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题目材料：
Comets—detritus from the formation of outer solar system bodies—represent one possible source of Earth's water. Although hundreds of Earth masses of comets now reside in orbits far from the Sun, early in the history of the solar system comets were more commonly in orbits that intersected the orbits of Mars, Earth, and Venus (based on computer studies of solar system formation). Collisions of comets with the planets would have released the cometary ices and gases into the atmospheres of the target planets. Early in Earth's history, the first couple of hundred million years, cometary material including water might have been episodically added to the atmosphere. However, the ratio of deuterium to hydrogen(D/H) in the water ice portion of most (but not all) comets that have been measured is twice that in ocean water on the Earth. No plausible way has been found to lower the value after it has been added to the Earth. Therefore, comets do not appear to be the primary source of Earth's water.
Two alternative possibilities have been proposed. Bodies in the asteroid belt would have been richer in water than material near the Earth, and Jupiter perturbed that material into orbits that could have allowed accretion by the Earth. Most of this material would have been in the form of bodies as large as the moon or even Mars, so that these collisions would have been violent. Nonetheless, the net effect would have been the addition of water to the growing Earth. Carbonaceous meteorites, some of which may have been derived from the asteroid belt, have a D/H range that averages out to the value present in the Earth's oceans. However, some of the details of the elemental and isotopic abundances in the carbonaceous chondrites [a type of meteorite] limit to 1 percent the amount of this material that could have been added to the Earth. It is possible that other types of chondrites were present in the asteroid belt that today are poorly known, such as a new class of bodies represented by a handful of so-called “main belt comets, but for the moment this is speculative. Alternatively, water could have been absorbed [gathered on a surface in a condensed layer] on rocky grains closer to the Earth, and brought in through a gentle rain of this material. While laboratory studies show that enough water might have stuck to the grains to explain the abundance of the Earth's oceans, the presence of such a water-laden dust laver in the nebula remains speculative.
Two alternative possibilities have been proposed. Bodies in the asteroid belt would have been richer in water than material near the Earth, and Jupiter perturbed that material into orbits that could have allowed accretion by the Earth. Most of this material would have been in the form of bodies as large as the moon or even Mars, so that these collisions would have been violent. Nonetheless, the net effect would have been the addition of water to the growing Earth. Carbonaceous meteorites, some of which may have been derived from the asteroid belt, have a D/H range that averages out to the value present in the Earth's oceans. However, some of the details of the elemental and isotopic abundances in the carbonaceous chondrites [a type of meteorite] limit to 1 percent the amount of this material that could have been added to the Earth. It is possible that other types of chondrites were present in the asteroid belt that today are poorly known, such as a new class of bodies represented by a handful of so-called “main belt comets, but for the moment this is speculative. Alternatively, water could have been absorbed [gathered on a surface in a condensed layer] on rocky grains closer to the Earth, and brought in through a gentle rain of this material. While laboratory studies show that enough water might have stuck to the grains to explain the abundance of the Earth's oceans, the presence of such a water-laden dust laver in the nebula remains speculative.
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