. "4 Drilling and Sampling Technologies and the Potential for Contamination." Exploration of Antarctic Subglacial Aquatic Environments: Environmental and Scientific Stewardship. Washington, DC: The National Academies Press, 2007.
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Exploration of Antarctic Subglacial Aquatic Environments: Environmental and Scientific Stewardship
and higher sensitivity was expected with the use of smaller bore GC columns or possibly a less volatile PFT (Smith et al. 2000). Results of PFT measurements suggested that drilling fluid infiltration was in the range of nanoliters per gram of core material, leading researchers to conclude that depending on the coring methods used and the formations sampled, contamination may represent at most 1-10 bacteria per gram of core material. Intrusion of fluorescent microspheres into the core interior was not detected in the APC-cored unconsolidated sediments or RBC-cored consolidated sediment or rock. However, microspheres were detected in thin sections tested from the igneous rock samples, leading researchers to conclude that processing of the rock after sample extraction was the likely source of contamination (Smith et al. 2000). PFT and fluorescent microsphere tracers have seen continued use in deep sea subsurface sampling conducted by the IODP (D’Hondt et al. 2004), and methodologies for contamination-free sampling and monitoring continue to be investigated (Lever et al. 2006).
POTENTIAL FOR TESTING AND ASSESSING CONTAMINATION:EXPERIENCES FROM INTERPLANETARY RESEARCH
Requirements for cleaning are driven by mission requirements (as set out by the National Aeronautics and Space Administration [NASA]) and are based upon landing or regional concerns on the target planet or moon. Current sterilization levels are based on criteria established during the Viking pre-launch biological research conducted in the 1970s and are accepted and implemented by NASA as current requirements. Examples of specific measures to control contamination include reduction of the spacecraft’s biological burden, a spacecraft organic inventory and material archive, and documentation of spacecraft trajectories and restrictions on returned samples.
Reduction includes sterilization and cleaning by approved and certified techniques. Sterilization requires that the entire landed system be sterilized at least to Viking post-sterilization biological burden levels or to levels of biological burden reduction driven by the nature and sensitivity of the particular life detection experiments, whichever are more stringent. Subsystems that are involved in the acquisition, delivery, and analysis of samples used for life detection must be sterilized to these levels, and a method for preventing recontamination of the sterilized subsystems is also required. Terminal sterilization methods (the terminal Viking bake-out process) were conducted after the lander had been sealed inside the aeroshell and bioshield capsule. The entire capsule was placed inside a thermal chamber (specially designed for the sterilization process) and the methods were verified by proxy; a sterilized spacecraft or component is never assayed directly. The acceptable terminal sterilization bioburden level was derived from the Viking pre-sterilization level of 3 × 105 spores per vehicle, which was reduced after bake-out by four orders of magnitude to a calculated value of 30 spores per vehicle. Such an approach using heat treatment is clearly impracticable in general for Antarctic sampling at present, although it may prove possible to apply it to the final sampling assembly if this is sealed inside a shell for transfer down the drill hole.
Cleaning requirements for spacecraft or parts that do not require terminal sterilization are driven by the pre-bake-out bioburden levels determined analytically on the Viking landers to be 3 × 105 spores per vehicle. Bioburden reduction techniques include clean-room assembly practices, dry heat to reduce the number of microbes, and physical cleaning of spacecraft components and surfaces. Clean-room assembly requirements