1 Introduction Knowledge of the spatial and temporal distribution of tropical water ice is important to constraining the scope of habitability, climate, Mars geology, and to prepare for human exploration, for example, Drake et al. (2010). Analyses of geologically young equatorial layered ejecta craters, many of which are plausibly produced by impacts into an ice-rich layer, suggests that the ice is not just of the past, for example, Li et al. (2015); Reiss et al. (2005); Reiss et al.
