Clocking Neptune's Spin by Tracking Atmospheric Features
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A day on Neptune lasts precisely 15 hours, 57 minutes and 59 seconds, according to the first accurate measurement of its rotational period made by University of Arizona planetary scientist Erich Karkoschka.
His result is one of the largest improvements in determining the rotational period of a gas planet in almost 350 years since Italian astronomer Giovanni Cassini made the first observations of Jupiter's Red Spot.
Unlike the rocky planets -- Mercury, Venus, Earth and Mars -- which behave like solid balls spinning in a rather straightforward manner, the giant gas planets -- Jupiter, Saturn, Uranus and Neptune -- rotate more like giant blobs of liquid. Since they are believed to consist of mainly ice and gas around a relatively small solid core, their rotation involves a lot of sloshing, swirling and roiling, which has made it difficult for astronomers to get an accurate grip on exactly how fast they spin around.
In addition to getting a better grip on Neptune's rotational period, the study could lead to a better understanding of the giant gas planets in general.
"We know Neptune's total mass but we don't know how it is distributed," Karkoschka explained. "If the planet rotates faster than we thought, it means the mass has to be closer to the center than we thought. These results might change the models of the planets' interior and could have many other implications."
The discovery is published in Icarus, the official scientific publication of the Division for Planetary Sciences of the American Astronomical Society.
Erich Karkoschka. Neptune's Rotational Period Suggested by the Extraordinary Stability of Two Features. Icarus, 2011;