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Author(s): Marius Millot [sup.1] , Federica Coppari [sup.1] , J. Ryan Rygg [sup.1] [sup.2] , Antonio Correa Barrios [sup.1] , Sebastien Hamel [sup.1] , Damian C. Swift [sup.1] , Jon [...], Since Bridgman's discovery of five solid water (H.sub.2O) ice phases.sup.1 in 1912, studies on the extraordinary polymorphism of H.sub.2O have documented more than seventeen crystalline and several amorphous ice structures.sup.2,3, as well as rich metastability and kinetic effects.sup.4,5. This unique behaviour is due in part to the geometrical frustration of the weak intermolecular hydrogen bonds and the sizeable quantum motion of the light hydrogen ions (protons). Particularly intriguing is the prediction that H.sub.2O becomes superionic.sup.6-12--with liquid-like protons diffusing through the solid lattice of oxygen--when subjected to extreme pressures exceeding 100 gigapascals and high temperatures above 2,000 kelvin. Numerical simulations suggest that the characteristic diffusion of the protons through the empty sites of the oxygen solid lattice (1) gives rise to a surprisingly high ionic conductivity above 100 Siemens per centimetre, that is, almost as high as typical metallic (electronic) conductivity, (2) greatly increases the ice melting temperature.sup.7-13 to several thousand kelvin, and (3) favours new ice structures with a close-packed oxygen lattice.sup.13-15. Because confining such hot and dense H.sub.2O in the laboratory is extremely challenging, experimental data are scarce. Recent optical measurements along the Hugoniot curve (locus of shock states) of water ice VII showed evidence of superionic conduction and thermodynamic signatures for melting.sup.16, but did not confirm the microscopic structure of superionic ice. Here we use laser-driven shockwaves to simultaneously compress and heat liquid water samples to 100-400 gigapascals and 2,000-3,000 kelvin. In situ X-ray diffraction measurements show that under these conditions, water solidifies within a few nanoseconds into nanometre-sized ice grains that exhibit unambiguous evidence for the crystalline oxygen lattice of superionic water ice. The X-ray diffraction data also allow us to document the compressibility of ice at these extreme conditions and a temperature- and pressure-induced phase transformation from a body-centred-cubic ice phase (probably ice X) to a novel face-centred-cubic, superionic ice phase, which we name ice XVIII.sup.2,17. The atomic structure of H.sub.2O is documented at several million atmospheres of pressure and temperatures of several thousand degrees, revealing shockwave-induced ultrafast crystallization and a novel water ice phase, ice XVIII, with exotic superionic properties.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.021 Byline: Sarah E. Dodson-Robinson (a), Peter Bodenheimer (b) Keywords: Planetary formation; Uranus; Neptune; Origin, Solar System; Abundances, Interiors Abstract: The core accretion theory of planet formation has at least two fundamental problems explaining the origins of Uranus and Neptune: (1) dynamical times in the trans-saturnian solar nebula are so long that core growth can take >15Myr and (2) the onset of runaway gas accretion that begins when cores reach [approximately equal to]10M.sub.[direct sum] necessitates a sudden gas accretion cutoff just as Uranus and Neptune's cores reach critical mass. Both problems may be resolved by allowing the ice giants to migrate outward after their formation in solid-rich feeding zones with planetesimal surface densities well above the minimum-mass solar nebula. We present new simulations of the formation of Uranus and Neptune in the solid-rich disk of Dodson-Robinson et al. (Dodson-Robinson, S.E., Willacy, K., Bodenheimer, P., Turner, N.J., Beichman, C.A. [2009]. Icarus 200, 672-693) using the initial semimajor axis distribution of the Nice model (Gomes, R., Levison, H.F., Tsiganis, K., Morbidelli, A. [2005]. Nature 435, 466-469; Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R. [2005]. Nature 435, 462-465; Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F. [2005]. Nature 435, 459-461), with one ice giant forming at 12AU and the other at 15AU. The innermost ice giant reaches its present mass after 3.8-4.0Myr and the outermost after 5.3-6Myr, a considerable time decrease from previous one-dimensional simulations (e.g. Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y. [1996]. Icarus 124, 62-85). The core masses stay subcritical, eliminating the need for a sudden gas accretion cutoff. Our calculated carbon mass fractions of 22% are in excellent agreement with the ice giant interior models of Podolak et al. (Podolak, M., Weizman, A., Marley, M. [1995]. Planet. Space Sci. 43, 1517-1522) and Marley et al. (Marley, M.S., Gomez, P., Podolak, M. [1995]. J. Geophys. Res. 100, 23349-23354). Based on the requirement that the ice giant-forming planetesimals contain >10% mass fractions of methane ice, we can reject any Solar System formation model that initially places Uranus and Neptune inside of Saturn's orbit. We also demonstrate that a large population of planetesimals must be present in both ice giant feeding zones throughout the lifetime of the gaseous nebula. This research marks a substantial step forward in connecting both the dynamical and chemical aspects of planet formation. Although we cannot say that the solid-rich solar nebula model of Dodson-Robinson et al. (Dodson-Robinson, S.E., Willacy, K., Bodenheimer, P., Turner, N.J., Beichman, C.A. [2009]. Icarus 200, 672-693) gives exactly the appropriate initial conditions for planet formation, rigorous chemical and dynamical tests have at least revealed it to be a viable model of the early Solar System. Author Affiliation: (a) University of Texas, Astronomy Department, 1 University Station C1400, Austin, TX 78712, USA (b) UCO/Lick Observatory, University of California at Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA Article History: Received 8 May 2009; Revised 30 October 2009; Accepted 16 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.10.018 Byline: Ignacio Mosqueira (a), Paul R. Estrada (a), Sebastien Charnoz (b) Keywords: Satellites, Formation; Iapetus; Disks; Planetesimals; Impact processes Abstract: Ever since their discovery the regular satellites of Jupiter and Saturn have held out the promise of providing an independent set of observations with which to test theories of planet formation. Yet elucidating their origins has proven elusive. Here we show that Iapetus can serve to discriminate between satellite formation models. Its accretion history can be understood in terms of a two-component gaseous subnebula, with a relatively dense inner region, and an extended tail out to the location of the irregular satellites, as in the SEMM model of Mosqueira and Estrada (2003a,b) (Mosqueira, I., Estrada, P.R. [2003a]. Icarus 163, 198-231; Mosqueira, I., Estrada, P.R. [2003b]. Icarus 163, 232-255). Following giant planet formation, planetesimals in the feeding zone of Jupiter and Saturn become dynamically excited, and undergo a collisional cascade. Ablation and capture of planetesimal fragments crossing the gaseous circumplanetary disks delivers enough collisional rubble to account for the mass budgets of the regular satellites of Jupiter and Saturn. This process can result in rock/ice fractionation as long as the make up of the population of disk crossers is non-homogeneous, thus offering a natural explanation for the marked compositional differences between outer solar nebula objects and those that accreted in the subnebulae of the giant planets. For a given size, icy objects are easier to capture and to ablate, likely resulting in an overall enrichment of ice in the subnebula. Furthermore, capture and ablation of rocky fragments become inefficient far from the planet for two reasons: the gas surface density of the subnebula is taken to drop outside the centrifugal radius, and the velocity of interlopers decreases with distance from the planet. Thus, rocky objects crossing the outer disks of Jupiter and Saturn never reach a temperature high enough to ablate either due to melting or vaporization, and capture is also greatly diminished there. In contrast, icy objects crossing the outer disks of each planet ablate due to the melting and vaporization of water-ice. Consequently, our model leads to an enhancement of the ice content of Iapetus, and to a lesser degree those of Titan, Callisto and Ganymede, and accounts for the (non-stochastic) compositions of these large, low-porosity outer regular satellites of Jupiter and Saturn. For this to work, the primordial population of planetesimals in the Jupiter-Saturn region must be partially differentiated, so that the ensuing collisional cascade produces an icy population of a[sup.3]1m size fragments to be ablated during subnebula crossing. We argue this is likely because the first generation of solar nebula [approximately equal to]10km planetesimals in the Jupiter-Saturn region incorporated significant quantities of.sup.26Al. This is the first study successfully to provide a direct connection between nebula planetesimals and subnebulae mixtures with quantifiable and observable consequences for the bulk properties of the regular satellites of Jupiter and Saturn, and the only explanation presently available for Iapetus' low density and ice-rich composition. Author Affiliation: (a) Carl Sagan Center, SETI Institute, 515 N. Whisman Rd., Mountain View, CA 94043, United States (b) Equipe AIM, Universite Paris Diderot/CEA/CNRS, CEA/SAp, Centre de l'Orme Les Merisiers, 91191 Gif-Sur-Yvette Cedex, France Article History: Received 19 August 2009; Revised 1 October 2009; Accepted 27 October 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.001 Byline: M. Fama, M.J. Loeffler, U. Raut, R.A. Baragiola Keywords: Ices, IR spectroscopy; Infrared observations; Kuiper belt; Satellites, Surfaces Abstract: We study radiation-induced amorphization of crystalline ice, analyzing the results of three decades of experiments with a variety of projectiles, irradiation energy, and ice temperature, finding a similar trend of increasing resistance of amorphization with temperature and inconsistencies in results from different laboratories. We discuss the temperature dependence of amorphization in terms of the 'thermal spike' model. We then discuss the common use of the 1.65[mu]m infrared absorption band of water as a measure of degree of crystallinity, an increasingly common procedure to analyze remote sensing data of astronomical icy bodies. The discussion is based on new, high quality near-infrared reflectance absorption spectra measured between 1.4 and 2.2[mu]m for amorphous and crystalline ices irradiated with 225keV protons at 80K. We found that, after irradiation with 10.sup.15 protonscm.sup.