Planet bulk composition

In keeping with the theme of this book, we are most concerned with estimating planet bulk compositions, which then can be recast into mean density and moment of inertia. A number of different cosmochemical models have been attempted to estimate the bulk compositions of planets for which we have very few, or more likely no samples. 

Equilibrium condensation model for planet bulk compositions, assuming that temperature and pressure decreased outward from the Sun, as shown by the heavy curved line. Modified from Barshay and Lewis (1976). 

Effects of condensation are also seen in the bulk compositions of the planets and their satellites. The outer planets, Uranus and Neptune, have overall densities consistent with their formation from icy and stony solids. The satellites of Uranus have typical densities of 1.3g/cm which would tend to indicate a large ice com-... 

A collision with a Mars-sized object may have resulted in the formation of the Earth s moon. Our moon is by no means the largest satellite in the solar system, but it is unusual in that it and the moon of Pluto are the largest moons relative the mass of the planets they orbit. Geochemical studies of returned lunar samples have shown that close similarities exist between the bulk composition of the moon and the Earth s mantle. In particular, the abimdances of sidero-... 

The solar system formed from a well-mixed collection of gas and dust inherited from its parent molecular cloud. The bulk composition of this material, as best we can know it, is given by the solar system abundances of elements and isotopes (Tables 4.1 and 4.2). From this bulk material, the planets, asteroids, and comets formed, each with its own unique composition. The processes that produced these compositions separated, or fractionated, elements and isotopes from one another. By studying these elemental and isotopic fractionations, we can potentially identify the processes that separated the elements and can leam about the physical conditions involved. This is particularly important for understanding the early solar system, because its processes and conditions are not directly observable. 

Some of these processes also cause measurable isotopic effects. Evaporation into empty space can cause the residual liquid or solid to become enriched in heavy isotopes. Processes that do not necessarily produce chemical fractionations can also produce isotopic effects. Diffusion is an example of such a process. Also, if the various constituents that go into making asteroids and planets have different isotopic compositions, the formation of these bodies can result in bulk compositions that are isotopically fractionated. Oxygen isotopes... 

Bouvier, A., Vervoort, J. D. and Patchett, P. J. (2008) The Lu-Hf and Sm-Nd isotopic composition of CHUR constraints from unequilibrated chondrites and implications for the bulk compositions of the terrestrial planets. Earth and Planetary Science Letters, 273, 48-57. 

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. 

In estimating the bulk compositions of the other terrestrial planets, there are not nearly so many constraints. Determination of a planet s mass (obtained from its gravitational effect on the orbits of moons or nearby spacecraft) and volume (calculated from its diameter as measured by telescopes) enables the calculation of its mean density. A meaningful comparison of planet mean densities requires that we correct for the effects of self-compression due... 

No description of our sofar system s formation wouid be complete without a discussion of the profound changes wrought by its orbital and collisional evolution. Although these physical processes may not seem to be related to cosmochemistry, they have changed the spatial distribution of planets and small bodies of differing compositions within the solar system, and in some cases, even the bulk compositions of large bodies. Understanding these processes can help us appreciate how some of the cosmochemical conundrums and complexities of the solar system arose. 

Tiny solid cosmic particles - often referred to as dust - are the ultimate source of solids from which rocky planets, planetesimals, moons, and everything on them form. The study of the dust particles genesis and their evolution from interstellar space through protoplanetary disks into forming planetesimals provides us with a bottom-up picture on planet formation. These studies are essential to understand what determines the bulk composition of rocky planets and, ultimately, to decipher the formation history of the Solar System. Dust in many astrophysical settings is readily observable and recent ground- and space-based observations have transformed our understanding on the physics and chemistry of these tiny particles. 

Planets 10 Lunar and martian meteorites, planetary bulk composition Exoplanets ... 

While the amount of dust and small particles that underwent thermal processing remains difficult to constrain both in the entire proto-solar nebula and in protoplanetary disks around other stars, in the Asteroid Belt over 80% of the pre-chondritic components have been melted. These heating events may play a crucial role in defining the bulk composition of planetesimals and planets by reprocessing much or all... 

Ma (Wadhwa et al. 2007 and references therein), which is actually the age of a group of inclusions within chondrites known as calcium-aluminum-rich inclusions (CAIs). The word primitive refers to the fact that the bulk compositions of all chondrites, within a factor of two, are solar in composition for all but the most volatile elements (Weisberg et al. 2006). This fact indicates that chondrites have not been through a planetary melting or differentiation process in their parent body, indicating that they have recorded the materials that were present and the processes that operated within the disk before or during planet formation. 

Differences between the bulk compositions of chondrites, planets and asteroids can be attributed to accretion from different batches of CAIs, chondrules and other components, which are spread along variation lines on the standard three-isotope plot. The preservation of oxygen isotopic anomalies shows that there were numerous oxygen reservoirs for the manufacture of chondrules and CAIs that were quite separate. 

MaePherson G. J. and Huss G. R. (2000) Convergent evolution of CAIs and chondrules evidence from bulk compositions and a cosmochemical phase diagram. In Lunar Planet. Sci. XXXI, 1796, The Lunar and Planetary Institution, Houston (CD-ROM). 

Kitts K. and Lodders K. (1998) Survey and evaluation of eucrite bulk compositions. Meteorit. Planet. Sci. 33, A197-A213. 

Bulk Compositions of Planets and Meteorite Parent Bodies 421... 

Condensation calculations in which the thermodynamic equilibrium condensate mineral assemblage is calculated as a function of temperature in a gas of fixed pressure and composition (usually solar) provide an extremely useful framework for interpreting bulk compositions of CAIs, chondrules, meteorites, and planets. Calculations of this sort date back to Urey (1952). Lord (1965) and Larimer (1967) calculated gas-solid equilibria for solar system composition, but did not correct the gas for removal of condensed material. The first detailed high-temperature condensation sequence was defined by Grossman (1972). The results of a calculation of Yoneda and... 

The bulk compositions of the terrestrial planets and all meteorite parent bodies except that of the Cl chondrites are depleted in volatile elements to various degrees. These depletions are reasonably smooth functions of 50% condensation... 

These are the most reasonable estimates of the bulk compositions of the planets made on the basis of the compositions of rocks from their surfaces or, for Earth, surface and mantle. 

Ringwood A. E., Seifert S., and Wanke H. (1987) A komatiite component in Apollo 16 highland breccias imphcations for the nickel-cobalt systematics and bulk composition of the Moon. Earth Planet. Sci. Lett. 81, 105—117. 

Warren P. H. and Kallemeyn G. W. (1984) Pristine rocks (8th foray) Plagiophile element ratios, crustal genesis, and the bulk composition of the Moon. Proc. 15th Lunar Planet. Sci. Conf C16-C24. 

Baker M. B. and Beckett J. R. (1999) The origin of abyssal peridotites a reinterpretation of constraints based on primary bulk compositions. Earth Planet. Set Lett. 171, 49-61. 

There is a wide range of meteorite types, which are readily divided into three main groups the irons, the stony irons and the stones (see also Volume 1 of the Treatise). With this simple classification, we obtain our first insights into planetary dilferentiation. All stony irons and irons are differentiated meteorites. Most stony meteorites are chondrites, undifferentiated meteorites, although lesser amounts are achondrites, differentiated stony meteorites. The achondrites make up —4% of all meteorites, and <5% of the stony meteorites. A planetary bulk composition is analogous to that of a chondrite, and the differentiated portions of a planet—the core, mantle, and crust— have compositional analogues in the irons, stony irons (for core-mantle boundary regions), and achondrites (for mantle and crust). 

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