Abstract EANA2024-106

The Viking Labeled Telease (LR) experiment (1976) aimed to detect the presence of active microbial life on the surface of Mars. When a nutrient medium with 14C-labeled organic components was added to a sample of Martian regolith, the radiolabelled carbon was quickly emitted (most likely as CO2), just as if microbial respiration had occurred. In control experiments, preheating the soil to 50°C prior to nutrient injection resulted in a weaker emission; preheating it to 160°C resulted in no emission; and long-term storage in the dark at 10°C resulted in no emission [1]. The same results were obtained at both Viking landing sites, 6400 km apart [1]. There is a near-consensus that they represent a “false positive” for life detection and that the nutrients were oxidised abiotically. However, it is still unclear precisely which oxidants were responsible. Attention initially focused on perchlorate salts [2] (found in particular abundance by the Phœnix mission [3]) but these are too thermally stable to explain the results of the control experiments. Hence, Quinn et al. [4] showed experimentally that the real suspect may be hypochlorite (ClO–), which is suitably thermally unstable and should be present in Martian soil as a radiolytic and photolytic degradation product of perchlorate, albeit possibly at lower concentrations than they used in their experiments [5]. Here, we describe the current state of understanding of perchlorates on Mars and study the nature of their photolytic degradation products. We review their potential to explain the results of the Viking LR experiment, with a particular focus on hypochlorite, which is a prime candidate due to its instability at elevated temperatures. A new experimental direction is outlined to assess the properties and steady states of the products.

 

References

[1] Levin, G., V., Straat, P. A. (2016) The Case for Extant Life on Mars and Its Possible Detection by the Viking Labeled Release Experiment. Astrobiology 16(10), 798-810, doi: 10.1089/ast.2015.1464.

[2] Navarro‐González, R., Vargas, E., de La Rosa, J., Raga, A. C., & McKay, C. P. (2010). Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars. Journal of Geophysical Research: Planets, 115(E12).

[3] Hecht, M. H., Kounaves, S., P., Quinn, R., C., West S., J., Young, S., M., M., Ming, D., W., Catling, D., C., Clark, B., C., Boynton, W., V., Hoffman, J., DeFlores, L., P., Gospodinova, K., Kapit, J., Smith, P., H. (2009). Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site. Science 325,64-67, doi:10.1126/science.1172466

[4] Quinn, R. C., Martucci, H. F. H., Miller, S. R., Bryson, C, E., Grunthaner, F. J., & Grunthaner, P. J. (2013). Perchlorate Radiolysis on Mars and the Origin of Martian Soil Reactivity. Astrobiology 13(6), 515-20, doi: 10.1089/ast.2013.0999.

[5] Hassler, D. M et al. (2014) Mars’ Surface Radiation Environment Measured with the Mars Science Laboratory’s Curiosity Rover. Science 343,1244797, doi:10.1126/science.1244797