-2, crystalline ice films thinner than the ion range become fully amorphous, and that the infrared absorption spectra show no significant changes upon further irradiation. The complete amorphization suggests that crystalline ice observed in the outer Solar System, including trans-neptunian objects, may results from heat from internal sources or from the impact of icy meteorites or comets. Author Affiliation: Laboratory for Atomic and Surface Physics, University of Virginia, 351 McCormick Road, P.O. Box 400238, Charlottesville, VA 22904-4238, USA Article History: Received 19 June 2009; Revised 24 October 2009; Accepted 2 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.10.014 Byline: Konrad J. Kossacki (a), Slawomira Szutowicz (b) Keywords: Comets, composition; Comets, nucleus; Ices Abstract: Our work was inspired by the recent brightening of Comet 17P/Holmes. The recently observed increase in brightness of this comet was correlated with emission of dust, probably larger in mass than the dust mantle of the nucleus. We analyzed the hypothesis that the comet can eject a large mass of dust due to non-uniform crystallization of amorphous water ice. For this purpose, we simulated the evolution of a model nucleus on the orbit of Comet 17P/Holmes. The nucleus is composed of water ice and dust and has the shape of an elongated ellipsoid. The simulations include crystallization of amorphous ice in the nucleus, changes in the dust mantle thickness, and changes in the nucleus orientation in space. Our computations indicate that: (i) ejection of the dust cover triggers crystallization of ice independently on the material properties of the nucleus; (ii) moderate changes in the nucleus orientation ([approximately equal to]50[degrees]) may result in an acceleration of the crystallization of ice in the northern hemisphere, while a rather large change in the orientation ([approximately equal to]120[degrees]) is needed to cause a significant jump of the crystallization front in the southern hemisphere, where the emission of dust during the recent brightening was strongest. We investigated the possible reason for an explosion and we have found that the crystallization of the water ice itself is probably not sufficient. Author Affiliation: (a) Institute of Geophysics of Warsaw University, Pasteura 7, 02-093 Warsaw, Poland (b) Space Research Center of Polish Academy of Sciences, Bartycka 18a, 00-716 Warsaw, Poland Article History: Received 1 April 2009; Revised 13 October 2009; Accepted 19 October 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.10.012 Byline: A. Kereszturi (a), D. Mohlmann (b), Sz. Berczi (a)(c), T. Ganti (a), A. Horvath (a)(f), A. Kuti (e), A. Sik (a)(d), E. Szathmary (a) Keywords: Mars, Surface; Ices Abstract: Springtime low albedo features, called Dark Dune Spots, on the seasonal frost covered dunes on Mars between 77[degrees]N and 84[degrees]N latitude have been analyzed. Two groups of these spots have been identified: 'small' and 'large' ones, where large spots have diameters above 4m, and complex internal structure. From these 'large' spots branching seepage-like features emanate and grow on the steep slopes. They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3m/day and 7m/day respectively, first only from the spots, later from all along the dune crest. During this 'seepage period' the temperature is between 150K and 180K at a 3-9km spatial resolution scale, indicating that CO.sub.2 ice-free parts must be present at the observed dark spots. Around the receding northern seasonal CO.sub.2 cap, an annulus of water ice lags behind, which is probably present in the spots too where the CO.sub.2 frost has sublimated. Our model estimates show in the present work and in Kereszturi et al. (Kereszturi, A., Mohlmann, D., Berczi, Sz., Ganti, T., Kuti, A., Sik, A., Horvath, A. [2009b]. Icarus 201, 492-503) that the warming driven by solar insolation may produce not only interfacial water, but also bulk brines around the dune grains. The brine can support the movement of liquids and dune grains, enhances the sublimation of CO.sub.2 frost, and produce the dark features, as well as liquid modifies the optical properties of the surface. Signs of movement of dune material after the total defrosting of the terrain is also visible but it is uncertain because of the limit of resolution. In our previous work (Kereszturi et al., 2009b) we showed that resembling seepage-like streaks at the southern hemisphere might have been formed by ephemeral interfacial water, as well as these northern features. Such wet environments may have astrobiological importance too. Author Affiliation: (a) Collegium Budapest, Institute for Advanced Study, H-1041 Budapest, Szentharomsag 2, Hungary (b) DLR Institut fur Planetenforschung, D-12489 Berlin, Rutherfordstr. 2, Germany (c) Eotvos Lorand University of Sciences, Institute of Physics, H-1117 Budapest, Pazmany 1/A, Hungary (d) Eotvos Lorand University of Sciences, Institute of Geography and Earth Sciences, H-1117 Budapest, Pazmany 1/c, Hungary (e) Eotvos Lorand University of Sciences, Institute of Physics, Department of Astronomy, H-1117 Budapest, Pazmany 1/c, Hungary (f) Konkoly Observatory, H-1525 Budapest, Pf. 67, Hungary Article History: Received 11 June 2009; Revised 18 September 2009; Accepted 19 October 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.013 Byline: Diedrich T.F. Mohlmann Keywords: Ices; Mars; Mars, Surface Abstract: It is investigated whether conditions for melting can be temporarily created in the upper sub-surface parts of snow/ice-packs on Mars at subzero surface temperatures by means of the solid-state greenhouse effect, as occurs in snow- and ice-covered regions on Earth. The conditions for this possible temporary melting are quantitatively described for bolometric albedo values A =0.8 and A =0.2, and with model parameters typical for the thermo-physical conditions at snow/ice sites on the surface of present Mars. It is demonstrated by numerical modelling that there are several sets of parameters which will lead to development of layers of liquid water just below the top surface of snow- and ice-packs on Mars. This at least partial liquefaction occurs repetitively (e.g. diurnally, seasonally), and can in some cases lead to liquid water persisting through the night-time in the summer season. This liquid water can form in sufficient amounts to be relevant for macroscopic physical (rheology, erosion), for chemical, and eventually also for biological processes. The creation of temporary pockets of sub-surface water by this effect requires pre-existing snow or ice cover, and thus is more likely to take place at high latitudes, since the present deposits of snow/ice can mainly be found there. Possible rheologic and related erosion consequences of the appearance of liquid sub-surface water in martian snow/ice-packs are discussed in view of current observations of recent rheologic processes. Author Affiliation: DLR Institut fur Planetenforschung, Rutherfordstr. 2, D 12489 Berlin, Germany Article History: Received 22 February 2009; Revised 10 November 2009; Accepted 13 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.09.007 Byline: Colin M. Dundas, Shane Byrne Keywords: Mars, Surface; Ices; Mars, Atmosphere; Impact processes Abstract: New impacts in the martian mid-latitudes have exposed near-surface ice. This ice is observed to slowly fade over timescales of months. In the present martian climate, exposed surface ice is unstable during summer months in the mid-latitudes and will sublimate. We model the sublimation of ice at five new impact sites and examine the implications of its persistence. Even with generally conservative assumptions, for most reasonable choices of parameters it is likely that over a millimeter of sublimation occurred in the period during which the ice was observed to fade. The persistence of visible ice through such sublimation suggests that the ice is relatively pure rather than pore-filling. Such ice could be analogous to the nearly pure ice observed by the Phoenix Lander in the 'Dodo-Goldilocks' trench and suggests that the high ice contents reported by the Mars Odyssey Gamma Ray Spectrometer at high latitudes extend to the mid-latitudes. Our observations are consistent with a model of the martian ice table in which a layer with high volumetric ice content overlies pore-filling ice, although other structures are possible. Author Affiliation: Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, United States Article History: Received 23 May 2009; Revised 26 August 2009; Accepted 8 September 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.07.036 Byline: Katrin Stephan (a), Ralf Jaumann (a)(b), Roland Wagner (a), Roger N. Clark (c), Dale P. Cruikshank (d), Charles A. Hibbitts (e), Thomas Roatsch (a), Harald Hoffmann (a), Robert H. Brown (f), G. Filiacchione (g), Bonnie J. Buratti (h), Gary B. Hansen (i), Tom B. McCord (j), Phil D. Nicholson (k), Kevin H. Baines (h) Keywords: Saturn; Satellites; Surfaces; Spectroscopy; Geological processes Abstract: We present a detailed analysis of the variations in spectral properties across the surface of Saturn's satellite Dione using Cassini/VIMS data and their relationships to geological and/or morphological characteristics as seen in the Cassini/ISS images. This analysis focuses on a local region on Dione's anti-saturnian hemisphere that was observed by VIMS with high spatial resolution during orbit 16 in October 2005. The results are incorporated into a global context provided by VIMS data acquired within Cassini's first 50 orbits. Our results show that Dione's surface is dominated by at least one global process. Bombardment by magnetospheric particles is consistent with the concentration of dark material and enhanced CO.sub.2 absorption on the trailing hemisphere of Dione independent of the geology. Local regions within this terrain indicate a special kind of resurfacing that probably is related to large-scale impact process. In contrast, the enhanced ice signature on the leading side is associated with the extended ejecta of the fresh impact crater Creusa ([approximately equal to]49[degrees]N/76[degrees]W). Although no geologically active regions could be identified, Dione's tectonized regions observed with high spatial resolution partly show some clean H.sub.2O ice implying that tectonic processes could have continued into more recent times. Author Affiliation: (a) DLR, Institute of Planetary Research, Rutherfordstrasse 2, 12489 Berlin, Germany (b) Freie Universitat, FR Planetologie und Fernerkundung, Malteserstr. 