As the universe further cooled and expanded, fluctuations in density initiated the condensation of galaxies and the formation of first-generation stars. Such density variations appear to have been present from the earliest stages of formation and were first detected as ripples in cosmic microwave background radiation by the Cosmic Background Explorer (COBE) spacecraft, launched in 1989. The early, massive first-
generation stars had very short lifetimes and ended in Type II supernovae, spreading their fused and remaining unfused molecules into the material occupying the gaps between the stars known as the interstellar medium (ISM). Hence, the composition of this medium changed over time, with the D/H ratio slowly reducing as deuterium was "burnt" in stars and the He/H ratio increasing for the same reason. In addition, the abundance of heavy elements (where the term "heavy" denotes atoms heavier than He) also increased as more and more hydrogen and then helium was fused in stars. Subsequent stars typically had lower mass and longer lives and shed their atmospheres more gently into the ISM when they died. The 15N/14N ratio of the material in the ISM is a good indicator of how much the material has evolved since 15N comes mainly from early primary production in stars ending in Type II supernovae, while 14N comes mainly from second-generation lower mass, longer lived stars. Hence, we expect the abundance of 14N to build up over time and thus that the 15N/14N ratio slowly reduces.
Hence, all the heavy atoms of our world, and indeed our bodies, such as carbon, oxygen, nitrogen, etc., were all produced in the cores of ancient stars that have long since perished and given up fractions of their atmospheres to the ISM. A review of the chemistry taking place in the ISM and particularly molecular clouds is given by Fraser et al. (2002). The estimated abundance, relative to hydrogen, of different elements in the solar system has been the subject of many studies over the years such as Cameron (1982) and the widely used abundance tables of Anders and Grevesse (1989). These abundances have been significantly updated in light of new 3D hydro-dynamical models of the solar photosphere, coupled with improved solar line data, non-local thermodynamic equilibrium modeling and observations (e.g., Grevesse and Sauval, 1998; Grevesse et al., 2007). Most recently, Grevesse et al. (2007) have revised their estimates of solar metallicity to significantly lower values. As an example, the solar C/H ratio of Anders and Grevesse (1989), referenced by numerous authors discussing the composition of the outer planets, has been revised to 68% of its previous value by Grevesse et al. (2007). Hence, when the abundances of elements in the giant planet atmospheres are referred to in various papers in terms of their ratio to solar abundance, the source of the solar data used needs to be clarified to avoid any confusion. In this book, we shall assume the solar abundances of Grevesse et al. (2007). It should be noted, however, that the current observed solar photospheric abundances are thought to be less than the original proto-solar abundances by up to 12% due to effects such as gravitational settling (Turcotte and Wimmer-Schwein-gruber, 2002; Turcotte et al., 1998). However, we shall refer here to the Grevesse et al. (2007) observed solar photospheric values.
The molecular form of the elements in the current ISM may be inferred from spectroscopic measurements and modeling. Oxygen is thought to be mostly found within molecules of water ice and carbon, mainly within molecules of CO and some CH4. Nitrogen is assumed to be mainly in the form of N2, although this molecule cannot be spectroscopically detected (Chapter 6), and sulfur is thought to exist mainly within H2S molecules. Ion-molecule reactions forming water molecules directly from the atoms in the ISM would have enriched the deuterium abundance, such that the D/H ratio in water molecules of the pre-solar cloud was increased to something like 7.3 x 10~4. All recent comets coming from the Oort Cloud (see Section 2.4.1) have been observed to have a D/H ratio of 3 x 10~4 in their water molecules, which has implications for how they formed as we shall see later in Section 2.6.1. For reference the D/H ratio in the Earth's oceans is -1.5 x 10~4. This is greater than would be expected if all the Earth's water came from the local solar nebula at the time of formation, but less than if it all came from comets as has sometimes been suggested. A combination of the two sources thus seems most likely.
2.3 FORMATION OF THE PROTO-SOLAR NEBULA 2.3.1 Collapse of the interstellar cloud
Our solar system formed at the edge of our galaxy in one of its spiral arms about 4.6 billion years ago (as determined by radioisotope dating analysis). During this time the solar system has completed approximately 20 orbits about the galactic center. Since the formation of the galaxy, the composition of the local ISM had been evolving and at the time the solar system formed was composed of approximately 71% by mass of hydrogen, 27% helium, and 2% heavy elements (i.e., elements with molecular weight greater than helium). About 1% of the heavy elements are thought to have existed in a condensed "dust" phase.
The density and temperature of the ISM varies considerably with position. The denser parts have a temperature of the order of 10 K, and a density of 10~14 kg m , from which the pressure can be calculated to be 3.5 x 10~10 Pa. The pressure in these "dense molecular clouds'' is thus considerably less than the best modern laboratory vacuum! These dense clouds typically have a size of a few light-years across and thus contain enough mass to form many hundreds of stars. Figure 2.1 shows a typical dense molecular cloud, Barnard 68, observed in 1999 by the European Southern Observatory Very Large Telescope (VLT). At visible wavelengths, the opacity of dust in these clouds is sufficient to obscure the light of stars behind the cloud. However, since the dust grains are small, the opacity decreases rapidly with wavelength (as we shall see in Section 6.4) such that at near-infrared wavelengths the cloud becomes almost transparent (as can be seen in Figure 2.1).
Jeans' theory of collapse
Under certain circumstances these dense molecular clouds may become unstable to gravitational collapse, leading to the formation of stars. The conditions for gravitational collapse of dense molecular clouds were first considered by Sir James Jeans in 1917. Ignoring all other forces, an isothermal cloud of mass M and temperature T will undergo gravitational collapse if its gravitational potential energy is greater than its internal thermal energy. The thermal energy of a cloud is given roughly by
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