Volatile element depletion terrestrial planets

Describe the building blocks that accreted to form the terrestrial planets, and explain how that may relate to their volatile element depletions. 

There is evidence from chondrites that the solar nebula was well mixed between 0.1 and 10 AU during its first several million years of the evolution, as shown by the homogeneity in concentrations of many isotopes of refractory elements (Boss 2004 Chapter 9). This is likely caused by the evaporation and recondensation of solids in the very hot inner nebula, followed by outward transport due to turbulent diffusion and angular momentum removal. Materials out of which terrestrial planets and asteroids are built have been heated to temperatures above 1300 K and are thus depleted in volatile elements. The inner solar nebula, with some exceptions, does not retain memories of the pristine interstellar medium (ISM) chemical composition (Palme 2001 Trieloff Palme 2006). 

Most meteorites are depleted in moderately volatile and highly volatile elements (see Figures 2-4). The terrestrial planets Earth, Moon, Mars, and the asteroid Vesta show similar or even stronger depletions (e.g., Palme et aL, 1988 Palme, 2001). The depletion patterns in meteorites and in the inner planets are qualitatively similar to those in the ISM. It is thus possible that the material in the inner solar system inherited the depletions from the ISM by the preferential accretion of dust grains and the loss of gas during the collapse of the molecular cloud that led to the formation of the solar system. There is, however, little support for this hypothesis ... 

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... 

The planets and asteroids are depleted in volatile elements compared to the Sun. The degree of fractionation decreases with distance the terrestrial planets and inner-belt asteroids are highly depleted in volatiles, the outer-belt asteroids are less so, while many satellites in the outer solar... 

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. 

One of the salient characteristics of the composition of the Earth is the depletion in volatiles compared to the parental solar nebula relative abundances. This is most pronounced in the noble gases (Fig. 1). However, the acquisition by the planet of these unreactive elements at even these levels pose considerable problems. A comparison between noble gases on the terrestrial planets and other solar system objects reveals significant differences in both elemental ratios and isotopic compositions and indicate that complex processes were involved in sequestering planetary volatiles from the nebula, as well as providing important indications of the sources and evolutionary history of planetary volatiles. Also, noble gas isotope inventories that are produced by nuclear... 

The surface of Venus has K/U ratios of the same order as those of terrestrial surface rocks (Surkov 1981), so that it also is depleted in volatile elements relative to primitive nebula values. This relative depletion of the more volatile elements thus appears to have been a widespread feature of the inner solar nebula rather than being a feature unique to the earth. Accordingly, a widespread loss of volatile elements occurred in the irmer solar system prior to the accretion of the terrestrial planets (Taylor 1987a). No information is yet available for the rare earth elements on Venus, but on the basis of the discussion above, they are probably similar to the terrestrial abundance levels. 

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