When it come to making your head spin, Jupiter revolves on its axis in less than 10 hours. Up until now, it was the only gas planet in our solar system that had an accurate spin measurement. But grab your top and cut it loose, because University of Arizona planetary scientist Erich Karkoschka has clocked Neptune cruising around at a cool 15 hours, 57 minutes and 59 seconds.
“The rotational period of a planet is one of its fundamental properties,” said Karkoschka, a senior staff scientist at the UA’s Lunar and Planetary Laboratory. “Neptune has two features observable with the Hubble Space Telescope that seem to track the interior rotation of the planet. Nothing similar has been seen before on any of the four giant planets.”
Like spinning gelatin, the gas giants – Jupiter, Saturn, Uranus and Neptune – don’t behave in an easy to study manner. By nature they deform as they rotate, making accurate estimates difficult to pin down.
"If you looked at Earth from space, you’d see mountains and other features on the ground rotating with great regularity, but if you looked at the clouds, they wouldn’t because the winds change all the time,” Karkoschka explained. “If you look at the giant planets, you don’t see a surface, just a thick cloudy atmosphere.”
Of course, 350 years ago Giovanni Cassini was able to estimate Jupiter’s rotation by observing the Great Red Spot – an atmospheric condition. Neptune has observable atmospheric conditions, too… But they’re just a bit more transitory. “On Neptune, all you see is moving clouds and features in the planet’s atmosphere. Some move faster, some move slower, some accelerate, but you really don’t know what the rotational period is, if there even is some solid inner core that is rotating.”
Roughly 60 years ago astronomers discovered Jupiter gave out radio signals. These signals originated from its magnetic field generated by the spinning inner core. Unfortunately signals of this type from the outer planets were simply lost in space before they could be detected from here on Earth. “The only way to measure radio waves is to send spacecraft to those planets,” Karkoschka said. “When Voyager 1 and 2 flew past Saturn, they found radio signals and clocked them at exactly 10.66 hours, and they found radio signals for Uranus and Neptune as well. So based on those radio signals, we thought we knew the rotation periods of those planets.”
In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Image: Erich Karkoschka)
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