When airless planetary bodies host water ice on their surfaces, water ice is continuously exposed to various processes. The primary contributors to the water ice redistribution in surface and near-surface regions include impact-driven mixing and thermal evolution. The cycle of water ice is complex, and thus integrated analysis requires consideration of the total effects on the water ice distribution. Here, we introduce a 1-Dimensional model that accounts for both impact mixing and thermal diffusion. The impact mixing process is modeled using an approach that statistically computes the time evolution of the average mixing depth [Hirabayashi et al., 2018]. At each time epoch, an impact cratering event can uniformly mix regolith and water ice. Thermal diffusion is modeled by tracking the amount of incoming and outgoing sublimated H2O molecules at depth [Schorghofer and Williams, 2020; Reiss et al., 2021]. The results show that depending on water supply and thermal conditions over the history of planetary surfaces, the water ice concentration becomes significantly different (see Figure 1 for a lunar case as an example). If water ice is continuously supplied, a top surface layer may have a higher water ice concentration than a subsurface layer (Figure 1a). This water concentration variation results from a shallower mixing depth within a shorter period. This condition can further be modified by thermal diffusion, depending on the surface temperature. At a top surface layer and a deeper layer, thermal diffusion is usually high, and thus the amount of water becomes low; water molecules tend to go up at the top layer and go down at a deeper depth (if vapor pressure is lower). If water ice is impulsively supplied at an earlier stage of surface evolution and has continuously been mixed with dry regolith, only a limited amount of water ice mixed with regolith remains at a deeper depth (Figure 1b).
Hirabayashi et al. (2018), JGR Planets, 123, 527-543.
Schorghofer and Williams (2020), PSJ, 1, 54
Reiss et al. (2021), JGR Planets, 126, e2020JE006742.