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Astronomers now know why Uranus, Neptune host the solar system’s strongest winds

The distant gas giants Uranus and Neptune hold many secrets that scientists have yet to unlock. While NASA’s Galileo and Cassini space probes have investigated both Jupiter and Saturn and transmitted a wealth of fascinating new data about these planets. The only time astronomers have been able to get a close view of these giant icy worlds was when NASA’s Voyager 2 passed by in the 1980s.

Atmospheric circulation on Uranus and Neptune is dominated by powerful east-west jet streams. These winds can reach supersonic speeds, with jet streams blowing hundreds of meters per second–10 to 15 times greater than on Earth. The question of the depth of these winds has puzzled scientists trying to understand their internal structures, energy output, and overall dynamics.

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Now, researchers, after pouring over data from the Hubble Space Telescope and the Voyager 2 space probe, have concluded that the weather on Neptune and Uranus is confined to a relatively thin atmospheric layers. The new study by lead author Yohai Kaspi of Israel’s Weizman Institute of Science and published in Saturday’s issue of the journal Nature could have important implications for understanding not only the dynamics, but how these planets were formed.

According to Kaspi, there were two approaches to explaining the existence of the strong winds on Neptune and Uranus. The first hypothesized that the jet streams are driven by shallow atmospheric processes near the surface. The second approach suggested that the winds were propelled by forces extending deep into the planets’ interiors. Kaspi and his colleagues resolved the issue by looking at the planets’ respective gravitational fields.

Looking at perturbations in Uranus and Neptune’s gravitational fields, the team separated out what could be attributed to the fact that the planets are not perfect spheres and what arose from small changes in the planets’ density, which indicates the effect of strong winds. By calculating how much the winds contribute to gravitational irregularity, the researchers found that the jet streams are active in only the outermost 0.2 percent of Neptune’s mass and in the outermost 0.15 percent of Uranus’ mass. In other words, the winds are not being propelled by forces active deep in the planets’ interiors.

Kaspi’s team is uncertain about what exactly does drive the jet streams, but a top contender is moist convection. When water vapor rises and condenses on Earth, it forms a cloud and releases latent heat into the atmosphere. This process can also happen on planets like Uranus and Neptune, according to Kaspi.

The new findings by Kaspi and colleagues could shed light on how the tremendously strong winds on Uranus and Neptune arise and evolve over time and help scientists create more accurate models of the interiors of these gas giants.