Planet accretion

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

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... 

The highly noncircular orbits of embryos and the long accretion timescales allowed considerable radial mixing of material over distances of 0.5-1.0 AU (Wetherill, 1994). It is likely that each of the inner planets accreted material from throughout the inner solar system, although the degree of radial mixing depends sensitively on the mass distribution of the embryos at this time (Chambers, 2001). The relative contributions from each part of the disk would have been different for... 

Not only is there a shortage of nebular gas in the Earth and terrestrial planets today but the moderately volatile elements also are depleted (Figure 1) (Gast, 1960 Wasserburg et al, 1964 Cassen, 1996). As can be seen from Figure 2, the depletion in the moderately volatile alkali elements, potassium and rubidium in particular, is far greater than that found in any class of chondritic meteorites (Taylor and Norman, 1990 Humayun and Clayton, 1995 Halliday and Porcelli, 2001 Drake and Righter, 2002). The traditional explanation is that the inner terrestrial planets accreted where it was hotter. 

An important development stemming from heterogeneous accretion models is that they introduced the concept that the Earth was built from more than one component and that these may have been accreted in separate stages. This provided an apparent answer to the problem of how to build a planet with a reduced metallic core and an oxidized sihcate mantle. However, heterogeneous accretion is hard to reconcile with modem models for the protracted dynamics of terrestrial planet accretion compared with the shortness of nebular timescales. Therefore, they have been abandoned by most scientists and are barely mentioned in modem geochemistry literature any more. 

Figure 1.3 (left) shows an artist s impression of the disk of gas and cosmic dust around the young star HD142527. The ALMA interferometer has allowed the observation of vast streams of gas flowing across the gap in the disk, expected to be created by giant planets accreting gas as they grow. 

Meteorites have great scientific value because most of them are fragments of asteroids which are themselves remnants of larger bodies that formed in the solar system at the time the planets accreted from planetesimals in the protoplanetary disk that formed around the Sun at the time of its formation about 4.6 x 10 years ago. Therefore, meteorites are our principal source of information about the origin of Earth and of the other planets of the solar system. 

When the Earth and other planets accreted aroxmd 4.5-4.6 billion years ago, the mixture of elements remaining reflected the cosmic abundances. Through a series of complex chemical reactions the Earth warmed up and differentiation of the constituent elements took place. 

Halliday, A.N. and Kleine, T. (2006) Meteorites and the timing, mechanisms, and conditions of terrestrial planet accretion and early differentiation, in Meteorites and the Early Solar System II (eds. D.S. Lauretta and H.Y. MeSween... 

In order to understand the Earth s character as a planet, it also is helpful to have an understanding of how the elements in our solar system were formed. Chapter 2 starts with the Big Bang theory and continues with how very small grains eventually came together and accreted to form the beginnings of what would eventually become the Earth and other planets, about 4.5 X 10 years ago (4.5 Gyr). The initial processes of the Earth s evolution involved heat... 

After planetary accretion was complete there remained two groups of surviving planetesimals, the comets and asteroids. These populations still exist and play an important role in the Earth s history. Asteroids from the belt between Mars and Jupiter and comets from reservoirs beyond the outer planets are stochastically perturbed into Earth-crossing orbits and they have collided with Earth throughout its entire history. The impact rate for 1 km diameter bodies is approximately three per million years and impacts of 10 km size bodies occur on a... 

Marcantonio F, Turekian KK, Higgins S, Anderson RF, Stute M, Schlosser P (1999) The accretion rate of extraterrestrial He based on oceanic °Th flux and the relation to Os isotope variation over the past 200,000 years in an Indian Ocean core. Earth Planet Sci Lett 170 157-168 Marchal O, Francois R, Stocker TF, Joos F (2000) Ocean thermohaline circulation and sedimentary 23ipa/230Th ratio. Paleoceanography 15(6) 625-641... 

Of the two models, homogeneous accretion is generally favoured. H. Wancke from the Max Planck Institute in Mainz (1986) described a variant of this model, in which the terrestrial planets were formed from two different components. Component A was highly reduced, containing elements with metallic character (such as Fe, Co, Ni, W) but poor in volatile and partially volatile elements. Component B was completely oxidized and contained elements with metallic character as their oxides, as well as a relatively high proportion of volatile elements and water. For the Earth, the ratio A B is calculated to be 85 15, while for Mars it is 60 40. According to this model, component B (and thus water) only arrived on Earth towards the end of the accretion phase, i.e., after the formation of the core. This means that only some of the water was able to react with the metallic fraction. 

However, one other interpretation has been discussed in the literature to explain the [Fe/H] excess observed for stars with planets. In fact, it has been suggested that the high metal content is the result of the accretion of planets and/or planetary material into the star (e.g. [12]). In such a case, the observed metallicity excess would itself be a by-product of the planetary formation process. 

On the one side, the traditional core accretion scenario (e.g. [1]) tells us that giant planets are formed as the result of the runaway accretion of gas around a previously formed icy core with about 10-20 times the mass of the Earth. Opposite to this idea, some authors have proposed that giant planets may form by a disk instability process [4]. 

The results presented above, showing that the probability of finding a planet is a strong function of the stellar metallicity, thus favor the core-accretion model as the main mechanisms responsible for the formation of giant planets (although they do not completely exclude the disk instability model - see e.g. [19]). Indeed, it has even be shown that according to the core-accretion model it is possible to predict the observed [Fe/H] distribution of planet-host stars [9]. 

The discovery of the average metal-rich nature of planet-harbouring stars with regard to disc stars (i.e. [1],[2], [3]) has revealed the key role that metallicity plays in the formation and evolution of planetary systems. If the accretion processes were the main responsible for the iron excess found in planet host stars, volatile abundances should show clear differences in stars with and without planets, since volatiles (with low Tc) are expected to be deficient in accreted materials [4]. Previous studies of the abundance trends of the volatiles N, C, S and Zn [5, 6] have obtained no anomalies for a large sample of planet host stars. 

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...

The formation of the planets around the proto-sun initially started as a simple accretion process, aggregating small particles to form larger particles. This process was common to all planets, even the gas giants Jupiter and Saturn and to a lesser extent Neptune and Uranus. The planetesimals form at different rates and as soon as Jupiter and Saturn had reached a critical mass they were able to trap large amounts of hydrogen and helium from the solar nebula. The centres of Jupiter... 

---