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Sources of Terrestrial and Martian Water
Humberto Campins
Michael J. Drake
January 30, 2006

Abstract: There is no agreement on the origin of water on Earth and Mars. A number of sources have been proposed but the pieces of this puzzle do not currently fit into a coherent picture. We list various geochemical measurements that can serve as discriminators and we use them to examine the principal proposed mechanisms for delivery of terrestrial and Martian water. Important new developments have occurred in our understanding of the presence of water during early and late stages of Earth's formation. It has long been thought that the region of the solar nebula where Earth formed was too dry for hydrous mineral phases to be stable and form a "wet" Earth, and that water was delivered by impacts of asteroids and/or comets with Earth after it formed. However, several recent measurements support the existence of water oceans on Earth shortly after its formation, as early as 4.3 to 4.4 x 10^9 years ago. The source or sources of this early water are not obvious at this time. Two new mechanisms proposed for the formation of a wet early Earth are adsorption of nebular gas onto fractal dust grains in Earth's formation region and migration of hydrated silicates from the outer asteroid belt region of the solar nebula. On the other hand, late stage delivery of significant quantities of water from asteroidal and cometary sources appears less likely that previously thought. Isotopic and molecular ratio considerations do not favor asteroids or comets as the main contributors to what is commonly termed the "late veneer" in Earth's formation. There are two important caveats to this last statement. First, our measurement of the composition of comets and asteroids may not be representative of their bulk atomic, isotopic and molecular composition. Second, comets and asteroids currently sampled spectroscopically and by meteorites may be unlike those falling to Earth during its formation. Independent of the role comets and asteroids may have played in the delivery of water, they appear to have been the principal source of organic compounds once the Earth's crust had solidified.

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On the Deuterium Abundance on Mars and Some Related Problems
Vladimir Krasnopolsky
Icarus, Volume 148, Issue 2,
pp. 597-602 (2000)

Abstract: Strong fractionation of deuterium in photolysis of H2O and above the hygropause reduces the production of HD relative to H2 on Mars by a factor of 3.7 total. The model by Y. L. Yung et al. (1988, Icarus 76, 146-159) for deuterium fractionation in chemical reactions on Mars corrected for this factor results in (HD/H2)/(HDO/H2O)=0.43. This value may fit the deuterium abundance observed by V. A. Krasnopolsky et al. (1998, Science 280, 1576-1580) if the eddy diffusion coefficient does not depend on solar activity: K=1.4¥1013n-1/2 cm^2 s-1 (model 2)... The three-reservoir model for hydrogen isotope fractionation suggested by Krasnopolsky et al. (1998) involves a reservoir composed primarily of water ice in the polar caps that isotopically interacts with the atmosphere. Assuming that water ice is half of the total volume of the polar caps and the polar-layered deposits, the total loss of water from Mars is equal to 65 and 120 m for models 1 and 2, respectively. Along with thermal and nonthermal escape, these values may include the loss of water by oxidation of regolith, if the released hydrogen escaped with isotopic fractionation. Although the solar-wind alpha particles are the main source of He on Mars, capture of the solar-wind H+ and D+ ions by Mars has a negligible effect on the thermospheric abundances of H and D. Improved observations of minor components in Mars' thermosphere may resolve the problem of eddy diffusion at various solar activity and choosing between the models.