74-100, 12249 Berlin, Germany (c) US Geological Survey, Denver Federal Center, Denver CO 80225, USA (d) NASA Ames Research Center, MS 245-3 Moffett Field, CA 94035-1000, USA (e) JHU Applied Physics Laboratory, Laurel, MD, USA (f) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA (g) INAF-IASF, via del Fosso del Cavaliere 100, Rome, Italy (h) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA (i) University of Washington, Seattle, USA (j) Space Science Institute, Winthrop, WA, USA (k) Cornell University, Astronomy Department, Ithaca, NY, USA Article History: Received 15 December 2008; Revised 6 July 2009; Accepted 24 July 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.11.006 Byline: G. Filacchione (a), F. Capaccioni (a), R.N. Clark (b), J.N. Cuzzi (c), D.P. Cruikshank (c), A. Coradini (d), P. Cerroni (a), P.D. Nicholson (e), T.B. McCord (f), R.H. Brown (g), B.J. Buratti (h), F. Tosi (d), R.M. Nelson (h), R. Jaumann (i), K. Stephan (i) Keywords: Saturn, Satellites; Spectroscopy; Ices, IR spectroscopy; Infrared observations; Image processing Abstract: We report the detailed analysis of the spectrophotometric properties of Saturn's icy satellites as derived by full-disk observations obtained by visual and infrared mapping spectrometer (VIMS) experiment aboard Cassini. In this paper, we have extended the coverage until the end of the Cassini's nominal mission (June 1st 2008), while a previous paper (, hereby referred to as Paper I) reported the preliminary results of this study. During the four years of nominal mission, VIMS has observed the entire population of Saturn's icy satellites allowing us to make a comparative analysis of the VIS-NIR spectral properties of the major satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus) and irregular moons (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso, Phoebe). The results we discuss here are derived from the entire dataset available at June 2008 which consists of 1417 full-disk observations acquired from a variety of distances and inclinations from the equatorial plane, with different phase angles and hemispheric coverage. The most important spectrophotometric indicators (as defined in Paper I: I/F continua at 0.55[mu]m, 1.822[mu]m and 3.547[mu]m, visible spectral slopes, water and carbon dioxide bands depths and positions) are calculated for each observation in order to investigate the disk-integrated composition of the satellites, the distribution of water ice respect to 'contaminants' abundances and typical regolith grain properties. These quantities vary from the almost pure water ice surfaces of Enceladus and Calypso to the organic and carbon dioxide rich Hyperion, Iapetus and Phoebe. Janus visible colors are intermediate between these two classes having a slightly positive spectral slope. These results could help to decipher the origins and evolutionary history of the minor moons of the Saturn's system. We introduce a polar representation of the spectrophotometric parameters as function of the solar phase angle (along radial distance) and of the effective longitude interval illuminated by the Sun and covered by VIMS during the observation (in azimuth) to better investigate the spatial distribution of the spectrophotometric quantities across the regular satellites hemispheres. Finally, we report the observed spectral positions of the 4.26[mu]m band of the carbon dioxide present in the surface material of three outermost moons Hyperion, Iapetus and Phoebe. Author Affiliation: (a) INAF-IASF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere, 100, 00133, Rome, Italy (b) US Geological Survey, Federal Center, Denver, CO, 80228, USA (c) NASA Ames Research Center, Moffett Field, CA 94035-1000, USA (d) INAF-IFSI, Istituto di Fisica dello Spazio Interplanetario, Area di Ricerca di Tor Vergata, via del Fosso del Cavaliere, 100, 00133, Rome, Italy (e) Cornell University, Astronomy Department, 418 Space Sciences Building, Ithaca, NY 14853, USA (f) Bear Fight Center, 22 Fiddlers Rd., Winthrop, WA 98862, USA (g) Lunar and Planetary Laboratory and Steward Observatory, University of Arizona, AZ 85721, USA (h) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Groove Drive, Pasadena, CA 91109, USA (i) Institute for Planetary Exploration, DLR, RutherfordstaAe 2,12489, Berlin, Germany Article History: Received 26 November 2008; Revised 16 September 2009; Accepted 9 November 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.09.015 Byline: Andrew P. Ingersoll (a), Alexey A. Pankine (b) Keywords: Enceladus; Ices; Geological processes; Interiors Abstract: Given the heat that is reaching the surface from the interior of Enceladus, we ask whether liquid water is likely and at what depth it might occur. The heat may be carried by thermal conduction through the solid ice, by the vapor as it diffuses through a porous matrix, or by the vapor flowing upward through open cracks. The vapor carries latent heat, which it acquires when ice or liquid evaporates. As the vapor nears the surface it may condense onto the cold ice, or it may exit the vent without condensing, carrying its latent heat with it. The ice at the surface loses its heat by infrared radiation. An important physical principle, which has been overlooked so far, is that the partial pressure of the vapor in the pores and in the open cracks is nearly equal to the saturation vapor pressure of the ice around it. This severely limits the ability of ice to deliver the observed heat to the surface without melting at depth. Another principle is that viscosity limits the speed of the flow, both the diffusive flow in the matrix and the hydrodynamic flow in open cracks. We present hydrodynamic models that take these effects into account. We find that there is no simple answer to the question of whether the ice melts or not. Vapor diffusion in a porous matrix can deliver the heat to the surface without melting if the particle size is greater than [approximately equal to]1cm and the porosity is greater than [approximately equal to]0.1, in other words, if the matrix is a rubble pile. Whether such an open matrix can exist under its own hydrostatic load is unclear. Flow in open cracks can deliver the heat without melting if the width of the crack is greater than [approximately equal to]10cm, but the heat source must be in contact with the crack. Frictional heating on the walls due to tidal stresses is one such possibility. The lifetime of the crack is a puzzle, since condensation on the walls in the upper few meters could seal the crack off in a year, and it takes many years for the heat source to warm the walls if the crack extends down to km depths. The 10:1 ratio of radiated heat to latent heat carried with the vapor is another puzzle. The models tend to give a lower ratio. The resolution might be that each tiger stripe has multiple cracks that share the heat, which tends to lower the ratio. The main conclusion is that melting depends on the size of the pores and the width of the cracks, and these are unknown at present. Author Affiliation: (a) California Institute of Technology, MS 150-21, Pasadena, CA 91125, USA (b) Jet Propulsion Laboratory/Caltech, MS 183-301, Pasadena, CA 91109, USA Article History: Received 10 December 2008; Revised 3 June 2009; Accepted 19 September 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.09.005 Byline: Joseph S. Levy (a), David R. Marchant (b), James W. Head (a) Keywords: Mars; Ices; Mars, Polar Geology; Astrobiology; Mars, Surface Abstract: Thermal contraction crack polygons are complex landforms that have begun to be deciphered on Earth and Mars by the combined investigative efforts of geomorphology, environmental monitoring, physical models, paleoclimate reconstruction, and geochemistry. Thermal contraction crack polygons are excellent indicators of the current or past presence of ground ice, ranging in ice content from weakly cemented soils to debris-covered massive ice. Relative to larger topographic features, polygons may form rapidly, and reflect climate conditions at the time of formation -- preserving climate information as relict landforms in the geological record. Polygon morphology and internal textural characteristics can be used to distinguish surfaces modified by the seasonal presence of a wet active layer or dry active layer, and to delimit subsurface ice conditions. Analysis of martian polygon morphology and distribution indicates that geologically-recent thermal contraction crack polygons on Mars form predominantly in an ice-rich latitude-dependent mantle, more likely composed of massive ice deposited by precipitation than by cyclical vapor diffusion into regolith. Regional and local heterogeneities in polygon morphology can be used to distinguish variations in ice content, deposition and modification history, and to assess microclimate variation on timescales of ka to Ma. Analyses of martian polygon morphology, guided by investigations of terrestrial analog thermal contraction crack polygons, strongly suggest the importance of excess ice in the formation and development of many martian thermal contraction crack polygons -- implying the presence of an ice-rich substrate that was fractured during and subsequent to obliquity-driven depositional periods and continually modified by ongoing vapor equilibration processes. Author Affiliation: (a) Department of Geological Sciences, Brown University, Providence, RI 02912, United States (b) Department of Earth Science, Boston University, 675 Commonwealth Ave., Boston, MA 02215, United States Article History: Received 16 June 2009; Revised 12 August 2009; Accepted 3 September 2009

