Evolution of the giant planets

As already mentioned, an alternative theory postulates a gravitational collapse of accretion disk material without first forming planetary cores (Cameron, 1978). In this case one would expect all of the outer planets to contain elements in nearly the same proportions, very similar to that of the Sun. Small increases of heavy elements over solar abundance ratios are possible from a post-accumulation infall of planetesimals, but the large enrichment of some of the heavy elements, as indicated in Table 9.3.1, caimot be explained. In contrast, the theory suggesting core formation to have occurred before the bulk of the low-Z gases arrived readily explains such an enrichment. 

Remote sensing measurements from spacecraft, astronomical observations from Earth, and even measurements from future entry probes determine the composition only of the outermost atmospheric layers down to pressures of at best 10 or 20 bars, in most cases to much lower pressure levels. The question arises how representative are these abundance measurements of the composition of the atmospheres as a whole On Uranus and Neptune pressures at the lower boundary of the atmospheres are estimated to be several hundred thousand and on Jupiter and Saturn several million bars large extrapolations are therefore necessary. 

The second most abundant constituent of the atmospheres of the giant planets, helium, does not react chemically and carmot condense even at the cold tropopauses of Uranus and Neptime. For a long time helium was expected to be uniformly mixed in all giant planet atmospheres. Smoluchowski (1967), Salpeter (1973), Hubbard  

Acting against these homogenizing forces are chemical reactions studied extensively by Lewis (1973), Lewis Prinn (1984), and Prinn Fegley (1989). Of course, formation of clouds by condensation also depletes the colder atmosphere above the clouds of the cloud forming substance. Another constituent may then dissolve into the clouds, removing that compound from the gaseous phase as well. The low N/H ratios measured on Uranus and Neptune may be caused by such a process. 

On Uranus and Neptune temperatures in the upper troposphere are low enough for the formation of CH4 clouds. To obtain the CH4 concentration representative of the atmosphere as a whole the CH4 abundance below the CH4 cloud deck must be determined. Fortunately, this was possible for Uranus from data obtained by the Voyager Radio Science Investigation (Lindal et al, 1987) and from ground-based microwave measurements (Lutz et al., 1976). Scattering within the clouds complicates the interpretation of ground-based near infrared measurements. 

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Pollack, J. B. Bodenheimer, P. (1989). Theories of the origin and evolution of the giant planets. In Origin and Evolution of Planetary and Satellite Atmospheres, ed. 

The recent advances in modem technology continue to open new opportunities for the observation of chemical reactions on shorter and shorter time scales, at higher and higher quantum numbers, in larger and larger molecules, as well as in complex media, in particular, of biological relevance. As an example of open questions, the most rapid reactions of atmospheric molecules like carbon dioxide, ozone, and water, which occur on a time scale of just a few femtoseconds, still remain to be explored. Another example is the photochemistry of the atmospheres of nearby planets like Mars and Venus or of the giant planets and their satellites, which can help us to understand better the climatic evolution of our own planet. 

The giant planets, especially Jupiter and Saturn, significantly influenced accretion in the inner Solar System, with important consequences for the properties of the terrestrial planets, described in Section 10.4.1. The influence of the giant planets is especially strong in the Asteroid Belt. Given that meteorites are our primary samples of primitive Solar System material, understanding the role of dynamical and collisional processes in the formation and evolution of the Asteroid Belt is of fundamental importance for theories of planet formation (Section 10.4.2). 

The Thermal Infrared Emission of the Giant Planets and Implications for Evolution... 

Models of planetary evolution assume that at the time of planetary formation the solar system had a single universal and well-mixed composition from which aU parts of the solar system were derived (see Podosek, 1978). Information as to the elemental and isotopic characteristics of this primordial composition is presently available from the Sun, meteorites, and the atmospheres of the giant planets (Wider, 2002). In the case of the Sun, distinction is usually made between the present-day composition, which is available via spectral analysis of the solar atmosphere and capture of the solar wind, either directly in space or by using metallic foU targets, and the proto-Sun (the composition at the time of planetary accretion) whereby the lunar regolith and/or meteorites are utilized as archives of ancient solar wind. As discussed below, the distinction is only really important for helium due to production of He by deuterium burning. 

