Physical conditions on early earth: Implications for the origin of life

Abstract

The history of the earth can be divided into the earlier part of the accretion, the later part of accretion and core formation, and the remaining history. The early part of accretion is irrelevant to the origin of life because of the extreme conditions associated with the late stages of accretion. The impact of several lunar to Mars-sized objects appears inevitable. These large impacts surrounded the earth with rock vapor tens to hundreds of kilometers thick. Although the solid surface was quickly reestablished any ocean or atmosphere existing before the last very large impact was either buried or lost. The iron core of the earth formed during accretion and released enough gravitational energy to melt the interior of the earth. Moderate sized impacting bodies formed local pockets of impact melt. Following the last very large impact the earth (and the moon) were near their present sizes and the energy flux from impacts was sufficiently less than the present solar heat flux. A cool surface with oceans and an atmosphere then could persist. The internal tectonics and degassing mechanisms of the earth were different than during the rest of earth history. In particular, the eruption of total ly melted rock to the surface was a much more efficient means of heat loss than plate movements and the return of the frozen material to the interior may have efficiently cooled the interior. Once the last pockets of very hot material in the mantle erupted and were used up, plate tectonics became the dominate mode of heat transfer from the earth 's interior. The interior temperature at the s tar t of plate tectonics need not have been particularly high as cold total ly frozen material acted to cool the mantle during the early epic of total melting and volcanism. Mare-sized impacts were infrequent during the period of heavy bombardment and mare formulation on the moon that the surface geology of the earth was probably dominated by internal processes rather than cratering. In particular, the resurfacing of the earth was probably comparable to that of present-day active volcanic regions where life is abundant in spite of frequent disasters. The mare-forming imparts on the earth would have produced heavy casualities in any life that existed, but the worst effects were local and not a danger to life globally. Conversely, impacts were sufficiently infrequent that meteoritic iron, shocked or melted rocks, and shock-induced atmospheric gasses were not dependable material for the sustenance of early life. The origin of an atmosphere and an ocean following the last major impact was probably rapid, but quantitative observations exist only for radiogenic rare gasses and not for CO 2 and H20. As at present, competing processes tended to degas volatiles and to return them to the earth 's interior. Volcanic activity and metamorphism released gasses and lower temperature alteration particularly at mid-oceanic ridges returned volatiles into the interior. Although the