Ice has a memory. It remembers in detail and it remembers for a million years or more. Ice remembers forest fires and rising seas. Ice remembers the chemical composition of [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.08.016 Byline: Constantin Romanescu, Jochen Marschall, Deena Kim, Ajeeta Khatiwada, Konstantinos S. Kalogerakis Keywords: Ices; Ices; IR spectroscopy; Spectroscopy Abstract: Optical constants in a broad temperature and wavelength range are important input parameters in radiative transfer models used in studies of planetary atmospheres. In the laboratory, the refractive index values of ices at the HeNe laser wavelength (632.8nm) are often used to monitor the growth rate and thickness of ice films. In this report we present laboratory measurements determining the refractive index at 632.8nm of ammonia and hydrocarbon ices in the temperature range 80-100K. Thin ice films are vapor-deposited on a cryogenically cooled mirror located inside a high-vacuum apparatus. The real component of the refractive index of these ice films is determined by a two-angle interferometric technique. Optical modeling calculations of the transmittance and reflectance through the thin ice films assist in the interpretation of the experimental results. We discuss our results and compare them with other measurements available in the literature. The results reported here are relevant to the spectroscopy of icy objects in the solar system; they are needed to perform laboratory characterization of ices, derive optical constants, and model spectra. Author Affiliation: Molecular Physics Laboratory, SRI International, Menlo Park, CA 94025, USA Article History: Received 20 March 2009; Revised 12 July 2009; Accepted 20 August 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.034 Byline: Sarah F. Newman (a), B.J. Buratti (a), R.H. Brown (b), R. Jaumann (c), J. Bauer (a), T. Momary (a) Keywords: Geological processes; Ices; IR spectroscopy; Saturn; Satellites Abstract: Saturn's satellite Dione is becoming an increasingly important object in the outer Solar System, as evidence for its current activity accumulates. Infrared observations of the surface can provide clues to the history of the body and currently active processes. Using data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS), we perform three sets of analyses that are sensitive to the ice state, temperature, thermal history, grain size and composition of surface ice. These are calculation of a 'crystallinity factor', spectral ratios and water ice band depths. In our analysis, we focus on the dichotomy between the wispy and dark terrain on Dione's trailing hemisphere, to better understand the source of the different materials and their current properties. Our results suggest two different scenarios: (1) the ice from the wispy region has a higher crystallinity and water ice content than the dark region or (2) the wispy region contains larger grains. Both of these models imply recent geologic activity on Dione. Author Affiliation: (a) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Mail Stop 183-501, Pasadena, CA 91109, USA (b) Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., P.O. Box 210092, Tucson, AZ 85721-0092, USA (c) German Aerospace Center (DLR), Germany, RutherfordstraAe 2, 12489 Berlin, Germany Article History: Received 12 July 2007; Revised 23 February 2009; Accepted 9 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.04.037 Byline: J.-B. Madeleine (a), F. Forget (a), James W. Head (b), B. Levrard (c), F. Montmessin (d), E. Millour (a) Keywords: Mars; Atmosphere; Dynamics; Climate; Geological processes Abstract: Recent geological observations in the northern mid-latitudes of Mars show evidence for past glacial activity during the late Amazonian, similar to the integrated glacial landsystems in the Dry Valleys of Antarctica. The large accumulation of ice (many hundreds of meters) required to create the observed glacial deposits points to significant atmospheric precipitation, snow and ice accumulation, and glacial flow. In order to understand the climate scenario required for these conditions, we used the LMD (Laboratoire de Meteorologie Dynamique) Mars GCM (General Circulation Model), which is able to reproduce the present-day water cycle, and to predict past deposition of ice consistent with geological observations in many cases. Prior to this analysis, however, significant mid-latitude glaciation had not been simulated by the model, run under a range of parameters. In this analysis, we studied the response of the GCM to a wider range of orbital configurations and water ice reservoirs, and show that during periods of moderate obliquity (I[micro] =25-35[degrees]) and high dust opacity (I.sub.dust =1.5-2.5), broad-scale glaciation in the northern mid-latitudes occurs if water ice deposited on the flanks of the Tharsis volcanoes at higher obliquity is available for sublimation. We find that high dust contents of the atmosphere increase its water vapor holding capacity, thereby moving the saturation region to the northern mid-latitudes. Precipitation events are then controlled by topographic forcing of stationary planetary waves and transient weather systems, producing surface ice distribution and amounts that are consistent with the geological record. Ice accumulation rates of [approximately equal to]10mmyr.sup.-1 lead to the formation of a 500-1000m thick regional ice sheet that will produce glacial flow patterns consistent with the geological observations. Author Affiliation: (a) Laboratoire de Meteorologie Dynamique, CNRS/UPMC/IPSL, 4 place Jussieu, BP99, 75252 Paris Cedex 05, France (b) Department of Geological Sciences, Brown University, Providence, RI 02912, USA (c) Astronomie et Systemes Dynamiques, IMCCE-CNRS UMR 8028, 77 Avenue Denfert-Rochereau, 75014 Paris, France (d) Service d'Aeronomie, CNRS/UVSQ/IPSL, Reduit de Verrieres, Route des Gatines, 91371 Verrieres-le-Buisson Cedex, France Article History: Received 11 October 2008; Revised 14 April 2009; Accepted 17 April 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.05.011 Byline: Susan W. Kieffer (a), Xinli Lu (a), Greg McFarquhar (b), Kenneth H. Wohletz (c) Keywords: Enceladus; Ices Abstract: The discovery of plumes of H.sub.2O vapor and ice particles erupting from the south pole of Enceladus, the tiny frigid satellite of Saturn, sparked controversy over whether these plumes are produced by boiling, or by sublimation with subsequent recondensation of the sublimated vapor [Porco, C.C., Helfenstein, P., Thomas, P.C., Ingersoll, A.P., Wisdom, J., West, R., Neukum, G., Denk, T., Wagner, R., Roatsch, T., Kieffer, S., Turtle, E., McEwen, A., Johnson, T.V., Rathbun, J., Veverka, J., Wilson, D., Perry, J., Spitale, J., Brahic, A., Burns, J.A., DelGenio, A.D., Dones, L., Murray, C.D., Squyres, S., 2006. Science 311, 1393-1401]. Porco et al.'s analysis that the masses of ice (I) and vapor (V) in the plume were comparable was taken to argue against the occurrence of sublimation and recondensation, leading to the hypothesis that the reservoir was boiling water, possibly as close as 7m to the surface. Thus, it has been advocated that Enceladus should be a target for astrobiology exploration. Here we show, with recalculations using the original data and methodologies, as well as with new sensitivity studies, that the mass of ice in the column is significantly less than the mass of water vapor, and that by considering three additional effects, I/V is likely to be Author Affiliation: (a) Department of Geology, University of Illinois, 1301 W Green St., Urbana, IL 61801, USA (b) Department of Atmospheric Sciences, University of Illinois, 105 S. Gregory St., Urbana, IL 61801, USA (c) Los Alamos National Laboratory, EES11 MS 665, Los Alamos, NM 87545, USA Article History: Received 26 November 2008; Revised 5 May 2009; Accepted 14 May 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.03.017 Byline: Dave R. Stegman (a), J. Freeman (b), David A. May (b) Keywords: Enceladus; Thermal histories; Rotational dynamics; Interiors; Tectonics Abstract: We consider the scenario in which the presence of ammonia in the bulk composition of Enceladus plays a pivotal role in its thermochemical evolution. Because ammonia reduces the melting temperature of the ice shell by 100 K below that of pure water ice, small amounts of tidal dissipation can power an 'ammonia feedback' mechanism that leads to secondary differentiation of Enceladus within the ice shell. This leads to compositionally distinct zones at the base of the ice shell arranged such that a layer of lower density (and compositionally buoyant) pure water ice underlies the undifferentiated ammonia-dihydrate ice layer above. We then consider a large scale instability arising from the pure water ice layer, and use a numerical model to explore the dynamics of compositional convection within the ice shell of Enceladus. The instability of the layer can easily account for a diapir that is hemispherical in scale. As it rises to the surface, it co-advects the warm internal temperatures towards the outer layers of the satellite. This advected heat facilitates the generation of a subsurface ocean within the ice shell of Enceladus. This scenario can simultaneously account for the origin of asymmetry in surface deformation observed on Enceladus as well as two global features inferred to exist: a large density anomaly within the interior and a subsurface ocean underneath the south polar region. Author Affiliation: (a) School of Earth Sciences, The University of Melbourne, Carlton, VIC 3010, Australia (b) School of Mathematical Sciences, Monash University, Clayton, VIC 3800, Australia Article History: Received 26 September 2007; Revised 5 March 2009; Accepted 10 March 2009

A method to retrieve total vertical amounts of cloud liquid and ice in stratiform precipitating systems is described. The retrievals use measurements from the vertically pointing [K.sub.a]- and W-band cloud radars operated by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program and auxiliary measurements from a scanning National Weather Service radar and a ground-based disdrometer. Separation between the cloud liquid and rain is based on estimations of the total attenuation of millimeterwavelength radar signals in the liquid hydrometeor layer. Disdrometer measurements are used for the retrieval constraints. Because the liquid phase hydrometeor retrievals use only differential measurements, they are immune to the absolute radar calibration uncertainties. Estimates of the ice cloud phase are performed using empirical relations between absolute radar reflectivity and ice water content. Data from the nearby scanning weather-service radar, which operates at a lower frequency, are used to correct cloud radar measurements observed above the freezing level for attenuation caused by the layers of liquid and melting hydrometeors and also by wet radomes of cloud radars. Polarimetric and vertical Doppler measurements from ARM cloud radars provide a distinct separation between regions of liquid and ice phases, and therefore the corresponding retrievals are performed in each region separately. The applicability of the suggested method is illustrated for a stratiform precipitation event observed at the ARM Southern Great Plains facility. Expected uncertainties for retrievals of cloud liquid water path are estimated at about :200-250 g [m.sup.-2] for typical rainfall rates observed in stratiform systems (~3-4 mm [h.sup.-1]). These uncertainties increase as rainfall rate increases. The ice water path retrieval uncertainties can be as high as a factor of 2.