Outer Planets’ Stormy Weather: All on the Surface

Have you complained about the weather recently? On the gas giants at the edges of our solar system, Uranus and Neptune, hurricane-like storm systems as big around as Earth blow 1000 km/h winds for years on end.

Voyager II image of Neptune, showing storm features. Image: NASA

But wait…What exactly constitutes weather on a giant gas planet? Here on planet Earth, there is a clearly delineated gas layer enveloping the solid/liquid layer we call home. Our weather arises from our planet’s rotation and the solar heating of its surfaces. On a rapidly-rotating gas planet (a day on Neptune is 16 hours long), which receives little warmth from the sun, how do storms arise? Is the whole planet a swirling, turbulent mass, or is the weather confined to an outer layer, like on Earth?

We now have, if not an exact answer, an upper limit for the thickness of weather systems on those planets. And it turns out that they are surprisingly Earth-like (frigid 1000 km/h winds aside): The winds blow in an atmospheric layer that is no more – and probably less – than 1000 km thick. That is much less than one percent of the mass of these planets.

The researchers who calculated this limit – at the Weizmann Institute, Tel Aviv University and the University of Arizona – used slight variations in the gravitational field – gravity maps of the planets – to work out the depth of the weather. On Earth, large masses like mountains slightly increase the planet’s gravitational pull on nearby objects. On gas planets, wind-driven variations in the gas density are what create rises and dips in the gravitational map. By calculating the theoretical gravitational fields on idealized planets of the same size, but lacking winds, and comparing these with the observed gravity, the team was able to ascertain the contribution of wind to the overall pattern.

According to lead author Dr. Yohai Kaspi, having a better grasp of what is occurring on the planets’ surfaces will give us clues to the events taking place deeper within the planets, as well as information leading to theories about their formation and possibly even hints as to the makeup of the many largish exoplanets that have been detected in recent years. Kaspi is on the scientific team of NASA’s Juno mission to Jupiter, and is planning to apply this method sometime around 2016, when the spacecraft will be taking detailed measurements of that planet’s gravity.

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