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Origin and Evolution of Earth: Research Questions for a Changing Planet
that have little water are derived from the inner asteroid belt (inward of 2.5 AU), while the volatile element-rich meteorites, some with as much as 20 percent water as well as complex organic compounds, come from farther than 3 AU. These objects from the asteroid belt region may have been the source of Earth’s water and carbon. There is also evidence that much later in the history of the Solar System—500 million to 600 million years after its formation—a large but unknown amount of rocky debris was flung into the inner Solar System, bringing a last barrage of large impacts and finishing off the major construction of the inner planets. However, it is unlikely that this “late heavy bombardment” added enough material to significantly affect Earth’s overall composition.
The aspect of Earth’s composition that is likely best known is the proportion of refractory elements, which form solids at the high temperatures thought to have prevailed in the inner Solar System as the terrestrial planets were forming. Included among the refractory elements are most of Earth’s major components—Si, Mg, Al, and Ca. The relative amounts of refractory elements do not vary much among different classes of the benchmark chondritic meteorites, which is generally taken as a strong argument that Earth is not much different from the meteorites. For the more volatile elements, which evaporate more easily, there are wide and puzzling variations throughout the Solar System. Oxygen is one example. Si, Mg, and Fe readily combine with oxygen to form SiO2, MgO, and FeO. On Earth almost all of the Si and Mg occur as oxides, but only about 20 percent of the Fe is combined with O; the rest is metallic Fe that resides in Earth’s core. The size of the core is therefore a rough measure of the amount of oxygen that Earth has. Most meteorites have different Fe/FeO ratios, and at least two of the other terrestrial planets have a different ratio of metallic core to silicate mantle. Elements of intermediate volatility also raise important questions of chemical evolution. Potassium, for example, is relatively volatile, and estimates suggest that Earth has about 10 percent of what was available in the nebula. But exactly how much? The answer is critical because the isotope 40K is radioactive and provides 20 to 40 percent of the heat produced in the early Earth. This heat plays a role in powering the convection in the mantle that drives plate tectonics (Questions 4 and 5).
The chondritic model and the Solar System’s apparent ability to sort chemical elements according to their volatility have proven useful for understanding many aspects of planet formation. But our increasing ability to probe the chemical and isotopic compositions of meteorites and our planet is causing some serious rethinking of long-held models. Unanticipated compositional differences have been discovered between Earth and meteorites and between different types of meteorites. Perhaps the most striking difference is that of the isotopes of oxygen—the most abundant element on Earth (Figure 1.6). Chondritic meteorites have a peculiarly variable proportion of the isotope 16O, and almost every class of meteorites has different proportions of the three oxygen isotopes. Chondritic meteorites, long thought to be the best model for the original Earth, are not like Earth with respect to oxygen isotopes. The one class of meteorites that is like Earth in this respect—enstatite chondrites—would probably be no one’s first choice for Earth’s main building blocks because they do not match Earth for most other ele-
FIGURE 1.6 Representation of the range of values of oxygen isotope ratios on Earth, the Moon, Mars, and different classes of meteorites, including carbonaceous chondrites (CI, CK, CM, CO, CR, CV); ordinary chondrites (H, L, LL); other chondrite groups (R); primitive achondrites (Acapulcoite [Aca], Brachinite [Bra], Lodranite [Lod], Winonaite [Win], Ureilite [Ure]); Howardite, Eucrite, Diogenite [HED] achondrites; aubrite achondrites (Aub); stony-iron meteorites (Pallasites [Pal], Mesosiderite [Mes]); and iron meteorites (IAB-IIICD irons). SOURCE: <http://www4.nau.edu/meteorite/>. Used with permission.