Ice giants

In the Nordic creation myth, which can be found at the beginning of the Edda, we encounter Ginnungagap, a timeless, yawning void. It contains a type of supreme god, Fimbultyr, who willed the formation of Niflheim in the north, a cold, inhospitable land of fog, ice and darkness, and in the south Muspelheim (with light and fire). Sparks from Muspelheim flew onto the ice of Niflheim. This caused life to emerge, and the ice giant Ymir and the huge cow Audhumbla were formed. 

Boss A. P., Wetherill G. W., and Haghighipour N. (2002) Rapid formation of ice giant planets. Icarus 156, 291—295. Briceno C., Vivas A. K., Calvet N., Hartmann L., Pacheco R., Herrera D., Romero L., Berlind P., and Sanchez G. (2001) The CIDA-QUEST large-scale survey of Orion OBI evidence for rapid disk dissipation in a dispersed stellar population. Science 291, 93-96. 

Quahtatively speaking, aU accretion involves several stages, although the relative importance must dilfer between planets and some mechanisms are only likely to work under certain conditions that currently are underconstrained. Although the exact mechanisms of accretion of the gas and ice giant planets are poorly understood (Boss, 2002), all such objects need to accrete very rapidly in order to trap large volumes of gas before dissipation of the solar nebula. 

The ice giants Ganymede and Callisto are in many ways at the other end of the evolutionary spectmm from rocky, youthful, volcanic lo, having bulk compositions with approximately equal proportions of ice and rock, and ancient, cratered surfaces. Because of these similarities in some global properties, Ganymede and Callisto are usually considered together. The major... 

Comet-like materials are presumed to be the budding blocks of Uranus and Neptune (the ice giants) they may have played a role in the formation of Jupiter and Saturn (the gas giants) and they also played some role in transporting outer solar system volatile materials to inner planets (Delsemme, 2000). The inner solar system flux of comets may have been much higher in the past and comets may have played a role in producing the late heavy bombardment on terrestrial planets (Levison et al., 2001). Comets also exist outside the solar system and there is good evidence that they orbit a major fraction of... 

Figure 18 D/H ratios of several comets compared to the oceans (SMOW), planets, the solar nehula (PSN), and the interstellar medium. Low-temperature fractionation processes increase D/H. Jupiter and Saturn have compositions close to the original nehular composition, hut low-temperature formation of ice caused the enhancements seen in Uranus and Neptune (the ice giants) and comets. The discrepancy between the plotted LP comets and SMOW argues against these comets providing Earth with a major fraction of its water. Other comets, formed in warmer environments, near Jupiter, could he more similar to SMOW (source Huehner, 2002).

Mantle 13.4 Earth masses consists of ice, liquid water, ammonia and other substances with a high electrical conductivity. As is the case for Neptune, ice dominates the structure of that planet. Therefore, they are called ice giants. 

A water pump purchased from Little Giant Pump Company, Oklahoma City, Oklahoma, was used by the submitters to circulate ice-water through the condenser. The checkers used a dry ice condenser. 

Giant molecular clouds the GMCs have a lifetime of order 106—10s years and are the regions of new star formation. The Orion nebula (Orion molecular cloud, OMC) is some 50 ly in diameter and 1500 ly from Earth. The temperature within the cloud is of order 10 K and the atomic density is 106 cm-3. The chemical composition is diverse and contains small diatomic molecules, large polyatomic molecules and dust particles covered with a thick ice mantle. 

The density estimates in Table 7.1 show a distinction between the structures of the planets, with Mercury, Venus, Earth and Mars all having mean densities consistent with a rocky internal structure. The Earth-like nature of their composition, orbital periods and distance from the Sun enable these to be classified as the terrestrial planets. Jupiter, Saturn and Uranus have very low densities and are simple gas giants, perhaps with a very small rocky core. Neptune and Pluto clearly contain more dense materials, perhaps a mixture of gas, rock and ice. 

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. 

Pluto and some moons of the giant planets contain considerable amounts of ices and deserve special mention. In some cases they even exhibit active or recent processes that liberate liquids and gases derived from ices, suggesting a tentative link with cometary activity. These bodies, which are too large to be called planetesimals, include former KBOs now relocated into the planetary region, as well as objects that probably accreted in the giant planet region. 

The giant planets are composed mostly of hydrogen and helium. Uncompressed mean densities provide constraints on the proportion of rock to ice or gas, although the enormous internal pressures in some of these planets produce phase changes in hydrogen that complicate this determination (discussed below). 

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). 

Models of the interiors of the giant planets depend on assumed temperature-pressure-density relationships that are not very well constrained. Models for Jupiter and Saturn feature concentric layers (from the outside inward) of molecular hydrogen, metallic hydrogen, and ice, perhaps with small cores of rock (rocky cores are permissible but not required by current data). Uranus and Neptune models are similar, except that there is no metallic hydrogen, the interior layers of ice are thicker, and the rocky cores are relatively larger. 

A Little Giant" submersible pump (available from Little Giant Pump Co., Oklahoma City, OK) is used to circulate ice water through the immersion well. 

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