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5 - The Mysterious Periodicities of Saturn
- Edited by Kevin H. Baines, University of Wisconsin, Madison, F. Michael Flasar, NASA-Goddard Space Flight Center, Norbert Krupp, Tom Stallard, University of Leicester
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- Book:
- Saturn in the 21st Century
- Published online:
- 13 December 2018
- Print publication:
- 06 December 2018, pp 97-125
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- Chapter
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Summary
The rotation rate of a planet is a fundamental parameter, no less than its mass or composition, and planetary investigators require this rate to assess various other phenomena such as planetary wind speeds, internal and atmospheric models, ring dynamics and so forth. Saturn presents a conundrum, however, because none of its various planetary periods indicates the “true” rotation of the planet. Thus, although the planet displays an abundance of periodicities near 10.7 hours, the exact rotation period of Saturn is unknown. In the magnetosphere, “planetary-period oscillations” (PPOs) appear in charged particles, magnetic fields, energetic neutral atoms, radio emissions and motions of the plasma sheet and magnetopause. In Saturn’s rings, the spoke phenomenon can exhibit periodicities near 10.7 hours, and ring phenomena themselves may be related to the interior rotation of the planet. In the high-latitude ionosphere, modulations near this period appear in auroral motions and intensities. In the upper atmosphere, some cloud features rotate near this period, although wind speeds are generally faster, and the well-known polar hexagon rotates with a period close to 10.7 hours. Some of the magnetospheric/ionospheric oscillations differ in the northern and southern hemispheres and their periods do not remain constant, sometimes varying on long time scales of a year or longer and sometimes on much shorter time scales. These variations in the period argue against a cause related to changes interior to Saturn, and because the magnetic and spin axes of Saturn are reported to be axisymmetric (unlike those of any other known planet), Saturn’s periodicities cannot be explained as “wobble” caused by a geometric tilt or by a nondipolar magnetic anomaly. Several models have been proposed to account for the observed periodicities, including rotating atmospheric vortices, periodic plasma releases and a flapping magnetodisk, but none can satisfactorily explain all of Saturn’s periodicities nor their common origin, and none can determine the exact rotation rate of the planet. This chapter reviews Saturn’s periodicities, theories thereof, and how they might be used to determine the elusive rotation rate of the planet.
Constraints on clade ages from fossil outgroups
- Matthew M. Hedman
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- Journal:
- Paleobiology / Volume 36 / Issue 1 / Winter 2010
- Published online by Cambridge University Press:
- 08 April 2016, pp. 16-31
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- Article
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This paper presents a method for constraining the age of a clade with the ages of the earliest fossil specimens in that clade's outgroups. Given a sufficiently deep, robust, well-resolved, and stratigraphically consistent cladogram, this method can yield useful age constraints even in the absence of specific information about the fossil preservation and recovery rates of individual taxa. The algorithm is applied to simulated data sets to demonstrate that this method can yield robust constraints of clade ages if there are sufficient fossil outgroups available and if there is a finite chance that additional outgroups may be discovered in the future. Finally, the technique is applied to actual fossil data to explore the origin of modern placental mammals. Using data from recently published cladograms, this method indicates that if all Mesozoic eutherians are regarded as outgroups of Placentalia, then the last common ancestor of modern placental mammals and their Cenozoic allies lived between 65 and 88–98 million years ago, depending on the assumed cladogram and the number of outgroups included in the analysis.