A reassessment of perchlorate in samples from gale crater , mars

Introduction: Perchlorate anion (ClO4) was discovered in Martian soil by the Phoenix Lander in 2009 [1], with important implications for potential Martian biology, photochemistry, aqueous chemistry, and the chlorine cycle on Mars. Perchlorate is highly soluble, resulting in the potential for the formation of brines via freezing point depression, and it is also highly deliquescent. These combined capabilities have resulted in perchlorate being invoked in a wide range of observations and processes including RSL [2], putative subpolar cap liquid water [3], and the survivability and detection of organics at the Martian surface [4,5]. Support for the presence of perchlorate on Mars with a distribution beyond the north polar soils where it was originally detected is lent by the detection of perchlorate in Shergottite EETA79001 [6]. Subsequent analyses concluding a more global distribution of perchlorate based on Viking data [7] and orbital spectroscopy [8] have been forcefully challenged [9,10]. The first samples ingested by the SAM instrument on the Curiosity rover yielded temporally correlated releases of O2, HCl, and chlorinated hydrocarbons, interpreted as evidence for the presence of perchlorate throughout the rocks in Gale Crater [11]. The large O2 releases present in many samples subsequently analyzed have also been interpreted as evidence of perchlorate distributed through the Gale stratigraphic sequence [12]. SAM chlorine isotope measurements reveal highly variable chlorine isotopic signatures which indirectly support the presence of perchlorate by analogy to the isotope signatures of perchlorate on Earth [13]. Perchlorate Stability: The stability of perchlorate calls into question whether it could survive for billions of years as a component of ancient bedrock. Although it has a high level of kinetic stability, perchlorate is thermodynamically unstable [14]. Over long timescales the effect of ionizing radiation from radioactive decay and exposure to cosmic rays is expected to have a dramatic effect on perchlorate survival. Given the range of exposure ages and K contents measured in Gale, and reasonable estimates of U and Th content, we estimate a total fluence of 25-80 MGy over 3.5 Ga. Depending on the valence of the cation in the perchlorate salt, this range would result in the destruction of 75~100% of any ~3.5 Ga perchlorate based on perchlorate radiolysis data from the literature [15] (Figure 1). These considerations clash with the amounts of putative perchlorate in Gale Crater. Given the measured perchlorate content of 1.19 wt% in a SAM sample [12], the original perchlorate content of the rock would have to have been >10 wt%; the destruction of such a large fraction of the rock would result in compaction and disruption features, which are not observed in the undisturbed sedimentary layering in Gale.