Radiation pressure on bacterial clumps in the solar vicinity and their survival between interstellar transits

Abstract

Radiation pressure cross-sections for clumps of hollow bacterial grains with thin coatings of graphite are calculated using rigorousGuttler formulae. The carbonized skins are expected to form through exposure to solar ultraviolet radiation, but a limiting thickness of about 0.03 μm is determined by opacity effects. The ratios of radiation pressure to gravity P/G are calculated for varying sizes of the clumps and for varying thickness of the graphite coatings. Bacterial clumps and individual desiccated bacteria without coatings of radii in the range 0.3–8 μm have P/G ratios less than unity, whereas particles with coatings of 0.02μmthickness have ratios in excess of unity. Such coatings also provide protection from damaging ultraviolet radiation. Putative cometary bacteria, such as have beenrecently collected in the stratosphere, are thus not gravitationally bound in the solar system provided they possess carbonised exterior coatings. They are rapidly expelled from the solar system reaching nearby protosolar nebulae in timescales of a few million years. Even with the most pessimistic assumptions galactic cosmic rays are unable to diminish viability to an extent that vitiates the continuity of panspermia.