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Earth and Titan are two planetary bodies formed far from each other.Nevertheless the chemical composition of their atmospheres exhibits commonindications of being produced by the accretion, plus ulterior in-situprocessing of cometary materials. This is remarkable because while the Earthformed in the inner part of the disk, presumably from the accretion of rockyplanetesimals depleted in oxygen and exhibiting a chemical similitude withenstatite chondrites, Titan formed within Saturn's sub-nebula from oxygen- andvolatile-rich bodies, called cometesimals. From a cosmochemical andastrobiological perspective the study of the H, C, N, and O isotopes on Earthand Titan could be the key to decipher the processes occurred in the earlystages of formation of both planetary bodies. The main goal of this paper is toquantify the presumable ways of chemical evolution of both planetary bodies, inparticular the abundance of CO and N2 in their early atmospheres. In order todo that the primeval atmospheres and evolution of Titan and Earth have beenanalyzed from a thermodynamic point of view. The most relevant chemicalreactions involving these species and presumably important at their earlystages are discussed. Then we have interpreted the results of this study inlight of the results obtained by the Cassini-Huygens mission on these speciesand their isotopes. Given that H, C, N and O were preferentially depleted frominner disk materials that formed our planet, the observed similitude of theirisotopic fractionation, and subsequent close evolution of Earth's and Titan'satmospheres points towards a cometary origin of Earth atmosphere. Consequently,our scenario also supports the key role of late veneers (comets and water-richcarbonaceous asteroids) enriching the volatile content of the Earth at the timeof the Late Heavy Bombardment of terrestrial planets.

Clues on the importance of comets in the origin and evolution of the atmospheres of Titan and Earth