The SPORES experiment of the EXPOSE-R mission: Bacillus subtilis spores in artificial meteorites

Abstract

The experiment SPORES ‘Spores in artificial meteorites’ was part of European Space Agency's EXPOSE-R mission, which exposed chemical and biological samples for nearly 2 years (March 10, 2009 to February 21, 2011) to outer space, when attached to the outside of the Russian Zvezda module of the International Space Station. The overall objective of the SPORES experiment was to address the question whether the meteorite material offers enough protection against the harsh environment of space for spores to survive a long-term journey in space by experimentally mimicking the hypothetical scenario of Lithopanspermia, which assumes interplanetary transfer of life via impact-ejected rocks. For this purpose, spores of Bacillus subtilis 168 were exposed to selected parameters of outer space (solar ultraviolet (UV) radiation at λ>110 or >200 nm, space vacuum, galactic cosmic radiation and temperature fluctuations) either as a pure spore monolayer or mixed with different concentrations of artificial meteorite powder. Total fluence of solar UV radiation (100–400 nm) during the mission was 859 MJ m⁻². After retrieval the viability of the samples was analysed. A Mission Ground Reference program was performed in parallel to the flight experiment. The results of SPORES demonstrate the high inactivating potential of extraterrestrial UV radiation as one of the most harmful factors of space, especially UV at λ>110 nm. The UV-induced inactivation is mainly caused by photodamaging of the DNA, as documented by the identification of the spore photoproduct 5,6-dihydro-5(α-thyminyl)thymine. The data disclose the limits of Lithopanspermia for spores located in the upper layers of impact-ejected rocks due to access of harmful extraterrestrial solar UV radiation.