Comparison of terrestrial and extraterrestrial D/H Ratios of organic matter and water

Kurzfassung

We compare the D/H ratios of terrestrial cell material published in the literature with the values estimated for related organic substances of the solar nebula. In SOM (sedimentary organic matter) isoprenoid hydrocarbons are taken as biomarkers. Their D/H ratios measured for various oil sources indicate deuterium depletion relative to SMOW (Standard Mean Ocean Water ). Similar values are found for LOM (living organic matter). A different situation occurs for EOM (extraterrestrial organic matter). We consider hydrogen cyanide and formaldehyde. These compounds are precursors of biochemical substances like adenine or glycerol. In order to estimate the D/H ratio of water and the selected molecules in the solar nebula a hydrodynamic model of a collapsing cloud is coupled to a set of kinetic equations to include deuterium chemistry. Sublimation and accretion on the surface of interstellar dust are considered. The initial state of the collapsing cloud is a cold molecular hydrogen sphere. In the course of the collapse a low mass protostellar core evolves which corresponds to an early phase of our sun. The resulting hot cloud envelope near the protostar is related to the early phase of the solar nebula. Our calculations for the early solar nebula (ESN) show that water, hydrogen cyanide and formaldehyde are deuterium enriched in comparison to SMOW. This result is confirmed by published measurements of D/H ratios for organic matter in comets, asteroids and in a lower amount meteorites (e.g. Murchison). In order to understand the deuterium depletion in SOM and LOM we draw the conclusion that two processes on the Earth surface are responsible for this effect. These processes are the exchange between organic matter and terrestrial water on the one hand and the isotope fractionation during the biochemical evolution of proto-cells on the other.