Terrestrial planets differentiation

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

These models provide an explanation for the thermal structure of the asteroid belt that is probably correct in principle but not in its details. The recognition that differentiated asteroids formed earlier than chondrites, perhaps within the terrestrial planet region, requires models in which asteroid accretion was initiated earlier than 2 Myr after CAI formation. 

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. 

The formation of the terrestrial planets is constrained by their bulk chemical compositions, but determining the compositions of entire planets is challenging. Because planets are differentiated into crust, mantle, and core, there is no place on or within a planet that has the composition of the entire body. Before considering the formation of the terrestrial planets, let s review how we go about estimating their bulk compositions. 

Differentiation of other terrestrial planets must have varied in important ways from that of the Earth, because of differences in chemistry and conditions. For example, in Chapter 13, we learned that the crusts of the Moon and Mars are anorthosite and basalt, respectively - both very different from the crust of the Earth. N either has experienced recycling of crust back into the mantle, because of the absence of plate tectonics, and neither has sufficient water to help drive repeated melting events that produced the incompatible-element-rich continental crust (Taylor and McLennan, 1995). The mantles of the Moon and Mars are compositionally different from that of the Earth, although all are ultramafic. Except for these bodies, our understanding of planetary differentiation is rather unconstrained and details are speculative. 

The terrestrial planets and the Moon are differentiated, with dense iron-rich cores and rocky mantles. The uncompressed densities of Earth and Venus are similar. Mercury has a high density which suggests it has relatively large core. Conversely, the Moon has a low density, indicating a very small core. There is little observational evidence that asteroids are differentiated except for Vesta and Ceres (Thomas et al. 2005). However, iron meteorites from the cores of differentiated asteroids are quite common, and the irons found to date come from several dozen different parent bodies (Meibom Clark 1999). Most meteorites come from asteroids that never differentiated. These chondritic meteorites consist of intimate mixtures of heterogeneous material millimeter-sized rounded particles that were once molten, called chondrules, similarly sized calcium-aluminum-rich inclusions (CAIs), and micrometer-sized matrix grains. 

An interesting consequence of this model is that bodies from the terrestrial-planet region may be scattered outwards while the planets are forming and implanted in the Asteroid Belt. With dynamical and collisional modeling of this process, Bottke et al. (2006) find that this may be the origin of most iron meteorites. This would explain the diversity of iron-meteorite types, why there is little observational or meteoritical evidence of mantle material from differentiated bodies in the Asteroid Belt, and the fact that most iron-meteorite parent bodies appear to have formed >1 Myr before the parent bodies of the chondritic meteorites (Kleine et al. 2005). 

The Moon s repertoire of geochemical processes may seem limited, but it represents a key link between the sampled asteroids (see Chapters 1.05 and 1.11) and the terrestrial planets. Four billion years ago, at a time when aU but monocrystalline bits of Earth s dynamic crust were fated for destruction, most of the Moon s crust had already achieved its final configuration. The Moon thus represents a unique window into the early thermal and geochemical state of a moderately large object that underwent igneous differentiation in the inner solar system, and into the cratering history of near-Earth space. 

Differentiation of terrestrial planets includes separation of a metallic core and possible later fractionation of mineral phases within either a solid or molten mantle (Figure 1). Lithophile and siderophile elements can be used to understand these two different physical processes, and ascertain whether they operated in the early Earth. The distribution of elements in planets can be understood by measuring the partition coefficient, D (ratio of concentrations of an element in different phases (minerals, metals, or melts)). 

It includes chapters on the origin of the elements and solar system abundances, the solar nebula and planet fomiution. meteorite classitlcation. the major types of meteorites, important processes in early solar system history, geochemistry of the terrestrial planets, the giant planets and their satellites, comets, and the formation and early differentiation of the Earth. This volume is intended to be the first reference work one would consult to learn about the chemistry of the solar system. 

As refractory lithophile elements, the REE play an important role in constraining the overall composition and history of the silicate fraction of planets, which for the terrestrial planets is also termed their primitive mantle (equivalent to the present-day crust plus mantle). Since there is no evidence for significant planetary-scale fractionation of refractory elements during the assembly and differentiation of planetary bodies, it is widely accepted that the primitive mantles of terrestrial planets and moon possess chondritic proportions of the REE. As such, the absolute concentrations of REE (and other refractory elements) in primitive mantles provide an important constraint on the proportions of volatile elements to refractory elements and on the oxidation state (i.e., metal/silicate ratio) of the body. To date, the only major planetary bodies for which REE data are directly available are the Earth, Moon, and Mars, and Taylor and McLennan" recently reviewed these data. 

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... 

Poitrasson, F., Levasseur, S., and Teutsch, N. (2005) Significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core-mantle differentiation of terrestrial planets. Earth Planet. Sci. Lett., 234, 151-164. 

Carlson, R.W. Boyet, M. Short-lived radionuclides as monitors of early crust-mantle differentiation on the terrestrial planets. Earth Planet Sd. Lett. 2009, 279, 147-156. 

Terrestrial planets have a solid surface, are differentiated (lighter elements near the surface, heavier elements in the core) and have relatively high densities. The main spectral signatures between 8 and 20 pm in the atmosphere of Venus Earth and Mars are shown in Fig. 3.1. 

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