Radiation Inactivation of Bacterial Spores on Mars

Introduction: The conditions on Mars are thought to have been more conducive for life during its early history, about 3 billion years ago. If life ever evolved on Mars, would it be possible to see the remnants of a long-extinct biosphere today? Or even more interesting, would it be possible to find Martian bacterial spores that survived for billions of years on Mars? Bacterial spores exhibit a remarkable resistance to adverse environmental conditions [1]. At the moment, however, no upper limit for the survival of bacterial spores has been established. We propose that the ionizing radiation dose on Mars (and in other environments in general) could well be the Achilles heel for the longterm survival of bacterial spores. To test this hypothesis, we have investigated how the radiation environment on Mars – on the surface, subsurface and potentially in fluid inclusions – would affect the survival of viable bacterial spores. Results: There are two distinct radiation environments on Mars: the first few meters are dominated by the effect of the Galactic Cosmic Radiation (GCR), while the deeper subsurface is characterized by radiation from the decay of radionuclides. The calculated radiation dose rate from the decay of radionuclides ranges from 350 μGy/year at 3 billion years ago to 130 μGy/year today [2]. The total accumulated dose over the last 3.1 billion years in the Martian sub-surface is close to 750 kGy [2]. This accumulated dose over time can be compared to the radiation inactivation constant of dry bacterial spores. The measured inactivation constant of dry B. thuringiensis, B. subtilis, and B. pumilus spores is in the range of 0.66-1 kGy [2], [3]. Dry bacterial spores show a temperature dependence in their radiation sensitivity, following a van-t’Hoff-Arrhenius law above 130 K [4]. The temperature corrected inactivation constant for a Martian sub-surface temperature of 220 K is 0.59-0.90 kGy for the referenced dry bacterial spores. The chances of finding viable bacterial spores in a 1-g sample of Martian soil is therefore less than 1 in a million after about 100 million years, depending on the radiation inactivation constant used, assuming an optimistic original bacterial concentration of 1x10/g of soil and a reduction of the population by 14 orders of magnitude (Fig. 1). Fig. 1 Calculated surviving fraction of dry bacterial spores in the Martian subsurface as a function of time. The dose rate is equivalent to early Mars.