Possibility of microscopic liquid water formation at landing sites on Mars and their observational potential

•We predict the appearance of microscopic liquid water on Mars on hygroscopic mineral surfaces.•The ideal periods are mostly during nighttime between 2–7 am.•At most of the former landing sites such microscopic liquid could have appeared.•We predict these ideal periods for the ExoMars rover’s candidate landing sites.•This period at the primary landing site (Oxia Planum) is LS = 115–225 and in Local Time = 2–5. Microscopic liquid brines, especially calcium-perchlorate could emerge by deliquescence on Mars during night time hours. Using climate model computations and orbital humidity observations, the ideal periods and their annual plus daily characteristics at various past, current and future landing sites were compared. Such results provide context for future analysis and targeting the related observations by the next missions for Mars. Based on the analysis, at most (but not all) past missions’ landing sites, microscopic brine could emerge during night time for different durations. Analysing the conditions at ExoMars rover’s primary landing site at Oxia Planum, the best annual period was found to be between Ls 115–225, and in LocalTime 2–5, after midnight. In an ideal case, 4 h of continuous liquid phase can emerge there. Local conditions might cause values to differ from those estimated by the model. Thermal inertia could especially make such differences (low TI values favour fast cooling and H2O cold trapping at loose surfaces) and the concentration of calcium-perchlorate salt in the regolith also influences the process (it might occur preferentially at long-term exposed surfaces without recent loose dust coverage). These factors should be taken into account while targeting future liquid water observations on Mars.