Temperature planet accretion

The equilibrium-condensation model assumes that solids thermally equilibrated with the surrounding nebular gas, and any uncondensed elements were somehow flushed from the system. Planets accreted from these solids would then have compositions dictated by condensation theory. Because temperature and pressure decreased away from the Sun, the condensed solids would have varied with heliocentric distance. Figure 14.7 shows planets... 

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. 

The earth was formed by a process of accretion about 4.6 billion years ago. Initially it was a molten mass lacking the gravitational pull to retain its gases at the prevalent elevated temperatures. And yet, within a mere 700 million years of the planet s birth, as calculated from the isotopic record of sediments, cellular life almost certainly existed. What raw materials were available to bring about this amazing turn of events What were the sources of energy used to drive the necessary reactions Where did the important reactions take place Was it in the atmosphere, in the oceans, on dry land, or all three ... 

For theories of planet formation and disk chemistry, the mid-plane temperature, Tm, is more important than the surface temperature. Also, to solve Eq. (3.4) or Eq. (3.6) we need to know v, which depends on Tm, not on Te. For irradiation-dominated disks Tm is roughly equal to the surface effective temperature Te. For disks in which accretional heating is important this is not the case. A rough estimate for the mid-plane temperature valid for any case is T4 = (3/8)T4viscrR + r4irr, where T,visc and 7). T are given above and tr = E/cr with /cr the Rosseland mean opacity of the gas-dust mixture at the temperature Tm. The expression for Tm is therefore not explicit, as it is also used for kr. For realistic dust opacities it requires a numerical iterative solution procedure. For a gray opacity, or for piecewise power-law opacities, such as those in Bell et al. (1997), it can be solved analytically. 

Detected rotational H2 and ro-vibrational CO, CO2, C2H2, HCN, as well as H2O and OH lines trace hot gas in the inner, planet-forming disk zone with T > 300 K (Brittain et al. 2003 Lahuis et al. 2006 Salyk et al. 2008), see Fig. 4.3. These lines are a good measure of temperature and high-energy radiation fields, and presumably sensitive to disk accretion, which could be a stress-test for advanced chemo-dynamical models. In Table 4.1 the various molecules used to study protoplanetary disks are overviewed. 

It was at one time thought that even the terrestrial planets themselves formed directly by condensation from a hot solar nebula. This led to a class of models called heterogeneous accretion models, in which the composition of the material accreting to form the Earth changed with time as the nebula cooled. Eucken (1944) proposed such a heterogeneous accretion model in which early condensed metal formed a core to the Earth around which silicate accreted after condensation at lower temperatures. In this context the silicate-depleted, iron-enriched nature of Mercury makes sense as a body that accreted in an area of the solar nebula that was kept too hot to condense the same proportion of silicate as is found in the Earth (Lewis, 1972 Grossman and Larimer, 1974). Conversely, the lower density of Mars could partly reflect collection of an excess of silicate in cooler reaches of the inner solar nebula. So the... 

Heterogeneous accretion models for the formation of the Earth advocate the initial accretion of refractory, less-oxidized components that make up the bulk of the planet (some 50-80%), followed by the accretion of a lower-temperature, more oxidized component (e.g., perhaps comparable to carbonaceous chondrites). The overall nature of the initially refractory material is not well characterized, but it could have affinities to ordinary or enstatite chondrites. These two-component mixing models seek to reconcile the observational constraints from chemical and isotopic studies of the silicate Earth. As of early 2000s, we do not have sufficient data to identify in detail the nature of these two components of accretion if they existed. 

The origins of volatile species on the terrestrial planets have been modeled as resulting from accretion, in variable planet-specific proportions, of rocky materials as well as three types of comets. These formed at different heliocentric distances and thus at different nebular temperatures, leading to distinctive elemental fractionation patterns in volatiles trapped in their ice from ambient nebular gases (e.g., Owen et al., 1991, 1992 Owen and... 

Gravitational attraction combined with rotation gradually formed the expanding cloud of material into relatively flat spiral galaxies containing millions of stars each. Complex interactions within the stars led to black holes and other types of stars, some of which exploded as supernovas and scattered their material widely. Further gradual accretion of some of this material into planets followed. At the lower temperatures found in planets, the buildup of heavy elements stopped, and decay of unstable radioactive isotopes of the elements became the predominant nuclear reactions. 

The planets of our solar system probably formed from a disc-shaped cloud of hot gases, the remnants of a stellar supernova. Condensing vapours formed solids that coalesced into small bodies (planetesimals), and accretion of these built the dense inner planets (Mercury to Mars). The larger outer planets, being more distant from the sun, are composed of lower-density gases, which condensed at much cooler temperatures. 

---