What is it about?
Planetary scientists have been trying to accurately measure the length of Saturn's day since William Herschel, the discoverer of the planet Uranus, inaugurated the quest in 1794. The usual modern method, sampling the magnetic field at close range by spacecraft, fails for Saturn because the planet's magnetic poles coincide exactly with its rotational poles---it is like trying to measure the rotation period of a lighthouse beacon that is pointed upwards. Fortunately, mass anomalies in Saturn's interior perturb nearby wave-bearing fluid regions in a conspicuous manner. This article describes the earliest such wave analysis, which uses Cassini observations of Saturn's atmosphere to yield the first clear, connected, and calibrated value: 10h 34m 13+/-20s.
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Why is it important?
The bulk rotation period of a planet---the length of its day---is one of its fundamental physical parameters, alongside its total mass, size, and shape. Without knowledge of Saturn's rotation period, accurate to within a few tens of seconds, it is impossible to make precise statements about its interior structure, which hampers understanding of how the planet formed and how it has evolved over the age of the Solar System.
Perspectives
The jet streams on both Saturn and Jupiter require deep roots to be stable against shear instability. In other words, it is easy to construct jet streams for these planets that are unstable and can thus be disqualified from further consideration, but is hard to construct stable jet streams. Thus, requiring a gas giant's jet streams to be shear stable is a powerful, top-down diagnostic, one that complements the bottom-up approach of using proximity orbits to accurately measure the planet's gravitational and magnetic fields, as has now been done by Cassini at Saturn and Juno at Jupiter.
Professor Timothy E. Dowling
University of Louisville
Read the Original
This page is a summary of: Saturn’s rotation period from its atmospheric planetary-wave configuration, Nature, July 2009, Springer Science + Business Media,
DOI: 10.1038/nature08194.
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