The planetary building blocks that formed in the terrestrial planet region were likely very dry, yet water is comparatively abundant on Earth. Here we review the various mechanisms proposed for the origin of water on the terrestrial planets. Various in-situ mechanisms have been suggested, which allow for the incorporation of water into the local planetesimals in the terrestrial planet region or into the planets themselves from local sources, although all of those mechanisms have difficulties. Comets have also been proposed as a source, although there may be problems fitting isotopic constraints, and the delivery efficiency is very low, such that it may be difficult to deliver even a single Earth ocean of water this way. The most promising route for water delivery is the accretion of material from beyond the snow line, similar to carbonaceous chondrites, that is scattered into the terrestrial planet region as the planets are growing. Two main scenarios are discussed in detail. First is the classical scenario in which the giant planets begin roughly in their final locations and the disk of planetesimals and embryos in the terrestrial planet region extends all the way into the outer asteroid belt region. Second is the Grand Tack scenario, where early inward and outward migration of the giant planets implants material from beyond the snow line into the asteroid belt and terrestrial planet region, where it can be accreted by the growing planets. Sufficient water is delivered to the terrestrial planets in both scenarios. While the Grand Tack scenario provides a better fit to most constraints, namely the small mass of Mars, planets may form too fast in the nominal case discussed here. This discrepancy may be reduced as a wider range of initial conditions is explored. Finally, we discuss several more recent models that may have important implications for water delivery to the terrestrial planets.

中文翻译：

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地球形成过程中水的输送

在类地行星区域形成的行星构件可能非常干燥，但地球上的水相对丰富。在这里，我们回顾了为类地行星上水的起源提出的各种机制。已经提出了各种原位机制，这些机制允许将水并入类地行星区域的本地星子或从本地来源进入行星本身，尽管所有这些机制都存在困难。彗星也被提出作为一个来源，尽管可能存在适合同位素约束的问题，并且输送效率非常低，以致于以这种方式输送即使是单个地球海洋也可能很困难。最有希望的输水路线是从雪线外堆积物质，类似于碳质球粒陨石，随着行星的生长而分散到类地行星区域。详细讨论了两个主要场景。首先是经典场景，其中巨行星大致从它们的最终位置开始，类地行星区域的微星和胚胎盘一直延伸到外小行星带区域。第二个是Grand Tack情景，巨行星早期的向内和向外迁移将雪线以外的物质植入小行星带和类地行星区域，在那里它可以被生长的行星吸积。在这两种情况下，都向类地行星输送了足够的水。虽然 Grand Tack 方案更适合大多数限制条件，即小质量的火星，在这里讨论的名义情况下，行星可能形成得太快。随着探索更广泛的初始条件，这种差异可能会减少。最后，我们讨论了几个最近的模型，这些模型可能对向类地行星输送水有重要意义。