Weather on the Outer Planets of Uranus and Neptune: The Limits of Extreme Winds

Uranus and Neptune are home to some of the most extreme winds that can be found in our solar system. Blowing at speeds of over 620 miles per hour, these hurricane-like storms can last for years as they tear across the two planets. Now, researchers have set an upper limit for the thickness of jet streams on the planets; they've learned a little bit more about the surface weather on these two worlds.

The climates of Neptune and Uranus are difficult to study. While they both feature strong winds in their upper atmospheres, researchers have long wondered what happens in the shallower areas located near the surface of these planets. In order to find out, researchers used a novel method for analyzing the gravitational field of the planets in order to determine an upper limit for the thickness of the atmospheric layer.

Deviations in the distribution of mass in planets cause measurable fluctuations in the gravitational field. For example, an airplane flying near a large mountain feels the slight extra gravitational pull of that mountain on Earth. In the same way, Uranus and Neptune can also feel pull.

These two planets actually experience rapid rotation, having periods of 17 and 16 hours. Because of this rapid rotation, their winds swirl around regions of high and low pressure. This allows researchers to deduce the relations between the distribution of pressure and density, and the planets' wind field. These physical principles, in turn, allowed the scientists to calculate the gravity signature of the wind patterns.

So what does this mean? The researchers were able to create a wind-induced gravity map of these planets. In other words, they were able to chart the weather. In fact, they found that the streams of gas observed in the atmosphere of these planets are limited to a "weather layer" of no more than about 620 miles in depth--that makes up only a fraction of a percent of the mass of these planets.

The findings reveal that there are still many open questions about these two planets. For example, what exactly is occurring beneath this weather layer? Although no spacecraft missions to Uranus and Neptune are planned for the near future, the discoveries will be useful in the analysis of another set of atmospheric circulation patterns: those of Jupiter. In addition, the study may help researchers learn exactly how these giant planets were formed.

The findings are published in the journal Nature.