DFG Priority Program 1115: Mars and the terrestrial planets: Experimental study of the survival of endolithic microorganisms during impact and ejection of Martian meteorites: First phase of the transfer of life between Mars and Earth

Kurzfassung

In general terms, this interdisciplinary project represents an experimental contribution to the theory of “Lithopanspermia”. This theory has its roots in the work of Svante Arrhenius who formulated the theory of “Panspermia” in 1903 and postulated that microscopic forms of life, e.g. spores, can be dispersed in space by the radiation pressure from the sun, thereby seeding life from one planet to another. “Panspermia” has been replaced in the more recent past by the more realistic “Lithopanspermia” as defined by Melosh in 1988 and by Mileikowsky et al. in 2000. Lithopanspermia assumes that impactexpelled rocks serve as transfer vehicles for microorganisms colonizing those rocks. Our project has been focused on the potential transfer of primitive life from Mars to Earth. In view of the geological and climatological development of the planet Mars there seems to be quite a chance for the origin and evolution of life in the early history of Mars. There is also convincing evidence that a great number of surface rock material was ejected from Mars by impact processes and a substantial portion of them has been transferred to the Earth. The mineralogy of the Martian meteorites, so far collected, indicates an exposure to shock waves in the pressure range of 5 GPa to 50 GPa. As we know from Earth, terrestrial rocks are frequently inhabited by microbial communities. Therefore, rocks ejected by impact processes from a planet in the habitable zone of the solar system, may carry with them endolithic microorganisms, if microbial life exists on this planet. In this scenario, the microorganisms have to cope with three major steps: (1) escape from the planet by impact ejection, (2) journey through space over extended time periods, and (3) landing on another planet. Whereas step 2 of the scenario has been studied in depth in space experiments, there have been only limited data on the survivability of microorganisms of the first step, i.e. the impact ejection. Based on this situation, we initiated a comprehensive and systematic experimental study to determine the survival rate of resistant terrestrial microorganisms (bacterial endospores, epilithic and endolithic microbial communities) in the pressure range indicated by the Martian meteorites, in order to tackle the question whether and to what extent endolithic microorganisms ejected by impact processes may survive the high pressures (5 to 50 GPa) and high temperatures (in the range of ~150 °C to ~600 °C) which were derived for Martian meteorites on the basis of experimentally calibrated shock effects in the constituent minerals. Shock recovery experiments with an explosive set-up were performed at the Ernst-Mach-Institute für Kurzzeitdynamik in which three types of microorganisms inside various types of host rocks were exposed to strong shock waves: the endospore Bacillus subtilis, the lichen Xanthoria elegans, and the cyanobacterium Chroococcidiopsis sp. 029. In these experiments, three fundamental parameters were systematically varied: (1) the peak shock pressure, (2) the type of host rock and (3) the pre-shock ambient temperature. The applied pressures were in the range from 5 to 50 GPa. The pre-shock temperatures were set at 293, 233, and 193 K. The host rocks included non-porous igneous rocks (gabbro and dunite), porous sandstone, rock salt (halite), and artificial Martian regolith (MRS07).