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Liquid water oceans in ice giants

Wiktorowicz, Sloane J. and Ingersoll, Andrew P. (2007) Liquid water oceans in ice giants. Icarus, 186 (2). pp. 436-447. ISSN 0019-1035. doi:10.1016/j.icarus.2006.09.003.

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Aptly named, ice giants such as Uranus and Neptune contain significant amounts of water. While this water cannot be present near the cloud tops, it must be abundant in the deep interior. We investigate the likelihood of a liquid water ocean existing in the hydrogen-rich region between the cloud tops and deep interior. Starting from an assumed temperature at a given upper tropospheric pressure (the photosphere), we follow a moist adiabat downward. The mixing ratio of water to hydrogen in the gas phase is small in the photosphere and increases with depth. The mixing ratio in the condensed phase is near unity in the photosphere and decreases with depth; this gives two possible outcomes. If at some pressure level the mixing ratio of water in the gas phase is equal to that in the deep interior, then that level is the cloud base. The gas below the cloud base has constant mixing ratio. Alternately, if the mixing ratio of water in the condensed phase reaches that in the deep interior, then the surface of a liquid ocean will occur. Below this ocean surface, the mixing ratio of water will be constant. A cloud base occurs when the photospheric temperature is high. For a family of ice giants with different photospheric temperatures, the cooler ice giants will have warmer cloud bases. For an ice giant with a cool enough photospheric temperature, the cloud base will exist at the critical temperature. For still cooler ice giants, ocean surfaces will result. A high mixing ratio of water in the deep interior favors a liquid ocean. We find that Neptune is both too warm (photospheric temperature too high) and too dry (mixing ratio of water in the deep interior too low) for liquid oceans to exist at present. To have a liquid ocean, Neptune's deep interior water to gas ratio would have to be higher than current models allow, and the density at 19 kbar would have to be ≈0.8 g/cm3. Such a high density is inconsistent with gravitational data obtained during the Voyager flyby. In our model, Neptune's water cloud base occurs around 660 K and 11 kbar, and the density there is consistent with Voyager gravitational data. As Neptune cools, the probability of a liquid ocean increases. Extrasolar “hot Neptunes,” which presumably migrate inward toward their parent stars, cannot harbor liquid water oceans unless they have lost almost all of the hydrogen and helium from their deep interiors.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Wiktorowicz, Sloane J.0000-0003-4483-5037
Ingersoll, Andrew P.0000-0002-2035-9198
Additional Information:© 2006 Elsevier Inc. Received 8 December 2005; revised 28 August 2006. Available online 15 November 2006. We thank D.J. Stevenson for a valuable debate regarding condensation in a water–hydrogen mixture. S.J.W. thanks C.J. Wiktorowicz for his help in determining uncertainty along the photospheric adiabat.
Subject Keywords:Atmospheres, evolution; Extrasolar planets; Neptune; Neptune, atmosphere; Neptune, interior
Issue or Number:2
Record Number:CaltechAUTHORS:20130124-071231171
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Official Citation:Sloane J. Wiktorowicz, Andrew P. Ingersoll, Liquid water oceans in ice giants, Icarus, Volume 186, Issue 2, February 2007, Pages 436-447, ISSN 0019-1035, 10.1016/j.icarus.2006.09.003. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:36550
Deposited By: Ruth Sustaita
Deposited On:24 Jan 2013 16:04
Last Modified:09 Nov 2021 23:22

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