Spacecraft-associated spores and four non-spore-forming bacterial isolates were prepared in Atacama Desert soil suspensions and tested both in solution and in a desiccated state to elucidate the shadowing effect of soil particulates on bacterial survival under simulated Martian atmospheric and UV irradiation conditions. All non-spore-forming cells that were prepared in nutrient-depleted, 0.2-m-filtered desert soil (DSE) microcosms and desiccated for 75 days on aluminum died, whereas cells prepared similarly in 60-m-filtered desert soil (DS) microcosms survived such conditions. Among the bacterial cells tested, Microbacterium schleiferi and Arthrobacter sp. exhibited elevated resistance to 254-nm UV irradiation (low-pressure Hg lamp), and their survival indices were comparable to those of DS-and DSE-associated Bacillus pumilus spores. Desiccated DSE-associated spores survived exposure to full Martian UV irradiation (200 to 400 nm) for 5 min and were only slightly affected by Martian atmospheric conditions in the absence of UV irradiation. Although prolonged UV irradiation (5 min to 12 h) killed substantial portions of the spores in DSE microcosms (ϳ5-to 6-log reduction with Martian UV irradiation), dramatic survival of spores was apparent in DS-spore microcosms. The survival of soil-associated wild-type spores under Martian conditions could have repercussions for forward contamination of extraterrestrial environments, especially Mars.
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Abstract. Nitrogen-containing cyclic organic molecules (N-heterocycles) play important roles in terrestrial biology, for example as the nucleobases in genetic material. It has previously been shown that nucleobases are unlikely to form and survive in interstellar and circumstellar environments. Also, they were found to be unstable against ultraviolet (UV) radiation. However, nucleobases were detected in carbonaceous meteorites, suggesting their formation and survival is possible outside the Earth. In this study, the nucleobase precursor pyrimidine and the related N-heterocycles pyridine and s-triazine were tested for UV stability. All three N-heterocycles were found to photolyse rapidly and their stability decreased with an increasing number of nitrogen atoms in the ring. The laboratory results were extrapolated to astronomically relevant environments. In the diffuse interstellar medium (ISM) these N-heterocycles in the gas phase would be destroyed in 10-100 years, while in the Solar System at 1 AU distance from the Sun their lifetime would not extend beyond several hours. The only environment where small N-heterocycles could survive, is in dense clouds. Pyridine and pyrimidine, but not s-triazine, could survive the average lifetime of such a cloud. The regions of circumstellar envelopes where dust attenuates the UV flux, may provide a source for the detection of N-heterocycles. We conclude that these results have important consequences for the detectability of N-heterocycles in astronomical environments.
Abstract.Benzene is an essential intermediate in the formation pathways of polycyclic aromatic hydrocarbons (PAHs) and carbon dust. Therefore, it is important to understand the interplay of formation and destruction in order to assess the lifetime of benzene in space. We performed UV photolysis and proton (0.8 MeV) bombardment experiments on benzene (C 6 H 6 ) isolated in inert argon matrices and in oxygen-rich solid mixtures in the laboratory. The destruction of benzene in different chemical environments was measured for both methods of energetic processing. Additionally, we quantitatively determined the absorbed photon fraction in the sample layers when exposed to our UV lamp with actinometry. This enabled us to derive destruction cross sections for benzene for both UV photolysis and proton bombardment allowing us to compare these two ways of energetic processing. The laboratory data were extrapolated to different interstellar environments and we found that benzene is efficiently destroyed in diffuse interstellar clouds, but could survive dense cloud environments longer than the average lifetime of the cloud. Benzene is likely to survive in the dense parts of circumstellar envelopes around carbon-rich AGB stars but only in a very finite region where UV photons are attenuated.
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