Subcritical water extraction methods for future Mars missions

• Main Author: Luong, Duy Tony
• Other Authors: Sephton, Mark
• Published: Imperial College London 2015
• Subjects: 523.43
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.686321

The discovery of water ice by the Phoenix Mars lander in 2008 and gypsum by Opportunity Rover in 2011 provides evidence that Mars was once a wet environment and therefore it could have sustained life in the past. Subsurface microbial life may be thriving on present Mars. The use of pyrolysis to liberate potential organic matter on Mars is known to cause mineral matrix-analytes interaction and result in total absence of detectable compounds. Solvent based extraction techniques were explored as alternatives to conventional pyrolysis for future Mars missions. Subcritical water extraction outperformed surfactant extraction but lagged behind organic solvent extraction where Martian regolith analogue was used. The study also showed that the optimum conditions of subcritical water extraction are an extraction temperature of 300 oC and an extraction duration of 20 minutes. Molecular transformations under hot aqueous conditions were also observed in the same study, for instance, the structural change of anthracene to aromatic diketone. Subcritical water extraction of organic matter bearing sedimentary rocks produced a range of organic compounds diagnostic of microbial and plant materials. The outcomes of subcritical water extraction of sedimentary rocks can help predict the subcritical water treatment responses of biotic chemical classes that may still survive on Mars. The chemical structures of plant organic compounds are similar to chemical structures of meteoritic macromolecules and the results of subcritical water extraction of type III organic matter can help distinguish the abiotic organic compounds from the biotic hydrocarbons on Mars. Further subcritical water experimentation using sulphate and iron rich samples collected from a low pH environment demonstrated the habitability of sulphate and iron rich environments on Mars and the capability of subcritical water system to isolate useful organic biosignatures from an active microbial community with inputs of plant materials. A comparison between two different extraction modes of subcritical water showed the superior performance of the static mode compared to dynamic extraction. Heavy oil sand extraction using subcritical water provided further evidence of the usefulness of subcritical water technology beyond the realm of space research.