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EFFECT OF THE LATE HEAVY BOMBARDMENT OF THE TERRESTRIAL PLANETS ON CHEMICAL EVOLUTION ON EARTH AND MARS Veme R. Oberbeck* and Guy Fogleman** (*Solar System Exploration Branch, **SETI Institute) MS: 239-12 NASA Ames Research Center Moffett Field, CA 94035

In order for life to have originated through chemical evolution, a number of favorable environmental factors must have been present. For example, the availability of prebiotic reactants and liquid water was necessary. It has only recently b e c o m e apparent, however, that impacts may also have constrained the process of chemical evolution. During the late heavy bombardment of the terrestrial planets before 3.8 b.y. ago, the time period between formation of impact basins of a given size increased with time. The time period between impacts of objects energetic enough to stop origination of life on Earth eventually increased until it equaled the time required to originate life through chemical evolution. If it is assumed that origination requires 105 to 107 years, then the earliest time that life could have arisen on Earth can be calculated (Maher and Stevenson, 1988). This approach was modified and used to calculate the maximum time available for life's origination and early evolution on Earth (Oberbeck and Fogleman, 1988b). To avoid underestimating the maximum time available, we used 2 X 1035 erg impacts (10 times the energy considered by Maher and Stevenson) as planetary sterilization events. We calculated that the maximum time interval available to originate and evolve microorganisms to the level of complexity represented by 3.5 b.y. old stromatolites was 130 million years. Thus, if 2 X 1035 erg impacts were able to stop the origination and early evolution of life, and if chemical evolution required the full ~30 million years that was available, impacts of this size would have been able to stop the process leading to the origination of life on Earth until the heavy bombardment ended. Impact is, therefore, an important physical process within the field of exobiology. It is necessary to understand large scale processes within the solar system, such as the late heavy bombardment of the terrestrial planets, when considering the possibility of life on other planets in this and other planetary systems. Also, processes such as stellar perturbations of cometary orbits may effect the time of delivery of prebiotic reactants. Thus interstellar processes might also constrain the origination of life within this and other planetary systems. If useful prebiotic reactants were supplied from space by comets (Or6 1961), stellar perturbations of the comets in Oort cloud and the cloud's location may have determined the time that prebiotic materials were supplied to the terrestrial planets. Monte Carlo models of the stellar perturbations of the Oort cloud suggest that, if the Oort cloud is as far away as 104 ALl, comets m a y have impacted the terrestrial 477

planets at one tenth and one half the present rate between 3.8 and 3.5 b.y. years ago, respectively (Weissman, 1982). Thus, comets could have supplied the prebiotic reactants needed for origination and early evolution of life on Earth. Prebiotic reactants like H2CO and HCN, produced b y photochemical reactions and lightning, respectively (Levine, 1985), m a y also have been available. If the only reactants needed for origination and early evolution of life were photochemical or electrical discharge products, these reactants could have been supplied at any time and life could have originated when the time period between damaging impacts equaled the time required for origination and evolution of life. Oberbeck and Fogleman (1989) used the above impact model with Martian cratering rates to show that, if prebiotic reactants were present on Mars, there may have been sufficient time to originate life between adverse impacts and before the water vanished from the surface of Mars at about 3.8 to 3.9 b.y. ago. First, prebiotic reactants from photochemical and electricaldischarge processes, and perhaps from comets, m a y have been present by thistime. Second, w e calculatethat 80 million years would have been available for origination and early evolution of lifeon Mars if liquid water did not vanish until 3.8 b.y. ago. Ifliferequires less than the m a x i m u m of 130 million years for origination and early evolution, or ifthere was more than the expected time of 80 millionyears between impacts on ancient Mars (which is possible because impact events are random), then lifem a y have originated on early Mars. Our discussion of the origination of life on Earth and Mars just after the late b o m b a r d m e n t illustrates the interaction between large scale physical processes and chemical and early evolution of life on a planet. This suggests that evaluation of the possibility for development of life in a planetary system will require a m o r e complete understanding of the way in which chemical evolution depends upon large-scale physical processes that influence planetary systems. Levine J. S. (1985): in The Photochemistry of A t m o s p h e r e s of Earth and Other Planets and Comets, ed. J. s. Levine, Academic Press Inc, 3-37. Maher K. A. and Stevenson, D. J., (1988): Nature, 331,612-614. Oberbeck V. R. and Fogleman, G. (1988b): "An Estimate of the Time Required for Chemical and Early and Biotic Evolution on Earth", NASA Ames Code SSS Preprint # NASA ARC/SSS 88-03 (December 1988). Submitted to Nature. Oberbeck V. R. and Fogleman, G. (1989): "On the Possibility of Life on Early Mars", NASA Ames Code SSS Preprint # NASA ARC/SSS 89-01 (January 1989). Ord, j.: 1961, Nature, 190, 389. Weissman, P. R. (1982): in Ceolosical Implications of Impacts of Large Asteroids and Comets on the Earth, ed. by Silver, L. 3".,and Schultz, P. H., Spec. Paper 190, U. S. Geol. Surv. Boulder, Colo. p.15. 478