CP-2156 Life In The Universe


Constraints on Early Life by Earth's Accretional and Preaccretional Development



[125] The initial state of Earth set the conditions for incipient life, but its accretional history and early evolution remain hypothetical. Further space explorations should help immensely to clarify present major uncertainties.


The thermal state of the early Earth and the properties of the primordial ocean-atmosphere determine conditions for incipient life. In the pre-Apollo era, even some obvious accretional constraints were taken lightly, and it was common to assume an Earth created with internal volatiles. This primordial undifferentiated planet would then gradually or spasmodically yield up an atmosphere with composition similar to present-day volcanic gases, including water for an ocean.

The accretionary history and early evolution of Earth still remain hypothetical. However, exploration of our Moon, Jupiter, and the terrestrial planets, including the discovery of 3.7-billion-year-old Earth crust, and of seafloor subduction and reappearance have brought some earlier known limitations into focus and added a wealth of new information and suggestions with direct or indirect bearing on the problem.

Several different types of accretional hypotheses operate more or less within this new observational framework, each one leading to drastically different early states of Earth but all sharing common concepts such as evolution via planetesimals and with sterilizing and pyrolyzing runaway accretion at some stage. Some hypotheses rely on special assumptions of strong activity of the early Sun to remove early heavy atmospheres of the [126] terrestrial planets. The recently established noble gas composition of the Venusian atmosphere places new boundary conditions on this aspect of the problem. Future space exploration plans include experiments which promise to further narrow the range of possible states for early Earth and other planets.