 METHANE EVOLUTION FROM UV-IRRADIATED SPACECRAFT MATERIALS UNDER SIMULATED MARTIAN CONDITIONS: IMPLICATIONS FOR THE MSL MISSION.  A. C. Schuerger 1, C. Clausen 2, and D. Britt 3.  1 University of Florida, Bldg. M6-1025, Space Life Sciences Lab, Kennedy Space Center, FL 32899; email: schuerg@ufl.edu; 2 Dept. of Chemistry, University of Central Florida, Orlando, FL 32816; email: clausen@mail.ucf.edu; 3Dept. of Physics, University of Central Florida, Orlando, FL 32816; email: britt@physics.ucf.edu.    Introduction: Methane (CH4) is an important trace gas in planetary atmospheres that contributes to heating of the atmosphere and can be produced from both biological and non-biological sources.  Because most methane on Earth is derived primarily from biological processes [1], there has been great interest in the detection of CH4 on Mars [2,3].  Recent publications [3,4,5] have demonstrated the presence of CH4 in the martian atmosphere with both spatial and temporal heterogeneity.  The Mars global average mixing ratio of CH4 appears to be constrained to ! 10 ppb (+/- 5 ppb), with spatial variations ranging between 0 and 45 ppb over the planetary surface [5].  However, the lifetime of CH4 in the Martian atmosphere has been estimated to be only ! 340 years [3,4] due to photocatalytic processes in the upper atmosphere.  Thus, if the CH4 detected on Mars is to be maintained, an active and contemporary source of CH4 evolution must persist near the surface. The primary atmospheric methane detection instrument on the Mars Science Laboratory (MSL) rover is the Sample Analysis at Mars (SAM) instrument package which includes a Tunable Laser Spectrometer (TLS) designed to measure trace levels of methane and its carbon isotopic composition [6].  The TLS is supported by a Chemical Separation and Processing Laboratory (CSPL) that will valve and pump an appropriate amount of gas through an inlet tube to the TLS.  The TLS uses a Herriott cell spectrometer with a channel at 3.27 microns for CH4.  Sensitivity is estimated at 2 ppb for direct measurements of methane and < 10 per mil for measurements of the methane 13 C/ 12 C ratio.  The target upper limit for terrestrial contamination in SAM is currently set at 40 ppb of total reduced carbon [6].   The objective of the research was to characterize the CH4 evolved from organic compounds and spacecraft materials likely to be found on the MSL rover.  The research was designed to determine if CH4 production from spacecraft materials under martian conditions might complicate the interpretation of the TLS data and prevent the MSL rover from accurately measuring CH4 abundance in the martian atmosphere.   Materials and Methods:  A Mars Simulation Chamber (MSC; Fig. 1) was used to create conditions similar to equatorial Mars (described in full by Schuerger et al. [7].  The MSC system can accurately simulate five key components of the surface environment of Mars including: (a) pressures down to 0.1 mb; (b) UVC, UVB, and UVA irradiation from 190 to 400 nm; (c) dust loading in the atmosphere from optical depths of 0.1 to 3.5; (d) temperatures from -100 to +30 C; and (e) an atmospheric mixture of CO2 (95.53%), N2 (2.7%), Ar (1.6%), O2 (0.13%), and H2O (0.03%).   Fig. 1.  Mars Simulation Chamber (MSC). Two Organic Reaction Vessels (ORV; Fig. 2) were placed on the upper surface of a liquid nitrogen thermal control plate, and fitted with gas sampling lines.  Pictured in Fig. 2 are the gas sampling lines for a Qudrapoule Residual Gas (RGA) analyzer (left lines on each ORV) and an external sampling port (right lines wrapped in aluminum foil on each ORV) used to collect CH4 samples.   Fig. 2.  Organic Reaction Vessels (ORV's) loaded with 1-g each of benzoic acid (left) and glucose (right). Spacecraft materials tested.  A diversity of organic compounds were tested for CH4 production including amino acids, aldehydes, fatty acids, nucleotides, DNA, adenosine triphosphate (ATP), and polycyclic aromatic hydrocarbons (PAH's).  In addition, several spacecraft materials were tested including a methylated siliconebased grease, kapton tape, a spectral imaging calibration target (grey-toned RTV), and iridited (i.e., ChemFilm) treated aluminum.  And lastly, intact bacterial spores from Bacillus subtilis HA101 were tested to determine if biological cells would evolve CH4 under UV-irradiation within the Mars chamber.   Experimental procedures.  Compounds were loaded into the ORV units (Fig. 2), the ORV's were closed, the Mars chamber was sealed, and all components equilibrated at the martian conditions required for each test.  