Violent winds on Uranus and Neptune confined to upper atmosphere

Uranus and Neptune have long been known to harbor swirling clouds and violent winds churning up their atmospheres.

Washington: Uranus and Neptune have long been known to harbor swirling clouds and violent winds churning up their atmospheres.

But given that Uranus` atmosphere is believed to be thick enough to swallow the entire Earth, it was not known just how far the weather perturbations reach into the planet`s interior.

Now a team of planetary scientists with the University of Arizona`s Lunar and Planetary Laboratory, including William Hubbard and Adam Showman, has published the results of new analyzes that put an upper limit to the weather zone on Uranus and Neptune.

According to their data, the atmosphere on both planets goes from screaming winds of infernal violence to dead-quiet at a much shallower depth than previously thought.

"Our analyses show that the dynamics are confined to a thin weather layer no more than about 680 miles deep," Hubbard said.

"This number is an upper limit, so in reality, it is possible that the atmosphere quiets down even shallower than that," he said.

For the study, which was led by Yohai Kaspi, a planetary researcher at the Weizmann Institute of Science in Rehovot, Israel, the team applied computer simulations and numerical analyzes to data collected by the spacecraft Voyager 2 during a fly-by in 1989.

Without a means to probe the atmosphere of gas giants directly, the researchers had to rely on indirect measurements to gather clues about weather patterns on the two planets.

"For Neptune and Uranus, the only spacecraft data we have were taken with Voyager 2`s equipment more than 20 years ago, and we won`t be able to get anything that lives up to today`s standards anytime soon," Hubbard, whose research focuses on studies of the structure and evolution of Jupiter, Saturn and extrasolar giant planets, said.

Instead, the team used deep circulation theories developed by Showman and Kaspi to predict what the gravitational fields of Neptune and Uranus should look like.

This method takes advantage of the fact that large weather perturbations in the atmospheres of giant planets modify their gravitational fields in a way that allows researchers to draw conclusions about the nature and extent of those weather phenomena.

"Basically, by applying these newly developed theories, we are able to tease out new information from old data," Hubbard said.

"The reason we can constrain the weather to the upper 680 miles or so is that we would see a much stronger distortion of the gravitational field if the weather extended much deeper," he said.

Unlike the jet streams on Uranus and Neptune, Hubbard said the winds are much more subtle on Jupiter and Saturn.

Hubbard explained that researchers believe the atmospheric disturbances are more numerous on Jupiter and Saturn but less strong compared to Uranus and Neptune, for reasons that may have to do with the planets` different compositions and their angles between the magnetic fields and rotational axis.

The findings are published in the journal Nature.