Icy planets

Machida, S. Hirai, H. Kawamura, T. Yamamoto, Y. Yagi, T. (2006). A new high-pressure structure of methane hydrate surviving to 86 GPa and its implications for the interiors of giant icy planets. Physics of the Earth and Planetary Interiors, 155 (1-2), 170-176. 

Additional reading on Earth s interior and dynamics may be found in references and and on icy planets and moons in the references and and on clathrate... 

In order to be successful, it is necessary that the standard model of planetary formation should explain the main features of the solar system, in particular the division of the planets into three main groups - the terrestrial planets, the giant planets and the outer icy planets (Table 2.2). In detail the standard model should also explain the following properties of the solar system (Lissauer, 1993) ... 

The terrestrial planets The giant planets The outer icy planets ... 

Currie, T, Plavchan, R, Kenyon, S.J. A Spitzer study of debris disks in the young nearby cluster NGC 2232 icy planets are common around 1.5-3 MsoIm stars. Astrophys. J. 688,... 

Effects of condensation are also seen in the bulk compositions of the planets and their satellites. The outer planets, Uranus and Neptune, have overall densities consistent with their formation from icy and stony solids. The satellites of Uranus have typical densities of 1.3g/cm which would tend to indicate a large ice com-... 

On the one side, the traditional core accretion scenario (e.g. [1]) tells us that giant planets are formed as the result of the runaway accretion of gas around a previously formed icy core with about 10-20 times the mass of the Earth. Opposite to this idea, some authors have proposed that giant planets may form by a disk instability process [4]. 

What scientists cannot determine is what caused carbon dioxide levels to drop in the first place during these icy periods in Earth s history. Some suggest perhaps photosynthesizing plants became too successful and took in too much carbon dioxide, causing less heat to be trapped near the planet s surface and consequently cooling the globe. Thus, while it is known cold climates coincide with low levels of atmospheric carbon dioxide, it is not known why. 

Ice mantles are important constituents of interstellar grains in molecular clouds, and icy bodies dominate the outer reaches of the solar system. The region of the solar system where ices were stable increased with time as the solar system formed, as accretion rates of materials to the disk waned and the disk cooled. The giant planets and their satellites formed, in part, from these ices, and probably also from the nebular gas itself. 

In this chapter we will consider the cosmochemistry of ice-bearing planetesimals. We will focus first on comets, because more is known about their chemistry than of the compositions of objects still in the Kuiper belt and Oort cloud. We will then explore asteroids whose ices melted long ago, and we will briefly consider some larger icy bodies, now represented by satellites of the giant planets. The importance of ice-bearing planetesimals to cosmochemistry stems from their primitive compositions, which have remained largely unchanged because of hibernation in a frozen state. 

Gravitational stirring of icy planetesimals by the giant planets could have sent many comets careening into the inner solar system, providing a mechanism for late addition of water to the terrestrial planets. Comets impacting the Earth and the other terrestrial planets would have delivered water as ice (Owen and Bar-Nun, 1995 Delsemme, 1999), whereas the accretion of already altered carbonaceous chondrite asteroids would have delivered water in the form of hydroxl-bearing minerals (Morbidelli el al., 2000 Dauphas et al., 2000). 

Among places where condensates accreted into significant solid bodies, such as planets, habitable realms have always been rarer than places that were either too cold or too hot for life to exist. Much of our Solar System s mass is still far too hot for life. Most of the deep interiors of the gas giants and rocky planets are too hot, as is, of course, the Sun itself. Most of the surface area of solid bodies in the Solar System are too cold - the icy satellites of the outer planets and the myriad comets and Kuiper Belt Objects on the far outer fringes of the Solar System. In this sense, places like the surfaces of Earth and Mars and Europa s subsurface ocean are indeed very rare places. 

Greenberg R, Geissler P (2002) Europa s dynamic icy crust. Meteor Planet Sci 37 1685-1710... 

Kargel JS, Consolmagno GJ (1996) Magnetic fields and the detectability of brine oceans in Jupiter s icy satellites. Lunar Planet Sci 27 643-644... 

Abstract Planet formation is a very complex process through which initially submicron-sized dust grains evolve into rocky, icy, and giant planets. The physical growth is accompanied by chemical, isotopic, and thermal evolution of the disk material, processes important to understanding how the initial conditions determine the properties of the forming planetary systems. Here we review the principal stages of planet formation and briefly introduce key concepts and evidence types available to constrain these. 

Sample return missions for satellites, which formed in subsystems of gaseous and icy giants and experienced no significant geological processes, and subsequent chronological studies of returned samples are the best and most direct way to estimate the timing of giant-planet formation. However, such sample return missions... 

Kuiper Belt a region in the outer Solar System beyond Neptune s orbit populated by small icy planetesimals or Kuiper Belt objects, and dwarf planets. Many short-period comets (possessing orbits of less than 200 years) are thrown into the Solar System from the Kuiper Belt. 

The protoplanetary nebula initially had a mass of at least O.OIM . This minimum mass is obtained by estimating the total amount of rocky and icy material in all the planets, and adding hydrogen and helium to give a nebula of solar composition (Weidenschilling, 1977a). However, planet formation is probably an inefficient process, suggesting that the protoplanetary nebula was initially more massive than this. 

Figure 1 Some known giant planets and brown dwarfs, illustrated with a limited azimuthal slice (pie slice) to correct scale. The interiors are color coded according to the principal materials in each zone. Ice and rock refer to elements common in materials that are icy or rocky at normal pressures. Metallic hydrogen indicates ionization primarily through pressure effects. Modeled central temperature in K, and pressure in 10 bar, is shown. The radii of all but G1229b are known directly for G1229b, modeling of the brightness versus wavelength must be used to derive the radius. From left to right, the masses (expressed relative to the mass of Jupiter) are 45,1, 0.7, 0.3, and 0.05...

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