worlds doubly mysterious.

Take, for example, the blustery jet streams on Uranus and Neptune. Both planets have one in each hemisphere, and another at the equator blowing hundreds of meters per second?10 times stronger than Earth's. Scientists had been wondering whether these streams simply skim the surface, or if they are driven by atmospheric dynamics that stretch deep down inside the planets. "The big question is, how deep are the winds that we see on the atmospheres?" says Yohai Kaspi of the Weizmann Institute of Science in Israel, lead author of a new study in Nature. "This is very important for understanding not only the dynamics, but how these planets are formed."

Harmony of the Spheres

Because Uranus and Neptune are simpler in structure than Jupiter or Saturn, Kaspi says, his team got an idea: They could piece together just how deep those jet streams go without needing to have a spacecraft visit either world. Basically, they used the available information?data from the Voyagers and instruments like the Hubble telescope, and models that simulate the planet's interior?to put a limit on the depth.

Using spherical gravity harmonics, Kaspi says, his team could find perturbations in Uranus and Neptune's gravity fields?instances in which the fields were uneven instead of acting like what you'd expect from a perfect sphere. Some of those instances appear because of so-called static characteristics?for instance, the fact that the planets are not, in fact, perfect spheres, but are slightly oblate because of their fast rotation. But there is also a second kind of perturbation, and these come from slight changes in the planet's density that reflect strong winds at work.

Illustration of Neptune's interior. Credit: Calvin J. Hamilton.

Here's the trick: If you can estimate the changes to the gravity fields of these planets, and you can estimate what percentage of those changes is because of their static qualities, you can find the difference. The difference tells you how much the winds contribute to gravity perturbation, and thus how strong they might be. It works particularly well on these two worlds, Kaspi says, because they have winds stronger than Jupiter's but are much smaller planets overall. That means that, relatively, the winds contribute more to these bumps in the data. "That's why you can do this trick on Uranus and Neptune," he says.

The final result: These winds are surprisingly shallow. The powerful jet streams are active in only the outermost 0.15 percent of Uranus's mass, and only in the outermost 0.2 percent of Neptune's. What goes on beneath evidently isn't feeding the blustery jet streams that scream around the planets' surfaces. The team can't be sure exactly what drives these streams, Kaspi says, but a leading candidate is moist convection. "Just like on Earth, when water vapor rises and condenses, it forms a cloud," he says. "This process releases latent heat into the atmosphere. This is a process that can happen on planets like Uranus and Neptune."

"We Know Almost Nothing"

Who cares about a few extraterrestrial jet streams? Understanding the four gas giants might be the key to understanding the solar system. The only time scientists have gotten a direct look inside a giant happened when the Galileo probe crashed into Jupiter in 2003 to end its mission. That provided unprecedented insight into the planet, Kaspi says, but left plenty of unanswered questions. Imagine if all the firsthand info you had about Earth's atmosphere came from crashing a probe into the Antarctic, or into the tropics on a hot summer day. The specifics of one location don't say very much about the planet as a whole.

The gas giants are so big that more than 99 percent of mass of the solar system (excluding the sun) is bound up in them. "We live on the crumbs," Kaspi says. Yet what the planets are like underneath is far from perfectly understood. Astronomers are not even sure if the worlds have solid cores, he says, but "the core is essential for understanding how these planets form."

Elucidating the limits of the winds, as this week's study did, will help scientists create more accurate models of the gas giants' interiors. And future missions should reveal even more. Kaspi and his colleagues are working with Juno, a spacecraft that launched in 2011 and is now en route to Jupiter to unravel the largest planet's gravity field, magnetic field, and more.

Source: http://www.popularmechanics.com/science/space/deep/the-screaming-jet-streams-of-uranus-and-neptune-15474397?src=rss

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