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The most abundant element in the sun is hydrogen; the third most abundant element is oxygen. It therefore follows that water molecules must have been a major constituent of the solar nebula from which the planets formed. Most water condensed on the "giant" planets in the outer parts of the solar nebula, but some water remained in the inner regions of the solar nebula, where it was acquired by Earth and other rocky "terrestrial" planets, by processes that remain largely unknown.
Clues come from meteorites, which can provide evidence for the chemical behavior of water at the time when planetesimalssmall rocky bodies from which planets accreted-grew in the early solar system. Two reports in this issue deal with chemical processes involving liquid water within asteroids or their planetesimal precursors. On page 1380, Brearley (1) presents transmission electron microscope evidence for the formation of iron-rich olivine at low temperatures and argues that these observations support an earlier proposal that this ubiquitous phase was formed by a hydration-dehydration sequence. And on page 1377, Zolensky et al. (2) present direct evidence for meteoritic water: tiny inclusions of brine within large crystals of halite (NaCI) inside a meteorite (see the figure). The existence of a water-soluble salt in this meteorite is astonishing. Also, this sample of aqueous solution trapped within the meteorite provides the first opportunity to study solar nebular water directly.
Ordinary chondrites and carbonaceous chondrites are two major classes of primitive stony meteorites. Both are characterized by abundant chondrules-millimeter-sized silicate spheroids that were once molten droplets in the solar nebula. In ordinary chondrites, the matrix between these chondrules is composed primarily of chondrule fragments. In contrast, in carbonaceous chondrites, the matrix is chemically and mineralogically distinct from chondrules and in...