Implications of the Presence of Surface Perchlorate for in Situ Detection of Organic Compounds during Future Missions

Introduction: One crucial discovery by the 2009 NASA Phoenix Lander was the detection of perchlorate at ~1% in the surface Martian regolith [1]. High concentrations of perchlorate may oxidize organics on Mars and be problematic during extraction of organics during future in situ investigations. High temperature pyrolytic extraction methods will likely result in catalytic degradation of organics [2] especially in the presence of oxidants or transition metals. If simulant studies reveal Martian soil chemistry to be incompatible with the survival of organics during pyrolytic extraction, then in situ instruments should utilize techniques that will not catalyze degradation during thermal volatilization (TV). Atacama Desert soils are known to contain large abundances of perchlorate [3], most likely derived from atmospheric photochemical deposition [4]. Previous GCMS analysis of samples from the hyperarid core of the Atacama Desert reported no organics above ppb-levels and claimed that this hyperarid region represented the dry limit for microbial life [5]. Amino acids at ppb-levels are detectable in the majority of samples from this region using subcritical water extraction (SCWE). It is unknown whether amino acid abundances are impacted by the presence of surface soil oxidants such as perchlorate in the Atacama. Discussion: Subcritical water extraction (SCWE) of amino acids has been successfully demonstrated on a variety of Atacama Desert soils (Fig 1) with high efficiencies compared to traditional quantification via acid hydrolysis. The characteristics of SCWE as a non-destructive and efficient extraction method is the reason it is the primary extraction step for the Urey organic detector [6] while it also may be integrated as a subsystem for other in situ instruments. Amino acid distributions and enantiomeric ratios can be used to determine whether their source is extant microbial life or diagenetic products of extinct life. Atacama Desert subsurface samples (>5 cm depth) show much better preservation of microbial biosignatures, and this has been interpreted as enhanced organic preservation via shielding from harsh surface conditions (e.g. destruction by UV-radiation, fenton oxidation, free radicals). It is unknown whether degradation of organics by surface oxidants, such as perchlorate, act as leveling reactions in the Atacama Desert. In order to examine whether amino acid abundances and distributions are affected by perchlorate concentrations, [ClO4] is currently being measured by ion chromatography (IC) within these Atacama Desert samples. Perchlorate concentrations are expected to be in low-ppm range due to levels observed in previous studies [3] and the extremely arid climate which enhances perchlorate deposition.