  Earth's volatile elements (H, C, and N) are essential to maintaining
habitable conditions for metazoans and simpler life forms. However, identifying
the sources (comets, meteorites, and trapped nebular gas) that supplied
volatiles to Earth is not straightforward because secondary processes like
mantle degassing, crustal recycling, and escape to space modified the
composition of the atmosphere. Here, we review two complementary approaches to
investigate the origin of Earth's atmosphere and oceans. The geochemical
approach uses volatile element abundances and isotopic compositions to identify
the possible contributors to the atmosphere and to disentangle the processes
that shaped it. In that respect, noble gases (He, Ne, Ar, Kr, and Xe), elements
that are chemically inert and possess several isotopes produced by
radioactivity, play a critical role. The dynamical approach uses our knowledge
of planetary dynamics to track volatile delivery to the Earth, starting with
dust transport in the disk to planet-building processes. The main conclusion is
that Earth acquired most of its major volatile elements by accretion of
planetesimals or embryos akin to volatile-rich meteorites. At the same time,
solar/meteoritic noble gases were captured by embryos and some gases were lost
to space, by hydrodynamic escape and large impacts. Comets did not contribute
much H, C, and N but may have delivered significant noble gases, which could
represent the only fingerprints of the bombardment of our planet with icy
bodies. The processes that governed the delivery of volatile elements to the
Earth are thought to be relatively common and it is likely that Earth-like
planets covered with oceans exist in extra-solar systems.