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.01.014 Byline: A. Kereszturi (a)(g)(h), D. Mohlmann (b), Sz. Berczi (a)(c), T. Ganti (a), A. Kuti (e)(g), A. Sik (a)(d), A. Horvath (a)(f) Keywords: Mars; surface; Ices Abstract: In springtime on HiRISE images of the Southern polar terrain of Mars flow-like or rheologic features were observed. Their dark color is interpreted as partly defrosted surface where the temperature is too high for CO.sub.2 but low enough for H.sub.2O ice to be present there. These branching streaks grow in size and can move by an average velocity of up to about 1 m/day and could terminate in pond-like accumulation features. The phenomenon may be the result of interfacial water driven rheologic processes. Liquid interfacial water can in the presence of water ice exist well below the melting point of bulk water, by melting in course of interfacial attractive pressure by intermolecular forces (van der Waals forces e.g.), curvature of water film surfaces, and e.g. by macroscopic weight, acting upon ice. This melting phenomenon can be described in terms of 'premelting of ice'. It is a challenging consequence, that liquid interfacial water unavoidably must in form of nanometric layers be present in water ice containing soil in the subsurface of Mars. It is the aim of this paper to study possible rheologic consequences in relation to observations, which seem to happen at sites of dark polar dunes on Mars at present. The model in this work assumes that interfacial water accumulates at the bottom of a translucent water-ice layer above a dark and insolated ground. This is warmed up towards the melting point of water. The evolving layer of liquid interfacial water between the covering ice sheet and the heated ground is assumed to drive downward directed flow-like features on slopes, and it can, at least partially, infiltrate (seep) into a porous ground. There, in at least temporarily cooler subsurface layers, the infiltrated liquid water refreezes and forms ice. The related stress built-up is shown to be sufficient to cause destructive erosive processes. The above-mentioned processes may cause change in the structure and thickness of the covering ice and/or may cause the movement of dune grains. All these processes may explain the observed springtime growing and downward extension of the slope streaks analyzed here. Author Affiliation: (a) Collegium Budapest, Institute for Advanced Study, H-1041 Budapest, Szentharomsag 2, Hungary (b) DLR Institut fur Planetenforschung, D-12489 Berlin, Rutherfordstr. 2, Germany (c) Eotvos Lorand University of Sciences, Institute of Physics, H-1117 Budapest, Pazmany 1/A, Hungary (d) Eotvos Lorand University of Sciences, Institute of Geography and Earth Sciences, H-1117 Budapest, Pazmany 1/c, Hungary (e) Eotvos Lorand University of Sciences, Institute of Physics, Department of Astronomy, H-1117 Budapest, Pazmany 1/c, Hungary (f) Konkoly Observatory, H-1525 Budapest, Pf. 67, Hungary (g) Hungarian Astronomical Association, H-1461 Budapest, Pf. 219, Hungary (h) Karoly Nagy Astronomical Foundation, H-1011 Budapest, Szekely u. 2-4, Hungary Article History: Received 16 September 2008; Revised 27 January 2009; Accepted 27 January 2009

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.11.023 Byline: Sarah E. Dodson-Robinson (a), Karen Willacy (b), Peter Bodenheimer (c), Neal J. Turner (b), Charles A. Beichman (a)(b) Keywords: Solar nebula; Planetary formation; Ices Abstract: To date, there is no core accretion simulation that can successfully account for the formation of Uranus or Neptune within the observed 2-3 Myr lifetimes of protoplanetary disks. Since solid accretion rate is directly proportional to the available planetesimal surface density, one way to speed up planet formation is to take a full accounting of all the planetesimal-forming solids present in the solar nebula. By combining a viscously evolving protostellar disk with a kinetic model of ice formation, which includes not just water but methane, ammonia, CO and 54 minor ices, we calculate the solid surface density of a possible giant planet-forming solar nebula as a function of heliocentric distance and time. Our results can be used to provide the starting planetesimal surface density and evolving solar nebula conditions for core accretion simulations, or to predict the composition of planetesimals as a function of radius. We find three effects that favor giant planet formation by the core accretion mechanism: (1) a decretion flow that brings mass from the inner solar nebula to the giant planet-forming region, (2) the fact that the ammonia and water ice lines should coincide, according to recent lab results from Collings et al. [Collings, M.P., Anderson, M.A., Chen, R., Dever, J.W., Viti, S., Williams, D.A., McCoustra, M.R.S., 2004. Mon. Not. R. Astron. Soc. 354, 1133-1140], and (3) the presence of a substantial amount of methane ice in the trans-saturnian region. Our results show higher solid surface densities than assumed in the core accretion models of Pollack et al. [Pollack, J.B., Hubickyj, O., Bodenheimer, P., Lissauer, J.J., Podolak, M., Greenzweig, Y., 1996. Icarus 124, 62-85] by a factor of 3-4 throughout the trans-saturnian region. We also discuss the location of ice lines and their movement through the solar nebula, and provide new constraints on the possible initial disk configurations from gravitational stability arguments. Author Affiliation: (a) NASA Exoplanet Science Institute, California Institute of Technology, MC 100-22, Pasadena, CA 91125, USA (b) Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA (c) Department of Astronomy and Astrophysics, University of California at Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA Article History: Received 21 June 2008; Revised 24 October 2008; Accepted 28 November 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2008.11.005 Byline: Igor A. Dmitrenko (a), Dorothea Bauch (a), Sergey A. Kirillov (b), Nikolay Koldunov (c)(d), Peter J. Minnett (e), Vladimir V. Ivanov (f), Jens A. Holemann (g), Leonid A. Timokhov (b) Keywords: Atlantic water; Arctic Ocean; Barents Sea water inflow; Ice and atmospheric forcing Abstract: Inflow of Atlantic water (AW) from Fram Strait and the Barents Sea into the Arctic Ocean conditions the intermediate (100-1000m) waters of the Arctic Ocean Eurasian margins. While over the Siberian margin the Fram Strait AW branch (FSBW) has exhibited continuous dramatic warming beginning in 2004, the tendency of the Barents Sea AW branch (BSBW) has remained poorly known. Here we document the contrary cooling tendency of the BSBW through the analysis of observational data collected from the icebreaker Kapitan Dranitsyn over the continental slope of the Eurasian Basin in 2005 and 2006. The CTD data from the R.V. Polarstern cruise in 1995 were used as a reference point for evaluating external atmospheric and sea-ice forcing and oxygen isotope analysis. Our data show that in 2006 the BSBW core was saltier (by [approximately equal to]0.037), cooler (by [approximately equal to]0.41[degrees]C), denser (by [approximately equal to]0.04kg/m.sup.3), deeper (by 150-200m), and relatively better ventilated (by 7-8[mu]mol/kg of dissolved oxygen, or by 1.1-1.7% of saturation) compared with 2005. We hypothesize that the shift of the meridional wind from off-shore to on-shore direction during the BSBW translation through the Barents and northern Kara seas results in longer surface residence time for the BSBW sampled in 2006 compared with samples from 2005. The cooler, more saline, and better-ventilated BSBW sampled in 2006 may result from longer upstream translation through the Barents and northern Kara seas where the BSBW was modified by sea-ice formation and interaction with atmosphere. The data for stable oxygen isotopes from 1995 and 2006 reveals amplified brine modification of the BSBW core sampled downstream in 2006, which supports the assumption of an increased upstream residence time as indicated by wind patterns and dissolved oxygen values. Author Affiliation: (a) Leibniz Institute of Marine Sciences, University of Kiel, Kiel, Wischhofstr. 1-3, Building 4, 24148 Kiel Germany (b) Arctic and Antarctic Research Institute, St. Petersburg, Russian Federation (c) Institute of Oceanography, Centre for Marine and Atmospheric Sciences, University of Hamburg, Hamburg, Germany (d) International Max Planck Research School on Earth System Modelling, Hamburg, Germany (e) Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA (f) International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK, USA (g) Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany Article History: Received 3 March 2008; Revised 3 November 2008; Accepted 10 November 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.11.002 Byline: John Wahr (a), Zane A. Selvans (b), McCall E. Mullen (a), Amy C. Barr (c), Geoffrey C. Collins (d), Michelle M. Selvans (e), Robert T. Pappalardo (f) Keywords: Europa; Satellites; general; Tides; solid body; Tectonics; Geophysics Abstract: The tidal stress at the surface of a satellite is derived from the gravitational potential of the satellite's parent planet, assuming that the satellite is fully differentiated into a silicate core, a global subsurface ocean, and a decoupled, viscoelastic lithospheric shell. We consider two types of time variability for the tidal force acting on the shell: one caused by the satellite's eccentric orbit within the planet's gravitational field (diurnal tides), and one due to nonsynchronous rotation (NSR) of the shell relative to the satellite's core, which is presumed to be tidally locked. In calculating surface stresses, this method allows the Love numbers h and l, describing the satellite's tidal response, to be specified independently; it allows the use of frequency-dependent viscoelastic rheologies (e.g. a Maxwell solid); and its mathematical form is amenable to the inclusion of stresses due to individual tides. The lithosphere can respond to NSR forcing either viscously or elastically depending on the value of the parameter [DELTA]a [mu]/I*I, where [mu] and I* are the shear modulus and viscosity of the shell respectively, and I is the NSR forcing frequency. [DELTA] is proportional to the ratio of the forcing period to the viscous relaxation time. When [DELTA]a'1 the response is nearly fluid; when [DELTA]a*1 it is nearly elastic. In the elastic case, tensile stresses due to NSR on Europa can be as large as [approximately equal to]3.3 MPa, which dominate the [approximately equal to]50kPa stresses predicted to result from Europa's diurnal tides. The faster the viscous relaxation the smaller the NSR stresses, such that diurnal stresses dominate when [DELTA]a[sup.3]100. Given the uncertainty in current estimates of the NSR period and of the viscosity of Europa's ice shell, it is unclear which tide should be dominant. For Europa, tidal stresses are relatively insensitive both to the rheological structure beneath the ice layer and to the thickness of the icy shell. The phase shift between the tidal potential and the resulting stresses increases with [DELTA]. This shift can displace the NSR stresses longitudinally by as much as 45[degrees] in the direction opposite of the satellite's rotation. Author Affiliation: (a) Department of Physics, University of Colorado, UCB 390, Boulder, CO 80309-0390, USA (b) Laboratory for Atmospheric and Space Physics and the NASA Astrobiology Institute, University of Colorado, UCB 392, Boulder, CO 80309-0392, USA (c) Department of Space Studies, Southwest Research Institute, 1050 Walnut St., Suite 