36th DPS Meeting, 8-12 November 2004
Session 5 Uranus and Neptune
Oral, Monday, November 8, 2004, 1:30-3:00pm, Clark

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[5.01] Oceans in Cold, Water-Rich Planets

S. J. Wiktorowicz, A. P. Ingersoll (Caltech)

The interiors of Uranus and Neptune are significantly different from those of the terrestrial and Jovian planets. Their large bulk densities are evidence for significant water content, so it is important to study the form in which that water exists. We examine a hypothetical planet by idealizing the density structure of Neptune and by making three major assumptions. First, we pin this planet’s photospheric (~0.5 bar) temperature and total pressure to Voyager measurements of Neptune. Second, we assume that this planet’s water and hydrogen are uniformly mixed throughout the interior. Third, we assume that the water-poor photosphere and the water-rich interior will be joined either by a wet adiabat terminating at a cloud base (with a dry, adiabatic, gaseous interior) or by a wet adiabat in vapor equilibrium with a liquid water-hydrogen interior. For a high-entropy interior (a “hot” planet), the planetary cloud deck terminates at a cloud base, whereas for a low-entropy, “cold” planet, the cloud deck transitions to a liquid ocean at depth. As planets radiatively cool, they lose entropy; therefore the cloud bases in young planets become ocean surfaces as these planets age. Planets with clouds will have different observational signatures from planets with oceans. For Neptune, better knowledge of its gravitational moments should help decide whether or not an ocean exists. For transiting extrasolar planets, radii and bulk densities can be determined. Observations of warm, water-rich atmospheres, in those whose bulk densities are compatible with water, imply hot, gaseous interiors. Detection of cold, hydrogen-rich atmospheres would be consistent with liquid water-hydrogen oceans beneath.

This work has benefited from the support of the NASA Planetary Atmospheres Program.

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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.