Planets/planetary compositions

The investigation of which chemical elements exist in nature and in what quantities has a long history. The determination of elemental abundances in various celestial objects is still a very active field in astronomy, planetary science, and meteoritics. There are multiple motivations for studying the solar system abundances of the chemical elements. One reason to study this overall composition of the solar system is to understand how the diversity of planetary compositions, including that of our home planet, can be explained, since all planets in the solar system share a common origin from the material of the protosolar disk (the solar nebula). 

In Earth s upper atmosphere (and, in planetary probe missions, the atmospheres of other planets), atmospheric composition can be measured in situ with mass spectrometers and related instrumentation. However, UV measurements provide a capability for remote sensing of atmospheric composition and its variation with altitude, geographic location, and time, which supplements and extends in situ measurements where available, and can be applied to many objects not yet visited by spacecraft (e.g., in observations of other planets by spacecraft in near-Earth orbit). In addition to atmospheric composition, UV measurements can remotely sense the atmospheric temperature structure and its spatial and temporal variations. Also, information on the fluxes, energies, and spatial distributions of incoming energetic particles (such as those that produce Earth s polar auroras) can be obtained. 

The miniaturized Mossbauer instruments have proven as part of the NASA Mars Exploration Rover 2003 mission that Mossbauer spectroscopy is a powerful tool for planetary exploration, including our planet Earth. For the advanced model of MIMOS II, the new detector technologies and electronic components increase sensitivity and performance significantly. In combination with the high-energy resolution of the SDD, it will be possible to perform XRF analysis in parallel to Mossbauer spectroscopy. In addition to the Fe-mineralogy, information on the sample s elemental composition will be obtained. 

Elemental oxygen also is present in the sun in less than 1% mass composition, as a fusion product of carbon-12, and hehum-4. No planet or its moon in the solar system, other than the earth is known to contain molecular oxygen in its atmosphere, although CO2 is a major component of many planetary... 

Some meteorites, and all planetary samples, have undergone melting and differentiation at some stage. Hence, the compositions of differentiated materials do not resemble solar system abundances. These samples can, however, tell us about various geochemical processes within asteroids and planets. 

Quantifying the chemical and isotopic fractionations in planetary samples and in meteorites, the closest analogues to the materials that formed planets, is a necessary first step. This involves careful measurement of chemical and isotopic compositions of the various bodies and an understanding of the composition of the material from which they formed. Once these fractionations have been identified, experiments (and theory, when the relevant experiments cannot be performed) that yield similar fractionations can point to the processes and conditions that produced them. 

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. 

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 necessary starting point for any study of the chemistry of a planetary atmosphere is the dissociation of molecules, which results from the absorption of solar ultraviolet radiation. This atmospheric chemistry must take into account not only the general characteristics of the atmosphere (constitution), but also its particular chemical constituents (composition). The absorption of solar radiation can be attributed to carbon dioxide (C02) for Mars and Venus, to molecular oxygen (02) for the Earth, and to methane (CH4) and ammonia (NH3) for Jupiter and the outer planets. 

The contrasting temperature-induced shifts of the pyroxene 1 and 2 pm bands could lead to erroneous estimates of the composition and, to a lesser extent, structure-type of a pyroxene-bearing mineral assemblage deduced from the remote-sensed reflectance spectrum of a hot or cold planetary surface if room-temperature determinative curves, such as that shown in fig. 10.5, are used uncritically. For example, remote-sensed spectra of planets with hot surfaces, such as Mercury and the Moon, would lead to overestimates of Fe2+ contents of the orthopyroxenes and underestimated Fe2+ contents of the clinopyroxenes (Singer and Roush, 1985). Planets with cold surfaces, such as Mars and the asteroids, could produce opposite results. On the other hand, the room-temperature data underlying the pyroxene determinative curve shown in fig. 10.5 may impose constraints on the compositions of pyroxenes deduced from telescopic spectra of a planet with very high surface temperatures, such as Mercury. 

Reflectance spectroscopy has proven to be the most powerful and versatile remote-sensing technique for determining surface mineralogy, chemical compositions and lithologies of planetary objects, as well as constituents of their atmospheres. Table 10.1 summarizes information that has been deduced for the terrestrial planets based on spectral properties of light in the visible and near-infrared regions reflected from their surfaces. 

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

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. 

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