300, Boulder, CO 80302, USA (d) Department of Physics and Astronomy, Wheaton College, 26 East Main Street, Norton, MA 02766-2322, USA (e) Division of Geological and Planetary Sciences, California Institute of Technology, MC 252-21, Pasadena, CA 91125, USA (f) Jet Propulsion Laboratory, California Institute of Technology, M/S 183-301, 4800 Oak Grove Dr., Pasadena, CA 91109, USA Article History: Received 4 March 2008; Revised 2 October 2008; Accepted 19 November 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.11.016 Byline: Michael T. Bland (a), Adam P. Showman (a), Gabriel Tobie (b)(c) Keywords: Ganymede; Thermal histories; Interiors; Resonances; orbital Abstract: The tectonically and cryovolcanically resurfaced terrains of Ganymede attest to the satellite's turbulent geologic history. Yet, the ultimate cause of its geologic violence remains unknown. One plausible scenario suggests that the Galilean satellites passed through one or more Laplace-like resonances before evolving into the current Laplace resonance. Passage through such a resonance can excite Ganymede's eccentricity, leading to tidal dissipation within the ice shell. To evaluate the effects of resonance passage on Ganymede's thermal history we model the coupled orbital-thermal evolution of Ganymede both with and without passage through a Laplace-like resonance. In the absence of tidal dissipation, radiogenic heating alone is capable of creating large internal oceans within Ganymede if the ice grain size is 1 mm or greater. For larger grain sizes, oceans will exist into the present epoch. The inclusion of tidal dissipation significantly alters Ganymede's thermal history, and for some parameters (e.g. ice grain size, tidal Q of Jupiter) a thin ice shell (5 to 20 km) can be maintained throughout the period of resonance passage. The pulse of tidal heating that accompanies Laplace-like resonance capture can cause up to 2.5% volumetric expansion of the satellite and contemporaneous formation of near surface partial melt. The presence of a thin ice shell and high satellite orbital eccentricity would generate moderate diurnal tidal stresses in Ganymede's ice shell. Larger stresses result if the ice shell rotates non-synchronously. The combined effects of satellite expansion, its associated tensile stress, rapid formation of near surface partial melt, and tidal stress due to an eccentric orbit may be responsible for creating Ganymede's unique surface features. Author Affiliation: (a) Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA (b) Universite Nantes Atlantique, Laboratoire de Planetologie et Geodynamique de Nantes, 2, rue de la Houssiniere, 44322 Nantes cedex 03, France (c) CNRS, UMR-6112, 2, rue de la Houssiniere, 44322 Nantes cedex 03, France Article History: Received 24 June 2008; Revised 4 November 2008; Accepted 9 November 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.earscirev.2008.12.002 Byline: Charles Harris (a), Lukas U. Arenson (b), Hanne H. Christiansen (c), Bernd Etzelmuller (d), Regula Frauenfelder (d), Stephan Gruber (e), Wilfried Haeberli (e), Christian Hauck (f), Martin Holzle (e), Ole Humlum (d), Ketil Isaksen (g), Andreas Kaab (d), Martina A. Kern-Lutschg (a), Michael Lehning (h), Norikazu Matsuoka (i), Julian B. Murton (j), Jeanette Notzli (e), Marcia Phillips (h), Neil Ross (k), Matti Seppala (l), Sarah M. Springman (m), Daniel Vonder Muhll (n) Keywords: European permafrost; climate change; geothermal monitoring; geothermal modelling; geophysics; slope stability; permafost hazards; permafrost engineering Abstract: We present a review of the changing state of European permafrost within a spatial zone that includes the continuous high latitude arctic permafrost of Svalbard and the discontinuous high altitude mountain permafrost of Iceland, Fennoscandia and the Alps. The paper focuses on methodological developments and data collection over the last decade or so, including research associated with the continent-scale network of instrumented permafrost boreholes established between 1998 and 2001 under the European Union PACE project. Data indicate recent warming trends, with greatest warming at higher latitudes. Equally important are the impacts of shorter-term extreme climatic events, most immediately reflected in changes in active layer thickness. A large number of complex variables, including altitude, topography, insolation and snow distribution, determine permafrost temperatures. The development of regionally calibrated empirical-statistical models, and physically based process-oriented models, is described, and it is shown that, though more complex and data dependent, process-oriented approaches are better suited to estimating transient effects of climate change in complex mountain topography. Mapping and characterisation of permafrost depth and distribution requires integrated multiple geophysical approaches and recent advances are discussed. We report on recent research into ground ice formation, including ice segregation within bedrock and vein ice formation within ice wedge systems. The potential impacts of climate change on rock weathering, permafrost creep, landslides, rock falls, debris flows and slow mass movements are also discussed. Recent engineering responses to the potentially damaging effects of climate warming are outlined, and risk assessment strategies to minimise geological hazards are described. We conclude that forecasting changes in hazard occurrence, magnitude and frequency is likely to depend on process-based modelling, demanding improved understanding of geomorphological process-response systems and their impacts on human activity. Author Affiliation: (a) School of Earth and Ocean Sciences, Cardiff University, CF10 3YE, UK (b) Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2W2 (c) Department of Geology, The University Centre in Svalbard, 9171 Longyearbyen, Norway (d) Department of Geosciences, University of Oslo, Blindern, NO-0316 Oslo, Norway (e) Department of Geography, University of Zurich, CH-8057 Zurich, Switzerland (f) Institute for Meteorology and Climate Reasearch, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany (g) The Norwegian Meteorological Institute, Blindern, 0313 Oslo, Norway (h) WSL,Swiss Federal Institute for Snow and Avalanche Research, SLF Davos, CH-7260 Davos Dorf, Switzerland (i) Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki 305-8572, Japan (j) Department of Geography, University of Sussex, Brighton, BN1 9SJ, UK (k) School of Geosciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK (l) Department of Geography, University of Helsinki, Helsinki, FIN-00014, Finland (m) Geotechnical Institute, ETH Zurich, CH-8093 Zurich, Switzerland (n) The Swiss Initiative in Systems Biology, ETH-Zurich, CH-8092 Zurich, Switzerland Article History: Received 13 July 2007; Accepted 5 December 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.10.016 Byline: M.R. Abernathy (a), S.C. Tegler (a), W.M. Grundy (b), J. Licandro (c), W. Romanishin (d), D. Cornelison (a), F. Vilas (e) Keywords: Kuiper belt; Spectroscopy; Trans-neptunian objects Abstract: We describe optical spectroscopic observations of the icy dwarf planet Eris with the 6.5-m MMT telescope and the Red Channel Spectrograph. We report a correlation, that is at the edge of statistical significance, between blue shift and albedo at maximum absorption for five methane ice bands. We interpret the correlation as an increasing dilution of methane ice with another ice component, probably nitrogen, with increasing depth into the surface. We suggest a mechanism to explain the apparent increase in nitrogen with depth. Specifically, if we are seeing Eris 50 degrees from pole-on [Brown, M.E., Schaller, L., 2008. Science 316, 1585], the pole we are seeing now at aphelion was in winter darkness at perihelion. Near perihelion, sublimation could have built up atmospheric pressure on the sunlit (summer) hemisphere sufficient to drive winds toward the dark (winter) hemisphere, where the winds would condense. Because nitrogen is more volatile and scarcer than methane, it sublimated from the sunlit hemisphere relatively early in the season, so the early summer atmosphere was nitrogen rich, and so was the ice deposited on the winter pole. Later in the season, much of the nitrogen was exhausted from the summer pole, but there was plenty of methane, which continued to sublimate. At this point, the atmosphere was more depleted in nitrogen, as was the ice freezing out on top of the earlier deposited nitrogen rich ice. Our increasing nitrogen abundance with depth apparently contradicts the Licandro et al. [Licandro, J., Grundy, W.M., Pinilla-Alonso, N., Leisy, P., 2006. Astron. Astrophys. 458, L5-L8] result of a decreasing nitrogen abundance with depth. A comparison of observational, data reduction, and analysis techniques between the two works, suggests the difference between the two works is real. If so, we may be witnessing the signature of weather on Eris. The work reported here is intended to trigger further observational effort by the community. Author Affiliation: (a) Department of Physics & Astronomy, Northern Arizona University, Flagstaff, AZ 86011, USA (b) Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA (c) Instituto de Astrofisica de Canarias, via Lactea s/n, E38205, La Laguna, Tenerife, Spain (d) Department of Physics & Astronomy, University of Oklahoma, Norman, OK 73019, USA (e) MMT Observatory, PO Box 210065, University of Arizona, Tucson, AZ 85721, USA Article History: Received 16 August 2008; Revised 27 October 2008; Accepted 29 October 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.10.010 Byline: Maxwell L. Rudolph, Michael Manga Keywords: Europa; Enceladus; Ices; mechanical properties; Satellites; surfaces Abstract: The proposed past eruption of liquid water on Europa and ongoing eruption of water vapor and ice on Enceladus have led to discussion about the feasibility of cracking a planetary ice shell. We use a boundary element method to model crack penetration in an ice shell subjected to tension and hydrostatic compression. We consider the presence of a region at the base of the ice shell in which the far-field extensional stresses vanish due to viscoelastic relaxation, impeding the penetration of fractures towards a subsurface ocean. The maximum extent of fracture penetration can be limited by hydrostatic pressure or by the presence of the unstressed basal layer, depending on its thickness. Our results indicate that Europa's ice shell is likely to be cracked under 1-3 MPa tension only if it is a[c]1/22.5 km thick. Enceladus' ice shell may be completely cracked if it is capable of supporting [approximately equal to]1-3 MPa tension and is less than 25 km thick. Author Affiliation: Department of Earth and Planetary Science, University of California, 307 McCone Hall, Berkeley, CA 94720-4767, USA