FIGURE 4.

Overview of potential redox substrate sources for nitrate-dependent iron oxidizing microorganisms in the early Mars environment. Nitrates are produced from an early atmospheric nitrogen reservoir by fixation from volcanic lightning (Stern et al., 2015), thermal shock during impacts (Summers and Khare, 2007), and irradiation from solar and cosmic sources (Smith et al., 2014). Reduced iron is released into aqueous environments by mineral dissolution, a process accentuated by hydrothermal activity (Emerson and Moyer, 2002; McSween et al., 2009). A fuller description of abiotic nitrogen fixation pathways is available in Summers et al. (2012).