Protection of bacterial spores in space, a contribution to the discussion on Panspermia

Spores of Bacillus subtilis were exposed to space in the BIOPAN facility of the European Space Agency onboard of the Russian Earth-orbiting FOTON sate llite. The spores were exposed either in dry layers without any protecting agent, or mixed with clay, red sandstone, Martian analogue soil or meteorit e powder, in dry layers as well as in so-called `artificial meteorites', i. e. cubes filled with clay and spores in naturally occurring concentrations. After about 2 weeks in space, their survival was tested from the number of colony formers. Unprotected spores in layers open to space or behind a qua rtz window were completely or nearly completely inactivated (survival rates in most cases less than or equal to 10(-6)). The same low survival was obt ained behind a thin layer of clay acting as an optical filter. The survival rate was increased by 5 orders of magnitude and more, if the spores in the dry layer were directly mixed with powder of clay, rock or meteorites, and up to 100% survival was reached in soil mixtures with spores comparable to the natural soil to spore ratio. These data confirm the deleterious effect s of extraterrestrial solar UV radiation. Thin layers of clay, rock or mete orite are only successful in UV-shielding, if they are in direct contact wi th the spores. The data suggest that in a scenario of interplanetary transf er of life, small rock ejecta of a few cm in diameter could be sufficiently large to protect bacterial spores against the intense insolation; however, micron-sized grains, as originally requested by Panspermia, may not provid e sufficient protection for spores to survive. The data are also pertinent to search for life on Mars and planetary protection considerations for futu re missions to Mars.