Article History: Received 30 October 2007; Revised 18 September 2008; Accepted 5 October 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.10.021 Byline: W.M. Grundy Keywords: Kuiper belt; Transneptunian objects; Centaurs; Ices; Spectroscopy Abstract: The extremely red colors of some transneptunian objects and Centaurs are not seen among the Jupiter family comets which supposedly derive from them. Could this mismatch result from sublimation loss of colorless ice? Radiative transfer models show that mixtures of volatile ice and non-volatile organics could be extremely red, but become progressively darker and less red as the ice sublimates away. Author Affiliation: Lowell Observatory, 1400 W. Mars Hill Rd., Flagstaff, AZ 86001, USA Article History: Received 23 July 2008; Revised 30 September 2008; Accepted 27 October 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.09.003 Byline: Konrad J. Kossacki (a), Wojciech J. Markiewicz (b) Keywords: Mars; surface; Water Abstract: In this paper we attempt to answer the question, how formation of a small-scale trench in the martian regolith affects local distribution of the subsurface ice. We are especially interested in the consequences of digging a trench to search for buried ice, as has been done during the Phoenix Mars Lander mission. However, the results may be also applicable for natural troughs, or cracks. We present results of simulations of diurnal exchange of water between the regolith and the atmosphere. Our model includes the heat and vapor migration in the regolith surrounding the trench, as well as formation of diurnal frost. We take into account scattering of light in the atmosphere and on the trench facets, as well as changes of atmospheric humidity on diurnal and seasonal time scales. Our calculations show, that the measurements of ice content in a sample obtained within one, or two days from the beginning of digging should not be affected. However, on somewhat longer time scale at the south facing site of the trench the regolith can be significantly depleted from ice. This effect should be taken into account if the excavation and taking samples from different depths will be performed in stages separated in time by a month, or more. Author Affiliation: (a) Institute of Geophysics of Warsaw University, Pasteura 7, 02-093 Warsaw, Poland (b) Max Planck Institute for Solar System Research, Max-Planck-Street 2, D-37191 Katlenburg-Lindau, Germany Article History: Received 26 February 2008; Revised 3 September 2008; Accepted 9 September 2008

Bullet rosette particles are common in ice clouds, and the bullets may often be hollow. Here the single-scattering properties of randomly oriented hollow bullet rosette ice particles are investigated. A bullet, which is an individual branch of a rosette, is defined as a hexagonal column attached to a hexagonal pyramidal tip. For this study, a hollow structure is included at the end of the columnar part of each bullet branch and the shape of the hollow structure is defined as a hexagonal pyramid. A hollow bullet rosette may have between 2 and 12 branches. An improved geometric optics method is used to solve for the scattering of light in the particle. The primary optical effect of incorporating a hollow end in each of the bullets is to decrease the magnitude of backscattering. In terms of the angular distribution of scattered energy, the hollow bullets increase the scattering phase function values within the forward scattering angle region from 1[degrees] to 20[degrees] but decrease the phase function values at side- and backscattering angles of 60[degrees]- 180[degrees]. As a result, the presence of hollow bullets tends to increase the asymmetry factor. In addition to the scattering phase function, the other elements of the phase matrix are also discussed. The backscattering depolarization ratios for hollow and solid bullet rosettes are found to be very different. This may have an implication for active remote sensing of ice clouds, such as from polarimetric lidar measurements. In a comparison of solid and hollow bullet rosettes, the effect of the differences on the retrieval of both the ice cloud effective particle size and optical thickness is also discussed. It is found that the presence of hollow bullet rosettes acts to decrease the inferred effective particle size and to increase the optical thickness in comparison with the use of solid bullet rosettes.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.11.023 Byline: Adrian J. Brown (a)(b), Shane Byrne (c), Livio L. Tornabene (c), Ted Roush (b) Keywords: Mars; Mars; polar geology; Ices; IR spectroscopy Abstract: We report on observations made of the [approximately equal to]36 km diameter crater, Louth, in the north polar region of Mars (at 70[degrees] N, 103.2[degrees] E). High-resolution imagery from the instruments on the Mars Reconnaissance Orbiter (MRO) spacecraft has been used to map a 15 km diameter water ice deposit in the center of the crater. The water ice mound has surface features that include roughened ice textures and layering similar to that found in the North Polar Layered Deposits. Features we interpret as sastrugi and sand dunes show consistent wind patterns within Louth over recent time. CRISM spectra of the ice mound were modeled to derive quantitative estimates of water ice and contaminant abundance, and associated ice grain size information. These morphologic and spectral results are used to propose a stratigraphy for this deposit and adjoining sand dunes. Our results suggest the edge of the water ice mound is currently in retreat. Author Affiliation: (a) SETI Institute, 515 N. Whisman Rd, Mountain View, CA 94043, USA (b) NASA Ames Research Center, Moffett Field, CA 94035, USA (c) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA Article History: Received 19 April 2007; Revised 19 November 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.01.016 Byline: J.D. Chittenden (a)(b), V. Chevrier (a), L.A. Roe (c), K. Bryson (a), R. Pilgrim (a), D.W.G. Sears (a)(b) Keywords: Mars; Ices; Mars; surface; Mars; atmosphere Abstract: We have studied the effect of wind velocity on the sublimation rate of pure water ice under martian conditions. Measurements were made for wind velocities ranging from 0.7 to 11.4 mas.sup.-1, a typical range observed by the meteorological instruments on the surface of Mars, and at -15[degrees]C a value typical of the daily high temperature for most of the year at the Pathfinder landing site. At this temperature, and for a low-humidity environment (relative humidity around 1%) sublimation rates increase following a linear trend of equation E.sub.S=0.68+0.025V (E.sub.S is the sublimation rate in mmah.sup.-1 and V is the wind speed in mas.sup.-1). In high relative humidity (30-35%) atmospheres, the effect of wind velocity is negligible, and the sublimation rate remains nearly constant at 0.33[+ or -]0.04 mmh.sup.-1. Pure forced convection theory did not provide a satisfying description of the data in terms of the range of values and their wind speed dependency. Therefore, a new semi-empirical expression for the sublimation rate that combines free and forced convection was developed using analogy with heat transfer models. Using this expression, sublimation rates of ice as a function of wind velocity for any temperature can be calculated. In general, temperature is more important that wind speed and atmospheric humidity in determining the rate of sublimation of ice on Mars. Author Affiliation: (a) W.M. Keck Laboratory for Space Simulation, Arkansas Center for Space and Planetary Science, MUSE 202, University of Arkansas, Fayetteville, AR 72701, USA (b) Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA (c) Department of Mechanical Engineering, University of Arkansas, Fayetteville, AR 72701, USA Article History: Received 19 April 2007; Revised 28 January 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.03.009 Byline: Vincent Chevrier (a), Daniel R. Ostrowski (a), Derek W.G. Sears (a)(b) Keywords: Mars; surface; Regolith; Ice; Clays Abstract: We have studied the sublimation of ice and water vapor transport through various thicknesses of clay ( Author Affiliation: (a) W.M. Keck Laboratory for Space Simulation, Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA (b) Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA Article History: Received 20 April 2007; Revised 28 February 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.11.007 Byline: Christophe Herbaut, Marie-NoA'lle Houssais, JerA[acute accent]me Sirven Keywords: Nordic Seas; Northern annular mode; Ice export; Freshwater transport; Atlantic inflow; Numerical modelling Abstract: The variability of the ice and freshwater transports through the main openings of the Nordic Seas is studied based on a 200-year simulation with a sea ice-ocean model forced by stochastic surface wind stress anomalies representative of Northern annular mode (NAM). The spectrum of the ice export through Fram Strait (FS), which constitutes the main contribution to the total freshwater export anomaly from the Arctic, shows no significant peak though half of the variance is concentrated at periods longer than a year. The standard deviation of the freshwater export to the subpolar gyre through Denmark Strait only amounts to 40% of the standard deviation of the total (ice+liquid) freshwater export through FS, with a comparatively larger variance in the low-frequency range, suggesting that the Greenland Sea could act as a low-pass filter. In the upper layer of the Iceland-Scotland Passage, positive phases of the NAM lead to a fast increase of the northward volume and salt transports. Within 2 years, the salt transport anomaly, however, changes sign due to advection of negative salinity anomalies which originate in the subpolar gyre and can be traced up to the Barents Sea. Author Affiliation: LOCEAN/IPSL UPMC, 4, place Jussieu, T45-55, 4eme etage, case 100, 75252 Paris Cedex 05, France Article History: Received 16 July 2004; Revised 18 October 2005; Accepted 29 November 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.05.003 Byline: Irene A. Garibotti (a), Maria Vernet (b), Martha E. Ferrario (c) Keywords: Phytoplankton; Spatial and interannual dynamics; Cryptophytes and diatoms; Western Antarctic Peninsula; Palmer LTER project Abstract: The distribution of phytoplankton composition, cell abundance and biomass from an area along the Western Antarctic Peninsula was studied during three summers, with the aim of understanding its dynamics over spatial and interannual scales. The studied area is characterized by seasonal sea-ice retreat and advance. Algae composition and concentration were found to be highly variable through the area as well as from year to year. Small unidentified phytoflagellates, diatoms and cryptophytes were the main phytoplankton groups, contributing