The Photochemistry of Unprotected DNA and DNA inside Bacillus subtilis Spores Exposed to Simulated Martian Surface Conditions of Atmospheric Composition, Temperature, Pressure, and Solar Radiation

DNA is considered a potential biomarker for life-detection experiments destined for Mars. Experiments were conducted to examine the photochemistry of bacterial DNA, either unprotected or within Bacillus subtilis spores, in response to exposure to simulated martian surface conditions consisting of the following: temperature (−10°C), pressure (0.7 kPa), atmospheric composition [CO2 (95.54%), N2 (2.7%), Ar (1.6%), O2 (0.13%), and H2O (0.03%)], and UV–visible–near IR solar radiation spectrum (200–1100 nm) calibrated to 4 W/m2 of UVC (200–280 nm). While the majority (99.9%) of viable spores deposited in multiple layers on spacecraft-qualified aluminum coupons were inactivated within 5 min, a detectable fraction survived for up to the equivalent of ∼115 martian sols. Spore photoproduct (SP) was the major lesion detected in spore DNA, with minor amounts of cyclobutane pyrimidine dimers (CPD), in the order TT CPD > TC CPD >> CT CPD. In addition, the (6-4)TC, but not the (6-4)TT, photoproduct was detected...