Planet formation molecular clouds

Figure 6.2 Formation of the solar system (a) unstable molecular cloud possessing some angular momentum (b) angular moment conservation produces the disc shape under collapse (c) matter accretion forms the planets (d) the mature system of planets seen today evolves after 4 Myr...

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

Stars form in dense cores within giant molecular clouds (see Fig. 1.4, Alves et al. 2001). About 1 % of their mass is in dust grains, produced in the final phases of stellar evolution. Molecular clouds are complex entities with extreme density variations, whose nature and scales are defined by turbulence. These transient environments provide dynamic reservoirs that thoroughly mix dust grains of diverse origins and composition before the violent star-formation process passes them on to young stars and planets. Remnants of this primitive dust from the Solar System formation exist as presolar grains in primitive chondritic meteorites and IDPs. 

Planetary systems are now generally believed to be by-products of the process of star formation. Star formation, therefore, is the natural starting point for discussions of planet formation. Almost all stars are born as members of stellar clusters that, in turn, are bom in molecular clouds. Formation of isolated stars seems to be possible according to observations, but this is a rare process. Whether the Sun and its associated planetary system formed in isolation or as member of a cluster is not known some indications hint to formation in a cluster (see Hester Desch 2005 Gounelle Meibom 2008 and Chapter 9, this volume). 

The chemical dynamics, reactivity, and stability of carbon-centered radicals play an important role in understanding the formation of polycyclic aromatic hydrocarbons (PAHs), their hydrogen-dehcient precursor molecules, and carbonaceous nanostructures from the bottom up in extreme environments. These range from high-temperature combustion flames (up to a few 1000 K) and chemical vapor deposition of diamonds to more exotic, extraterrestrial settings such as low-temperature (30-200 K), hydrocarbon-rich atmospheres of planets and their moons such as Jupiter, Saturn, Uranus, Neptune, Pluto, and Titan, as well as cold molecular clouds holding temperatures as low as 10... 

Most meteorites are depleted in moderately volatile and highly volatile elements (see Figures 2-4). The terrestrial planets Earth, Moon, Mars, and the asteroid Vesta show similar or even stronger depletions (e.g., Palme et aL, 1988 Palme, 2001). The depletion patterns in meteorites and in the inner planets are qualitatively similar to those in the ISM. It is thus possible that the material in the inner solar system inherited the depletions from the ISM by the preferential accretion of dust grains and the loss of gas during the collapse of the molecular cloud that led to the formation of the solar system. There is, however, little support for this hypothesis ... 

The chemistry of dense, dark molecular clouds prior to planet formation is the topic of this paper. Dr. Ziurys has discussed the inventory and measurement of gas phase interstellar molecules associated with dense molecular clouds in the chapter, "Identifying Molecules in Space" 8). Here, the focus is on the chemistry in and on the ices and the interaction of these ices with species in the gas. Since these ices represent the largest repository of interstellar molecules in dense clouds, they tie up a large fraction of the chemical inventory in molecular... 

Although the basic chemical and material building blocks for the planets and their satellites were fairly uniform during the initial formation of the solar nebula from inter-stellar cloud materials, chemical differentiation, and segregation occurred over time during accretion of the planets, and their moons such that the volatile chemical components of the solar nebula ended up as present day near-surface ice on Earth, and ice plus solid CO2 on Mars, and as ice and other molecular solids and fluids (such as hydrocarbons and ammonia) on most of the moons of Jupiter and Saturn, and as water ices and increasingly volatile species such as nitrogen in the outermost solar system. 

---