Boreskov

OP‐23THE EFFECT OF IMPACT‐INDUCED ACCRETIONARY PROCESSING OF THE EARTHGerasimov M.V.Space Research Institute, RAS, Profsoyuznaya, 84/32, Moscow, 117997, Russia,mgerasim@mx.iki.rssi.ru.Emergence of terrestrial life occurred very early in the Earth’s history. Investigationsindicate the presence of life forms in the found most ancient sedimentary samples with ages~3.8 G.a. (Schidlowski, 1993). The origin of life thus dates to the period between theplanetary accretion ~4.5 G.a. to the age of the most ancient terrestrial samples. The earlyemergence of life enforces to consider conditions on the early Earth in more detail since thismay give a key to the answer on the most fundamental question about the mechanism oflife origin.The accretion period of the Earth group planets (at least its latest stages) wascharacterized by massive impacts of planetesimals. Impacts of planetesimals provided anoutput of enormous energy that resulted in the early deep processing of planetary material.There are three major result of such processing: 1) the release of impact‐generatedatmosphere; 2) the release of water to form primordial ocean; and 3) development ofprimordial crust due to impact‐generated differentiation of planetary siliceous material.Low‐velocity impacts decompose volatile‐bearing minerals to release H 2 O, CO 2 and SO 2 .Hypervelocity impacts, which provide vaporization of colliding material, are characterized bycomplex vapor plume chemistry and result in formation of specific gases from volatileelements in the plume. Experimental study of impact plume chemistry (Mukhin et al.,1989,Gerasimov, 2002) showed that the released gas mixture was characterized by the presenceof both reduced and oxidized volatile elements components what provided an input ofhighly nonequilibrium species into ecosystem. Thermal decomposition of petrogenic oxidesprovides the release of sufficient quantities of molecular oxygen into primordial atmospherethough its presence could be temporal due to rather high sink (Gerasimov, 2002). Gasmixtures formed by impacts are in disequilibrium at normal conditions. Significant quantitiesof oxidized and reduced components could be present simultaneously in the evolved gasmixtures (e.g. H 2 and O 2 , SO 2 and H 2 S, CO 2 and CH 4 ). The most abundant reduced gases inquenched mixtures could be H 2 , H 2 S, CH 4 , and light hydrocarbons up to C 6 H 6 . The instabilityof the ecosystem was also provided by the redox behavior of siliceous materials. Iron was68