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题目材料：
Mercury, the planet closest to the Sun,is not much bigger than Earth's moon, yet its density is typical of that of a far larger planet. Since the density of terrestrial(nongaseous)planets increases with depth, they have high-density cores,probably composed largely of iron. Mercury's high density suggests that it may have the largest metallic core relative to its size of all the terrestrial planets. Astronomers believe that all the planets condensed from the solar nebula at about the same time,so why would Mercury have such an anomalous density? The composition of the solar nebula might have been dramatically different in the vicinity of Mercury's orbit- much more so than theoretical models would predict. Or the Sun may have been so energetic early in the life of the solar system that the more volatile, low-density elements on Mercury were vaporized and driven off by the Sun's heat. It is also possible that a massive object collided with Mercury soon after its formation, vaporizing the less dense material.
以上解析由 考满分老师提供。
