[IGPP Everyone] [EPSS Everyone] Planetary Science Seminar this week

[Kevin McKeegan]

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*PLANETARY SCIENCE SEMINAR*

*Thursday, May 17*

*noon in Slichter 3853*

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*  Jing Luan*

Dept. of Astronomy

University of California, Berkeley


*“Enceladus: three-stage limit cycle and current state”*


Enceladus is one of the most popular worlds that might accommodate life
outside our own earth. We study its evolutionary path, especially focus on
the physical processes that drive Enceladus to its current state. I will
also discuss possible applications of these physical processes to other
bodies in our solar system. Below is a brief summary of the evolution path
for Enceladus.



Eccentricity (e) growth as Enceladus migrates deeper into mean motion
resonance with Dione results in increased tidal heating. As the bottom of
the ice shell melts, the rate of tidal heating jumps and runaway melting
ensues. At the end of run-away melting, the shell's thickness has fallen
below the value at which the frequency of free libration equals the orbital
mean motion and e has damped to well below its current value. Subsequently,
both the shell thickness and e partake in a limit cycle. As e damps toward
its minimum value, the shell's thickness asymptotically approaches its
resonant value from below.  After minimum e, the shell thickens quickly and
e grows even faster.  This cycle is likely to have been repeated multiple
times in the past.



Currently, e is much smaller than its equilibrium value corresponding to
the shell thickness. Physical libration resonance resolves this mystery, it
ensures that the low-e and medium-thickness state is present for most of
the time between consecutive limit cycles. It is a robust scenario that
avoids fine tuning or extreme parameter choice, and naturally produces
episodic stages of high heating, consistent with softening of topographical
features on Enceladus.



--
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Kevin D. McKeegan
Professor of Cosmochemistry & Geochemistry
Dept. of Earth, Planetary, and Space Sciences
UCLA
Los Angeles, CA 90095-1567
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mckeegan at epss.ucla.edu
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