Ultraviolet irradiation was then applied to the top surfaces of the organic compounds at a rate of 4.1 W/s 2 (3.6 kJ/m 2/hr) for UVC (200-280 nm).  The atmospheric pressure was maintained at 6.9 mb with a Mars gas mixture (see above) containing CO2, Ar, N2, O2, and H2O vapor.  The temperature was maintained for most tests at +20 C.  The organic compounds were exposed exactly 8 hrs, and then gas samples collected from the head space over each material in each ORV.  The sampling procedure was tested and calibrated extensively with a series of controlled lab standards.  Results from the control assays indicated that (1) the accuracy and precision of quantitative CH4 measurements was in the range of +/- 0.5 ppm; (2) the concentrations of CH4 in all cal-gases (10, 25, 50, or 100 ppm in Mars gas, N2, or Ar) were accurately measured with the experimental system; and (3) CH4 in the air column was degraded by UV catalysis within the ORV units.  Background and CH4 degradation rates were factored into the estimates of CH4 production for each material.   Samples were collected from the ORV's in 75 ml Sulfinert! treated metal sample cylinders.  The sample cylinders were evacuated to a pressure of 0.01 mb, and then used to withdraw 112 cc of head space atmosphere within each ORV unit.  Samples were analyzed using a Perkin Elmer Clarus 500 gas chromatograph equipped with a flame ionization detector.  Methane calibration curves were generated by analyzing samples made up from ultrahigh purity argon gas doped with CH4 at 10, 25, 50, or 100 ppm.   Results: All organic compounds evaluated in these assays yielded CH4 when irradiated with 4.1 W/m 2 of UVC photons under martian conditions.  The PAH, pyrene (43.9 ppm) yielded the highest, and ATP (6.8 ppm) yielded the lowest CH4 concentrations after 8 hrs of UVC exposure at 6.9 mb.  The low yield for ATP is consistent with earlier work that described long residence times for ATP on spacecraft materials exposed to UV irradiation under martian conditions [7].  The average of all organic compounds tested was 14.4 ppm CH4 evolved per 8 hrs of UVC exposure (1 sol on Mars).  Bacterial endospores from Bacillus subtilis evolved CH4 at the rate of 4.2 ppm per 8 hrs, suggesting that both microbial and organic contamination on the upper deck of the MSL rover will yield CH4 when illuminated by solar UV on Mars.  And lastly, all spacecraft materials yielded CH4 upon UV irradiation under martian conditions.  The grey-RTV spectral imaging target (prepared and flown on the MER rovers and Phoenix lander; D. Britt, unpublished) yielded an average of 3.2 ppm CH4 above background levels (2.8 ppm).  Most surprising was that Kapton tape (used extensively in all recent Mars lander/rover missions) yielded 7.4 ppm CH4 during assays.   Discussion:  The results suggest that CH4 derived from UV irradiation of spacecraft surfaces on the MSL rover may be a significant contamination for the SAM instrument.  The sensitivity for the TSL instrument is 2 ppb, and the current standard for the level of terrestrial contamination for the MSL rover is 40 ppb [6].  However, we easily measured CH4 levels as high as 40 ppm (pyrene) from a range of compounds likely to be found on the MSL rover.  The difference between the sensitivity of the TSL instrument to CH4 and the levels of CH4 derived from spacecraft materials is over three orders of magnitude.  We conclude that CH4 contamination from UV-irradiated spacecraft materials (RTV, kapton tape), organics (PAH's, organic acids, biosignature molecules), and microbial bioloads (B. subtilis spores) are likely to raise the levels of CH4 around the MSL rover, and may impair the sensitivity of the SAM instrument.  Based on these results, we make three testable predications for the MSL mission: (1) CH4 will exhibit a diurnal periodicity due to UV irradiation of organic contamination and microbial bioloads on sun-exposed surfaces; (2) the CH4 contamination will be lessened if the surface winds are strong, or the SAM intake port is pointed upwind; and (3) background CH4 levels from the MSL rover should decrease over time as the rover is covered by aeolian dusts and the organic and microbial contamination is "burned-off" by solar UV irradiation.   References: [1] Keppler et al. (2006) Nature, 439, doi: 10.1038/nature04420. [2] Formisano et al. (2004) Science, 306, 1758-1761. [3] Krasnopolsky et al. (2004) Icarus, 172, 537-547. [4] Summers et al. (2002) Geophys. Res. Lett., 29(4), doi:10.1029/2002GL 015377. [5] Mumma et al., (2009) Science 323, 10411045. [6] Mahaffy (2008) Space Sci. Rev. 135, 255268. [7] Schuerger et al. (2008) Icarus, 194, 86-100.  
