Environmental observations of present day Mars and implications for habitability

The history of life on Earth, as we known it, began about 3.8 billion years ago, initially with single-celled life forms that evolved to the variety of life forms that we know now. As for Mars, it has been recently concluded, based on the observations of NASA's Curiosity rover at Gale crater, near the equator of Mars, that the physical, chemical, and energy conditions that were required to establish a habitable environment were present on Mars between 3.8 billion and 3.1 billion years ago [Hurowitz et al. 2017]. However, the habitability of present day Mars is still open to debate. An environment is defined as habitable if it is capable of supporting the activity (metabolic activity for survival, growth, and reproduction) of at least one organism [Cockell et al. 2016]. In order to investigate the present-day habitability of Mars, we review the Curiosity observations at Gale crater of the environmental and regolith properties that may affect life, based on our knowledge of life on Earth. We analyse the potential present day habitability of Mars as constrained by temperature, cosmic radiation, UV radiation, presence of carbon compounds, nitrogen, phosphate minerals, and Fe and S minerals, in a variety of redox states within the regolith, and, in particular, we discuss where and when liquid may be transiently stable on present day Mars [Martín-Torres et al. 2015]. We discuss where in the surface or subsurface of Mars life could exist and obtain energy from photosynthesis or chemosynthesis. Based on these parameters we discuss about the kind of instrument and sample measurements that may be required to assess unequivocally the habitability of present day Mars and the implications that this may have on the future exploration of Mars. References: Hurowitz et al. "Redox stratification of an ancient lake in Gale crater, mars", Science, Vol. 356, Issue 6341, 2017. doi: 10.1126/science.aah6849 Cockell et al. "Habitability: A Review". Astrobiology. 2016 Jan;16(1):89-117. Martín-Torres et al., "Transient Liquid water and water activity at Gale crater on Mars", Nature Geoscience, doi:10.1038/ngeo2412, 2015.