Our Astrochemical Heritage

The problem on the origin of the terrestrial water comes from the fact that the planetesimals that built up the Earth, if they were located at the same place where Earth is today, must have been dry. Therefore, either water came later, when Earth was mostly formed, or the planetesimals that formed the Earth were from a zone more distant than 1 AU. The first theory, also called "late veneer", was first proposed by [129] and [332] and postulates that water is mostly delivered to Earth from comets, especially during the Late Heavy Bombardment ([128]; [180]). For almost a decade, the theory had the problem, though, that the   abundance ratio in the six comets where it had been measured is about a factor of two too high ([224]; see 3.4 and Fig. 4). However, new Herschel measurements are changing the situation. The measure on the 103P/Hartley2 comet gives exactly the terrestrial value ([195]) whereas measurements toward C/2009 P1 give again a larger  value, ([51]). The other possibility is that Earth was partly built from water-rich planetesimals from the outer zone ([303]). Two arguments are in favor of this theory. First, the  ratio of carbonaceous chondrites is very similar to the terrestrial one (1.3-1.8 , [375]; see 3.4 and Fig. 4).

Second, numerical simulations of the young Solar System from several authors predict that up to 10% of the Earth may have been formed by planetesimals from the outer asteroid belt, providing enough water to Earth (e.g. [303]; [367]). The same simulations tend to exclude the cometary delivery as a major contribution. However, as any model, the predictions are subject to a number of uncertainties, a major one being how much water is in the outer asteroid belt planetesimals ([269]).



Finally, the question on the origin of Earth's water is somewhat linked to the question on the origin of the Earth's atmosphere. Even though the methods are different, also for the Earth's atmosphere it is discussed a cometary delivery versus a meteoritic origin. Likely, in this case, both sources are necessary (e.g. [126]).


We emphasize the key role played, in both theories, by the / ratio in the terrestrial water, comets and asteroids. In the following sections of this review, we will see why, when and how water becomes enriched of deuterium.

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