Atmospheric and oceanic clues to the origin of a habitable world

Abstract
The Earth is unique as the one planet in our solar system that has hosted abundant life over billions of years. The habitability of our world means that we possess an environment that has sustained stable liquid water over time and was provided early on with ample amounts of organic molecules to form and feed life until photosynthesis took over. The initial conditions required for habitability are not fully understood, but they involve a planet size sufficient to retain water and sustain volatile-recycling tectonics, as well as a planetary system in whch water and organics were available for delivery to the growing planet. Our atmosphere and Ocean hold clues, largely in the form of isotopic ratios and elemental abundances, to the mechanisms by which water and organics were delivered to the Earth. Yet it remains unresolved as to whether the Earth formed of planetesimals endowed with liquid water or acquired its water from remote sources. The most plausible remote water source is the primordial asteroid belt; comets may have provided the bulk of the organics. To better constrain these sources will require e`xtensive exploration of Mars, comets and asteroids, as well as new meteorite discoveries.