The satellites of the giant planets are of particular interest for the search of water and therefore also for astrobiology in general. A review about the evolution of the icy satellites of the giant planets has been given by Schubert et al., 2010 [298],... 

Water is one of the basic elements for life. It is even assumed that the evolution of life is only possible if there is liquid water present. A water molecule has some remarkable properties that make it quite unique in the universe. In the first chapter of this book we will review these basic properties of water and the role of water on Earth. All ancient civilizations realized the importance of water and their cities were constructed near great reservoirs of water. But is water unique on Earth Do we find water elsewhere in the solar system, on extrasolar planetary systems or in distant galaxies We will start the search for the presence of extraterrestrial water in our solar system. Surprisingly enough it seems that water in some form and sometimes in only minute quantities is found on any object in the solar system. Even on the planet nearest to the Sun, Mercury, there may be some water in the form of ice near its poles where never the light of Sun heats the surface. And there are objects in the solar system that are made up of a large quantity of water in terms of their mass, like comets and several satellites of the giant planets. 

Abstract Water ice consists about a half of mass and therefore about 0.75 of volume of most of the icy satellites. Differentiated, with water ice forming outer shells, and undifferentiated models of internal structure of the icy satellites of the giant planets are mentioned. It is stressed that the modelling of the evolution of satellites structure should be supported by laboratory experiments (i) concerning rheology of compaction of icy/mineral granular porous media, and (ii) concerning kinetics of phase transitions of water ice in these media. 

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. 

What makes a planet habitable In this section we will define habitability and see that there exist several zones where a planet or satellite of a giant planet may be located to provide habitable conditions for evolution of life (life in the sense we know from Earth). 

Cosmochemislry places important constraints on models for the origin of the solar nebula and the formation and evolution of planets. We explore nebula constraints by defining the thermal conditions under which meteorite components formed and examine the isotopic evidence for interaction of the nebula with the ISM and a nearby supernova. We consider how planetary bulk compositions are estimated and how they are used to understand the formation of the terrestrial and giant planets from nebular materials. We review the differentiation of planets, focusing especially on the Earth. We also consider how orbital and collisional evolution has redistributed materials formed in different thermal and compositional regimes within the solar system. 

Planet formation followed the planetesimal and protoplanet formation in the final stage of the disk evolution (Chapter 10). Gas giants, Jupiter and Saturn, captured disk gas due to their large gravities, and other planets, including Earth, may also have some evidence of disk-gas capture. In the second part of this section, we will seek constraints on the timing of dust and gas dispersal in the proto-solar disk from planets (Section 9.3.2). 

Figure 9.5 Summary of the timescales for the formation of chondrules, asteroids, and planets in the Solar System compared to the lifetime of disks around young stars. The Solar System chronology is based on the dating of the CAIs, which, we assume, formed within the first Myr of disk evolution. The inner-disk frequency is from infrared excess measurements of stars in different stellar groups (see Section 9.1.1). The timescale for the outer-disk dispersal is discussed in Sections 9.1.1 and 9.1.2. The Solar System chronology is summarized in Section 9.3. For the formation timescales of giant planets, we used those in Desch (2007) with the assumption that outer-disk planetesimals formed 2 Myr after CAIs.

Studies of the gas content of protoplanetary disks with ages between 1 and 30 Myr are necessary to determine how rapidly the gas disperses and make a more direct comparison to the evolution and dispersal of dust in disks. As we discussed in Section 9.1.2, the dispersal of gaseous disks also provides an upper limit for the formation time of giant planets that can be compared to the time necessary to form Jupiter and Saturn in our Solar System. From a Solar System perspective it is interesting to expand on the constraints placed on the gas dispersal from the age determination of meteorites with implantation of solar wind, which provide us a... 

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