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J. J. Fortney, W. B. Hubbard (LPL -- U. of Arizona)
We present the first models of Saturn to couple its evolution to both a radiative-atmosphere grid and to high-pressure phase diagrams of hydrogen with helium and other admixtures. The purpose of these models is to quantify the evolutionary effects of phase separation in the planet’s deep interior. We find that prior calculated phase diagrams in which Saturn's interior reaches a region of predicted He immiscibility do not allow enough energy release to prolong Saturn's cooling to its known age and effective temperature. We explore modifications to published phase diagrams that would lead to greater energy release, and find a modified H-He phase diagram that is physically reasonable, leads to the correct extension of Saturn's cooling, and predicts an atmospheric He mass fraction Yatmos = 0.185, in agreement with recent estimates. If the planet’s current Yatmos is greater than 0.21 an additional energy source other than He separation is needed to explain Saturn's current luminosity. We also explore the possibility of internal separation of elements heavier than He, and find that, alternatively, such separation could prolong Saturn's cooling under a realistic phase diagram and heavy element abundance. A new measurement of Saturn’s Yatmos by Cassini will shed light on Saturn and Jupiter’s interior processes. We discuss future work on the coupled problem of the evolution of Saturn and Jupiter. The evolutionary models presented here have a direct bearing on the luminosity, radii, and dectability of extrasolar giant planets in the ~ 0.2 to 1.0 MJupiter mass range—they’ll be brighter than you’d think. Reference: J.J. Fortney & W.B. Hubbard, 2003. Icarus, vol. 164.
This research was supported by NASA grants NAG5-10760 (Astrophysics Theory Program), NAG5-10629 (Origins of Solar Systems Program), NAG5-8906 (Planetary Geology and Geophysics Program) and a NASA GSRP Fellowship for JJF.
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.