Cloud Formation in Extra-solar Giant Planets

J.I. Lunine, W.B. Hubbard (LPL, The University of Arizona), A. Burrows, C. Sharp, D. Sudarsky (Steward Observatory, The University of Arizona), M. Marley (Dept. of Astronomy, New Mexico State University), T. Guillot (Dept. of Meteorology, University of Reading, U.K.), D. Saumon (Dept. of Physics and Astronomy, Vanderbilt University), R. Freedman (Sterling Software, NASA Ames Research Center)

Our group has constructed a new and comprehensive suite of non-grey models of extra-solar giant planets and brown dwarfs, with effective temperatures from Jupiter's up to 1300 K, to aid observers in both the search and characterization of such objects. This paper describes the conditions under which grain (cloud) formation occurs in the atmospheres of such objects, as well as the composition and physical properties of the condensate, as a function of effective temperature and surface gravity. The onset of condensation is determined by the saturation pressure of the condensable and the temperature pressure-profiles we construct for the atmospheres. We use simple grain growth timescales following on the earlier work of Lunine et al. (Ap.J. 338, 314, 1989) to predict mean particle or droplet size. Water cloud formation occurs in the radiatively-relevant region of the atmosphere from effective temperatures of 200 K up through 400 K. Magnesium silicate and iron clouds occur in the discernible atmosphere above effective temperatures of about 1600 K for Jovian-class surface gravities. Other, minor, cloud-forming species may affect the radiative balance to a lesser extent at intermediate effective temperatures. The most difficult issue to quantify is the areal uniformity, or lack thereof, of such clouds, with the Galileo probe results on Jupiter an important lesson in this regard. In the abstract by Marley et al. the effects of clouds on the spectra, and the potential for diagnosing cloud properties, are studied.