Methane evolution from UV-irradiated spacecraft materials under simulated martian conditions: Implications for the Mars Science Laboratory (MSL) mission

Fifteen organic and three inorganic compounds were tested for methane (CH4) evolution under simulated martian conditions of 6.9 mbar; UVC (200-280 nm) flux of 4W m(-2); 20 degrees C; simulated optical depth of 0.1; and a Mars gas composition of CO2 (95.3%), N-2 (2.7%), Ar (1.7%), O-2 (0.13%), and water vapor (0.03%). All three inorganic compounds (i.e., NaCl, CaCO3, graphite) failed to evolve methane at the minimum detection level 0.5 ppm, or above. In contrast, all organic compounds evolved methane when exposed to UV irradiation under simulated martian conditions. The polycyclic aromatic hydrocarbon, pyrene, released the most methane per unit of time at 0.175 nmol CH4 g(-1) h(-1), and a spectral reflectance target material used for the MER rovers and Phoenix lander released the least methane at 0.00065 nmol CH4 cm(-2) h(-1). Methane was also released from UV-killed bacterial endospores of Bacillus subtilis. Although all organic compounds evolved methane when irradiated with UV photons under martian conditions, the concentrations of residual organics, biogenic signature molecules, and dead microbial cells should be relatively low on the exterior surfaces of the MSL rover, and, thus, not significant sources of methane contamination. In contrast, kapton tape was found to evolve methane at the rate of 0.00165 nmol CH4 cm(-2) h(-1) (16.5 nmol m(-2) h(-1)) under the UV and martian conditions tested. Although the evolution of methane from kapton tape was found to decline over time, the large amount of kapton tape used on the MSL rover (lower bound estimated at 3 m(2)) is likely to create a significant source of terrestrial methane contamination during the early part of the mission. (C) 2011 Elsevier Inc. All rights reserved.