HR: 0800h
AN: P11A-0088    [Abstracts]
TI: Water-Ammonia Ionic Ocean on Uranus and Neptune-Clue from Tropospheric Hydrogen Sulfide Clouds
AU: * Atreya, S K
EM: atreya@umich.edu
AF: University of Michigan, Atmospheric, Oceanic, and Space Science, Ann Arbor, MI 48109 United States
AU: Egeler, P A
EM: pegeler@umich.edu
AF: University of Michigan, Atmospheric, Oceanic, and Space Science, Ann Arbor, MI 48109 United States
AU: Wong, A
EM: aswong@umich.edu
AF: University of Michigan, Atmospheric, Oceanic, and Space Science, Ann Arbor, MI 48109 United States
AB: Interior models of the ice-giants, Uranus and Neptune, predict a water-ammonia ionic ocean at tens of kilobar pressure [1,2]. If correct, its implication for planetary formation models is profound. In this presentation we demonstrate that the existence of an ionic ocean will manifest itself in the planets' tropospheric cloud structure, particularly in the form of a hydrogen sulfide, i.e. H₂S-ice, cloud. In fact, an H₂S cloud was introduced ad hoc in the 3-5 bar region to explain microwave absorption [3] and the methane [4] observations, but its presence cannot be proved in the absence of entry probes. Our equilibrium cloud condensation model (ECCM) shows that an H₂S-ice cloud does not form when conventional enrichment factors (20-30× solar at Uranus, and 30-50× solar at Neptune) are employed for all heavy elements (mass >4) [5]. However, a deep ``cloud'' composed of a weak solution of ammonia and water forms, and its base is at 370 and 500 bars, respectively, for 30× solar and 50× solar enrichment factors. If an ionic ``ocean'' exists much deeper, water vapor, as well as ammonia dissolved in it, would be severely depleted at levels above this ocean. The consequences of such water vapor and ammonia depletions are that (1) clouds of water and ammonia, if present, are much less prominent; (2) only small amount of H₂S vapor is removed by NH₃, to form an NH₄SH cloud; so that (3) a cloud of H₂S-ice can now form; and (4) an H₂O ``ocean'' in the 1-kilobar region [6] does not form. This scenario has important implications for the design of entry probe missions, as measurements to only 10-20 bars, rather than kilobar levels, will need to be made. The heavy elements, Ar, Kr, Xe, Ne, C, and S, as well as He, D/H, GeH<sub>4</sub>, AsH<sub>3</sub>, PH<sub>3</sub>, and CO can all be accessed at pressures less than 20 bars. These measurements are critical for constraining the formation models [5,7,8]. Measurement of water in the well-mixed region of Uranus and Neptune is technologically highly challenging, even if there were no ionic ocean. And, neither H<sub>2</sub>O nor NH<sub>3</sub> can be accessed if there is a deep ionic ocean. On the other hand, if all other heavy elements and above species and isotopes were measured, O and N are not critical for the formation models of Uranus and Neptune [5]. References: [1] Podolak, et al., 1991, in Uranus (J. Bergstralh, et al., eds.), Univ. of Arizona Press. pp 48-49.1991; [2] Ree, FH, Physica, 1986, 139-140B, 73-78; [3] de Pater, et al., 1991, Icarus, 91, 220; [4] Baines, K, Hammel, H, 1994, Icarus, 109, 20 ; [5] Atreya, SK, and Wong, AS, pp121-126, in ``Outer Planets", T. Encrenaz, et al., eds, Springer, 2005; [6] Wiktorowicz, SJ, Ingersoll, AP, 2004 DPS 36.0501; [7] Owen, T, Encrenaz, T, 2003, 106, 121; [8] Atreya, S.K., et al., 2003, Planet Space Sci., 47. 1243.
DE: 5210 Planetary atmospheres, clouds, and hazes (0343)
DE: 5749 Origin and evolution
DE: 6207 Comparative planetology
DE: 6255 Neptune
DE: 6293 Uranus
SC: Planetary Sciences [P]
MN: Fall Meeting 2005