ments. Moreover, it has recently been reported that the isotopes of neodymium, a lanthanide element that has proven critical for understanding planetary processes (Question 4), are also present in different amounts on Earth and chondritic meteorites (Figure 1.7), as are the isotopes of hafnium and barium.

Although we have long assumed that the isotopic compositions of the elements of the Solar System were mostly homogeneous, and measurements have borne this out in large measure, improved sensitivity is now showing small but significant differences between various planetary bodies. The O isotope differences,

FIGURE 1.7 Reported differences in 142Nd isotopic abundance between Earth, achondritic meteorites (Eucrites), and chondritic meteorites. The ε142Nd value is the difference in the proportion of 142Nd expressed in units of 0.01 percent. 142Nd is the radioactive decay product of the short-lived isotope 146Sm. The differences may reflect deep sequestration of ancient crust formed in the early Earth or differences in refractory element ratios between Earth and chondritic meteorites. SOURCE: Boyet and Carlson (2005). Reprinted with permission of the American Association for the Advancement of Science (AAAS).

for example, suggest that the nebular disk was not entirely homogeneous. While this is a problem in one sense, it is also an opportunity. If we can understand how this heterogeneity arose or was preserved, and what its structure was, we can learn more about how the materials of the nebula were sorted and gathered to produce the planets.

The Nd isotope discrepancy raises a different problem that has not yet been squarely addressed. Studies of asteroids and meteorites show that the process of accretion, whereby small chunks of rock gradually coalesce to form larger and larger bodies and eventually planets, is not one directional. When objects collide, they are almost as likely to blow each other apart as they are to coalesce. In addition, there is evidence that small accreting bodies become hot enough to melt, allowing crystals and liquid to separate. Thus, it was possible to differentiate (make heterogeneous by internal processes) smaller bodies and then blast material off them that is chemically different from the bulk object. This process would create differentiated objects that could eventually become part of the planets (or be lost into the Sun or ejected from the Solar System). In this view we cannot expect even the refractory elements to be present in exactly the same proportions everywhere, and this would have enormous implications. For example, if we relax the requirement that Earth be exactly chondritic for the elements Nd and Sm, we reach a different interpretation of the subsequent evolution of Earth’s mantle and crust (Question 4). If the Hf/W ratio of Earth is not chondritic, the timing of formation of Earth’s metallic core, as estimated by W isotope data, changes (see Question 2). We now know that even small bodies were able to partially melt and differentiate into core and mantle and that the mantle could potentially be removed from the core by an impact. So the timing and mechanism of formation of planetary metallic cores and the abundances of trace metals in planetary mantles have to be viewed in this context.

Was the Moon Formed by a Giant Impact?

More is known about the Moon than any terrestrial planetary body other than Earth because of the rock samples collected by the U.S. and Soviet lunar missions between 1969 and 1976. The peculiarities of these lunar rocks—their great antiquity, their nearly complete lack

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