Bacterial Spores Survive Simulated Meteorite Impact

Abstract

Lithopanspermia, i.e., the hypothesis of viable transport of microorganisms between the terrestrial planets by means of meteorites, requires that microorganisms, embedded in rocks, have to cope with three major steps: (i) escape from the planet by impact ejection, (ii) journey through space over extended time periods, and (iii) landing on another planet. Whereas step two of the scenario, the survival in space, has been studied in depth in space experiments, there are only limited data on the survivability of microorganisms of the first step, i.e. the impact ejection. Hypervelocity impacts of large objects, such as asteroids or comets are considered as the most plausible process capable of ejecting microbe-bearing surface material from a planet into space. The shock damage of rocks induced by the ejection process is quite substantial and leads to localized melting in the ejected rocks. However, due to a spallation effect, moderately shocked, solid rock fragments from the uppermost layer of the target can be accelerated to very high velocities (e. g., > 5 km/s) as documented by the meteorites that originated from the moon or Mars. To simulate this impact scenario, in shock recovery experiments with an explosive set-up, resistant microbial test systems (bacterial endospores of Bacillus subtilis), sandwiched between two quartz layers, were subjected to a shock pressure of 32 GPa, which is in the upper range indicated by the Martian meteorites and which can be assumed to hold also for “Earth” meteorites. Although the spore layer showed an intense darkening after the shock treatment, up to 500 spores per sample survived, resulting in a survival rate up to 10⁻⁴. The data demonstrate that a substantial fraction of spores are able to survive the severe shock pressure and temperature conditions which must be expected for collisionally produced rock fragments from a medium-sized terrestrial planet that have escape velocities of approximately 5 km/s.