The terrestrial planets

In this section we compare in a qualitative way thermal emission spectra of Venus, Earth, and Mars. As noted in Appendix 3, Venus and Earth are nearly equal in size Mars is somewhat smaller. Earth and Mars rotate at almost the same rate, while Venus is turning very slowly. The atmospheres of Venus and Mars consist mostly of carbon dioxide, while that of Earth is predominately nitrogen and oxygen with only a trace of carbon dioxide. The planets differ substantially in their mean distance from the Sun, atmospheric pressure at the surface, and mean surface temperature. 

Measured radiation from planetary objects up to Neptune 

On Earth the same spectral interval (from 667 cm to 800 cm Q is suitable for temperature retrieval in cloud-free areas (Kaplan, 1959). Indeed, this spectral region served in the first derivations of the vertical temperature profile from the Nimbus 3 meteorological satellite, and initiated a new era in weather forecasting 

In all spectra shown in Fig. 6.2.1, several other features can be attributed to CO2 besides the 667 cm band. These features, only weakly present in the Martian spectrum, include the bands at 961 cm and 1064 cm the distinct Q-branches near 545 cm 791 cm 830 cm and 865 cm and the strong features at 1918 cm and 1932 cm For comparison, a ground-based spectrum of Venus (Fig. 6.2.2) shows the CO2 bands between 750 cm and 1000 cm at a much higher spectral resolution of 0.2 cm (Kunde et al., 1977). If the spectral display of Fig. 6.2.1 extended to higher wavenumbers, then the strong vs-band of CO2 would appear at 2349 cm This is another spectral region where temperature sounding is feasible. Temperature retrieval in the atmosphere of the Earth is also 

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There appears to be a correlation between the mass of the planets and the mass and composition of their atmospheres. Generally, only those planets of high mass were able to retain much of their atmospheres. Nitrogen, hydrogen, and helium are probably abundant, though not yet detected, on the heavier planets. Table 25-V also reveals a considerable range in the surface temperatures of the planets. The higher temperatures on the terrestrial planets also contributed to the loss of their atmospheres. 

In addition to stable elements, radioactive elements are also produced in stars. The unstable but relatively long-lived isotopes °K, Th, and make up the internal heat source that drives volcanic activity and processes related to internal convection in the terrestrial planets. The short-lived transuranium elements such as Rn and Ra that are found on the Earth are all products of U and Th decay. 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

In the region of the terrestrial planets, there may have been several thousand planetesimals of up to several hundred kilometres in diameter. During about ten million years, these united to form the four planets—Mercury, Venus, Earth and Mars—which are close to the sun. Far outside the orbit of the planet Mars, the heavier planets were formed, in particular Jupiter and Saturn, the huge masses of which attracted all the hydrogen and helium around them. Apart from their cores, these planets have a similar composition to that of the sun. Between the planets Mars and Jupiter, there is a large zone which should really contain another planet. It... 

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. 

Among the terrestrial planets, the situation of the Earth is special. Its atmosphere (around 21% oxygen and 78% nitrogen by volume) is completely different from... 

Only the lightest gases, such as hydrogen and helium, could easily escape the gravitational field of the Earth. In contrast to earlier assumptions, it is now believed that the young Earth probably had either no atmosphere at all or only a very thin one, since the proportion of the primeval solar nebula from which the terrestrial planets were formed consisted mainly of non-volatile substances. 

The special position of the Earth among the terrestrial planets is also shown by the availability of free water. On Venus and Mars, it has not until now been possible to detect any free water there is, however, geological and atmospheric evidence that both planets were either partially or completely covered with water during their formation phase. This can be deduced from certain characteristics of their surfaces and from the composition of their atmospheres. The ratio of deuterium to hydrogen (D/H) is particularly important here both Mars and Venus have a higher D/H ratio than that of the Earth. For Mars, the enrichment factor is around 5, and in the case of Venus, 100 (deBergh, 1993). 

The density estimates in Table 7.1 show a distinction between the structures of the planets, with Mercury, Venus, Earth and Mars all having mean densities consistent with a rocky internal structure. The Earth-like nature of their composition, orbital periods and distance from the Sun enable these to be classified as the terrestrial planets. Jupiter, Saturn and Uranus have very low densities and are simple gas giants, perhaps with a very small rocky core. Neptune and Pluto clearly contain more dense materials, perhaps a mixture of gas, rock and ice. 

Kleine T., Munker C., Mezger K., and Palme H. (2002) Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry. Nature 418, 952-955. 

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. 

Among the elements that make up rocks and minerals, silicon, magnesium, and iron are of almost equal abundance followed by sulfur, aluminum, calcium, sodium, nickel, and chromium. Two of the most common minerals in meteorites and in the terrestrial planets are olivine ((Mg,Fe)2Si04) and pyroxene ((Mg,Fe,Ca)Si03). The composition obtained by averaging these two minerals is very similar to the bulk solar system composition, so it is really no surprise that they are so abundant. 

Wanke, H. and Dreibus, G. (1988) Chemical composition and accretion history of terrestrial planets. Philosophical Transactions of the Royal Society of London, A325,545—557. This paper describes how chemical fractionations resulted from accretion of different materials to form the terrestrial planets. 

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. 

In contrast to the terrestrial planets, the giant planets are massive enough to have captured and retained nebular gases directly. However, concentrations of argon, krypton, and xenon measured in Jupiter s atmosphere by the Galileo spacecraft are 2.5 times solar, which may imply that its atmosphere preferentially lost hydrogen and helium over the age of the solar system. 

Lunine, J. I. (2005) Origin of water ice in the solar system. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 309-319. A thoughtful review of the condensation of ices in the nebula and the delivery of ices to the terrestrial planets. 

Pepin, R. O. (2006) Atmospheres on the terrestrial planets clues to origin and evolution. Earth and Planetary Science Letters, 252, 1-14. 

Porcelli, D. and Pepin, R. O. (2004) The origin of noble gases and major volatiles in the terrestrial planets. In Treatise on Geochemistry, Vol. 4, The Atmosphere, ed. Keeling, R. F. Oxford Elsevier, pp. 319-344. 

Anhydrous planetesimals formed within the inner solar system, unlike the ice-bearing bodies discussed in the next chapter. These objects, composed of rock and metal, were the primary building blocks of the terrestrial planets. Relics of that population may survive today as asteroids that dominate the inner portions of the main belt. 

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. 

Bottke, W. F., Newvomy, D., Grimm, R. E., Morbidelli, A. and O Brien, D. R (2005) Iron meteorites as remnants of planetesimals formed in the terrestrial planet region. Nature, 439, 821-824. 

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. 

Uncompressed mean densities of the terrestrial planets and the Moon (Taylor and McLennan, 2009) vary with the relative volume proportions of cores and mantles. 

Mass fractions of cores (metallic iron) versus concentrations of FeO in mantles (oxidized iron) in the terrestrial planets and asteroid Vesta. After Righter et al. (2006). 

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