EFFECT OF PLANETARY IMPACT ON MICROBIAL LIFE

A growing body of evidence indicates that microbial life existed in terrestrial rocks that have a sedimentary formation age of 3.5 to 3.8 Gy, both in W. Australia and S. Africa. The occurrence of microbial fossils suggest that life emerged soon after the late heavy bombardment which concluded Earth impact accretion. The impact frustration of life, prior to this time, has been suggested by Maher and Stevenson, Sleep and Zahnle, Anbar and Mojzsis, and others. Previously, experiments and theory have considered and demonstrated the viability of bacteria and spores carried within dry or incompletely water-saturated media upon being subjected to impact-induced shocks. In many previous experiments, the viability of shocked organisms is dependent, in part, on the material strength of the rock media that host life. To remove the strength of rock effects, we have conducted shock recovery experiments of e. coli bacteria in a water-based growth medium and attempted to simulate the effects of early life occupying a subsurface world of water-filled cracks. This world is subjected to shock waves induced by planetary impact. Our experimental geometry is similar to that of G. Miller and J. Blank who conducted shock recovery experiments using stainless steel targets with cavities filled with fluids. We demonstrated recovery of viable e. coli bacteria living in a media exposed to 0.2 GPa initial shock pressure in water. The shock pressure was exerted for a duration of ~200 ns. Higher shock pressures sterilized all life and induced devolatilization up to 2 GPa. We developed a Turgor shock pressure decay model which can be extrapolated to lower dynamic stresses and longer durations. For example, we predict that for an 11 km/s, 3 km diameter silicate impactor inducing a shock with duration of 0.3 s, e. coli (in water within a gabbro matrix) will survive to shock pressures of 10 MPa at radii of > 200 km from the impact zone.