Inscrutable ice giants Neptune and Uranus have only a thin rind of windy weather over their fluid contents, a team of planetary scientists say. The research published in the journal Nature relies on decades-old data from the Voyager 2 spacecraft -- and may help scientists understand the atmospheric dynamics of alien gas-giant exoplanets beyond our solar system.
Although Neptune and Uranus are members of our own planetary neighborhood, their atmospheric dynamics have remained a mystery shrouded in thick cloud, said study coauthor Adam Showman, a planetary scientist at the University of Arizona in Tucson.
"We know what the winds are doing right at the top, where we see the clouds, because we can watch those clouds blow around and we can track those motions," Showman said. "What we have not known is what they’re doing in the deep interior or even really at all below the clouds -- that’s very hard to measure."
On Earth, the sun easily penetrates a thin atmosphere wrapped around a rocky surface. But because the ice giants are basically whole planets full of fluid, scientists aren’t even sure where the atmosphere begins, Showman said.
These are no mere zephyrs -- jets on Neptune and Uranus can blast at roughly 1,000 mph, compared with Earth’s weaker winds of about 65 to 225 mph.
"In an age in which space missions penetrate the far reaches of the solar system, it seems incredible that planetary scientists still argue about the depth of atmospheric circulations inside the ice giants, Uranus and Neptune, and the gas giants, Jupiter and Saturn," Peter Read of the University of Oxford wrote in a Nature commentary.
To sort out this question, the researchers looked at gravity field data collected by the Voyager 2 spacecraft when it flew by Uranus in 1986 and Neptune in 1989.
A gravity field will vary depending on the amount of mass underneath it. And since the researchers could tell what the gravity field should look like for a planet with no wind movement in its atmosphere, they used the variations in the field to determine how much winds were deforming the field, and how much mass the windy spots would account for.
They calculated that the windy layer for Uranus only accounted for the outermost 0.15% of the planet’s total mass; for Neptune, it came out to about the same, 0.2%. At most, they calculated, the winds couldn’t extend beyond roughly 680 miles deep.
Researchers have tried to understand whether movement in these gas giants' atmospheres is driven by heat rising up from their cores or by warming from the sun’s rays. The new study seems to show that these are shallower processes unlikely to come from deep inside the planet, according to the paper.
The research could help scientists better understand a class of "hot Neptunes" discovered by the planet-hunting NASA Kepler mission, Showman said.
"In terms of understanding those exoplanets, we would really want to understand our own Uranus and Neptune," he added.
Without data for Jupiter and Saturn’s gravity fields, the solar system’s greatest gas giants remain bulky mysteries. But NASA’s Juno spacecraft, launched in 2011, should send back a portrait of Jupiter’s gravity field, and the Cassini spacecraft will map Saturn’s field once it circles closer to the planet.
"What's neat is we found that with current data, we can essentially answer these same questions for Uranus and Neptune that NASA is sending billion-dollar spacecraft missions to answer for Jupiter and Saturn," Showman said.
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