the major proportion of total phytoplankton cell abundance and biomass concentration. Three annually recurrent phytoplankton assemblages were recognized in the area according to the algae composition and abundance: a diatom bloom associated with the sea-ice edge, an assemblage dominated by small unidentified phytoflagellates and cryptophytes, and a diatom-enriched assemblage in open waters. The distribution of these assemblages varied from year-to-year. During the summers preceded by early sea-ice retreat, the diatom bloom was spatially restricted and the other two assemblages occupied extended regions, whereas during the late sea-ice retreat year, the diatom bloom extended over a larger region and the other assemblages occupied smaller regions or were just absent. It was detected that these assemblages resemble different stages of the phytoplankton seasonal cycle, and that their distribution through the area can be related to a latitudinal and longitudinal gradient in the phytoplankton growth onset timing, associated with the progressive sea-ice retreat during spring. The local environmental conditions associated with each assemblage were also analyzed, but further study is needed for understanding the causes of the replacement of one assemblage by another through the area. On the other hand, the interannual variability in the distribution of the assemblages can be related to year-to-year differences in the timing of phytoplankton growth onset, associated with variations in the timing of the sea-ice retreat. Author Affiliation: (a) Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales, CRICyT, CONICET, C.C. 330, 5500 Mendoza, Argentina (b) Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093-0218, USA (c) Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, 1900 La Plata, Argentina Article History: Received 14 May 2004; Revised 8 February 2005; Accepted 16 May 2005

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.04.004 Byline: V.V. Ivanov (a), G.I. Shapiro (b)(c) Keywords: Dense water formation; Shelf edge dynamics; Overflow; Ice transport; Oceanic fronts; Arctic Ocean Abstract: In this paper, we examine the origin of a dense water cascade in the marginal ice zone west off the Novaya Zemlya Archipelago in the Barents Sea. We specifically consider how ice advection and melting contribute to the formation of density gradients favourable for cascading. The study is based on three hydrographic surveys taken in December 1987 by Russian research vessels, composite ice maps and meteorological records. A bulk numerical model is applied in order to quantify the dominant physical processes responsible for re-shaping the thermohaline structure, from a state that opposes cascading to one that enhances it. The overall effect of these processes significantly varies depending on the distance from the shore. The most intensive ice production and rapid density increase occurred in the shallow waters, where the salinity increase accounted for almost 90% of the total density increase. The transitional zone, covering the outer shelf and slope waters, is subject to strong freshening by melting ice, which is transported from the production zone. This effect diminishes in the deep water, where the amount of imported ice is small. The opposing trends in salinity in the production and transitional zones caused by the joint effects of freezing/melting and ice mobility play a crucial role in providing the necessary conditions for the onset of cascading. Without ice advection and melting, the density contrast between the shallow and deeper waters at the shelf break would have been -0.10 (kg/m.sup.3), which would inhibit the formation of dense water cascade. The difference in density fluxes on both sides of the front was 2.1x10.sup.-5 (kg/m.sup.2 s), which is comparable with average flux over Arctic polynyas, and resulted in a cross-frontal density difference of 0.06kg/m.sup.3, which favours dense water cascading. Author Affiliation: (a) International Arctic Research Center, University of Alaska, Fairbanks, AL 99775, USA (b) School of Earth, Ocean and Environmental Sciences, University of Plymouth, UK (c) Shirshov Institute of Oceanology, Moscow, Russia Article History: Received 10 November 2003; Revised 14 February 2005; Accepted 19 April 2005

The frequency of occurrence of the aspect ratio and roundness of particles in ice clouds from aircraft observations have been examined. Images of cloud particles were measured by a cloud particle imager (CPI) at 2.3-[micro]m resolution, installed on the National Research Council (NRC) of Canada Convair-580. Data were collected in winter midlatitude and polar stratiform clouds associated with frontal systems during three field projects in the Canadian and U.S. Arctic and over the Great Lakes. Approximately [10.sup.6] images of particles measured in ice clouds were included in the statistics. The frequency of occurrence of the aspect ratio and roundness were calculated in eight 5[degrees] temperature intervals from -40[degrees] to 0[degrees]C. In each temperature interval, the distributions were calculated for 12 size intervals in the range from 20 [micro]m to 1 mm. It was found that the roundness is a function of particle size and within each size interval it does not depend significantly on temperature. However, the aspect ratio of particles with 60 [micro]m < D < 1000 [micro]m is mainly a function of temperature and does not depend on size. The fraction of spherical particles in ice clouds rapidly decreases with particle size. The fraction of spherical particles in the size range 20 [micro]m < [D.sub.max], < 30 [micro]m on average does not exceed 50%. Ice clouds do not contain significant numbers of spherical particles larger than 60 [micro]m. The information on the habits of small ice particles obtained here gives an insight on the mechanisms of ice formation in clouds. The results can be used for parameterization of particle habits in radiation transfer, weather and climate models, and in remote sensing retrievals. It may also be of interest for calculations of collision efficiency in modeling of interaction of cloud particles with moving platforms related to in-flight icing.

A potential molecular mechanism of ice nucleation on AgI and AgI*AgCl particulates involves rearrangement of ordered water molecule clusters associated with hydrated [Ag.sup.+] ion patches. This nucleation mechanism is thought to occur rapidly at -5[degrees] to -20[degrees]C on substrate particles wetted by water. This hypothesis is based on analysis of the rates of ice crystal formation and ice nucleus activities observed in experiments previously conducted in a 1-[m.sup.3] isothermal cloud chamber using chemical kinetics. These experiments examined the chemistry of wetted AgI colloid particles involving electric charge separations between the particles and adjacent solution. The rate of nucleation is slowed by the presence of alkali and alkaline earth halides in concentrations greater than approximately [10.sup.-3] M in the water wetting the particles. Match of the crystal structure of the substrate with that of the ice may have no effect on this mechanism.

The polarization difference [DELTA][T.sub.b] between the vertical and horizontal components of thermal radiation emitted by clouds was studied using 37- and 85-GHz radiometers. The measurements were conducted during the Alliance Icing Research Project in Ottawa, Canada, during the winter of 1999/2000. Polarization differences ([DELTA][T.sub.b]) greater than 0.1 K were observed in approximately 30% of the cloudy periods. Characteristic values of the polarization difference at 85 GHz were about 2 K with a maximum value of about 4.5 K. Polarization difference at 37 GHz usually did not exceed 2.5 K and was typically 2-6 times less than that at 85 GHz. Both positive and negative polarization differences were observed. It is suggested that the microwave polarization results from scattering of atmospheric thermal radiation by cloud ice particles. The observations were interpreted with a model of radiative transfer in mixed-phase clouds. The characteristic polarization difference observed during groundbased measurements was found to agree with predictions of the radiative transfer model for typical values of cloud liquid and ice water content.

We describe a new continuous extraction system for trapped air from bubble ice or water for on-line determination of the isotopic composition of the main air components nitrogen and oxygen ([[delta].sup.15]N, [[delta].sup.18]O, and [[delta].sup.17]O). Studies of the composition of air from bubbles trapped in polar ice are providing fundamental information about ancient atmospheric composition and, therefore, are an important tool to learn more about Earth's climate. The new system proved to work reliably for standard air admixed and subsequently removed from a water stream. The precision (1 SD) of standard measurements is ~0.04 [per thousand] for [[delta].sup.15]N, ~0.1 [per thousand] for [[delta].sup.18]O, and ~0.15 [per thousand] for [[delta].sup.17]O. Ice measurements with the new on-line system are promising. Continuous measurements of nitrogen as well as oxygen isotope ratios can be performed with a spatial resolution of ~3 cm and nearly the same precision as for the standards. However, the measured [delta] values of ice are generally lower, as compared to ice measured with conventional techniques, as a result of a time-dependent dissolution process of air in water associated with kinetic fractionation, which affects standard and sample differently. By modeling the dynamics of the this dissolution process, we found a reason for the lack of accuracy and propose an improvement of the system that will lead to a better accuracy of the ice measurements.

The stability of floating ice shelves is an important indicator of ocean circulation and ice-shelf mass balance. A sub-ice-shelf sediment core collected during the Austral summer of 2000-2001 from site AM02 (69[degrees]42.8'S, 72[degrees]38.4'E) on the Amery Ice Shelf, East Antarctica, contains a full and continuous record of glacial retreat. The AM02 core site is ~80 km south of the floating ice shelf edge and contains a 0.5-m-thick Holocene surface layer of siliceous mud and diatom ooze of marine origin. Core data are supportive of sub-ice-shelf circulation models that predict the landward flow of oceanic water, and prove that the landward transport of hemipelagic sediments occurs beneath floating ice shelves over distances of at least ~80 km. An increase in sea-ice-associated diatom deposition in the upper part of the Holocene suggests that a major retreat of the Amery Ice Shelf to at least 80 km landward of its present location may have occurred during the mid-Holocene climatic optimum. Keywords: Antarctica, ice shelves, Holocene, diatoms, sedimentology.