The idea that the primary organic syntheses started in space is not new. Without even going back to van Hise (1904) or Chamberlin and Chamber-lin (1908), the possibility of an organic cosmochemistry was presumed by Oro (1961) and Bernal (1968). Whipple (1979), Chang (1979), and Delsemme (1979) concurred (in the same meeting) that comets may have provided most of the volatiles present nowadays on the Earth. Later, Delsemme (1981) took one step further, showing that the new paradigm on the origin of the solar system implies that the Earth first accreted from outgassed planetesimals then received a veneer of comet material as the result of the growth of the giant planets. This chapter has quantified this idea and shown that it connects a series of apparently unrelated problems, like the age record of the lunar craters, the reason why siderophile elements of the Earth's crust were not scavenged by the formation of the Earth's iron core, the reason why the "siderophile excess" and the "volatile excess" are linked, the source of the "planetary" component of the noble gas abundances on the Earth, the origin of the deuterium in the oceanic water, as well as the depletion of the volatile and siderophile metals in the upper mantle. To understand better our early atmosphere, we must now study comets.

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