Volatile element depletion asteroids

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 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... 

Evidence for condensation is seen in the meteorites, fragments of the asteroids that formed in the region between Mars and Jupiter. The stony meteorites that have elemental compositions that closely match those of the Sun (except for volatile elements such as H, He, N, etc.) are called chondrites after the presence of small spherical particles called chondrules. Although the chondrites generally contain close to undifferentiated solar composition, there is elemental fractionation in these objects that is related to condensation processes. Different chondrite groups are distinguished by Fe/Si ratios that vary by 50%, ratios of Ca, Al, and Ti to Si that vary by about 40%, and abundances of volatile elements, such as Cd, Bi, In, and Pb, that vary by orders of magnitude. The depletion of volatile elements is believed to be due to incomplete condensation. The correlated depletion of Ca, Al, and... 

In most respects, asteroid 4 Vesta is geochemically similar to the Moon. As judged from howardite-eucrite-diogenite (HED) meteorites (see Chapter 6), Vesta is an ancient, basalt-covered world (Keil, 2002). Its rocks are highly reduced, and its depletions in volatile and siderophile element abundances resemble those of lunar basalts. And like the Moon, Vesta is hypothesized to have had an early magma ocean. The exploration of Vesta is now in progress, and within a few years we may have enough data to discuss it in a similar way that we have considered the Moon. 

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