Potential impact-induced water-solid reactions on the Moon

Water (ice, liquid, or vapor) is a critical driver of future exploration, and methods of its detection and characterization are a high priority for upcoming lunar missions. Thus, we assess the potential for alteration products resulting from water-ice liberated during various impact events in the lunar polar regions. In this work, we estimate the maximum amount and duration of melted, vaporized, or sublimed water-ice during representative post-impact environments using a model of bulk heat transfer. Our model is sensitive to heat loss by radiation, initial and final near-surface temperatures, and pre-existing water-ice abundance and distribution. Mineral dissolution rates in aqueous solution are used as a metric for potential chemical alteration in the presence of liberated water-ice following an impact. We find that the modeled timescales and potential for water liberation and reactivity are compatible with near-surface chemical alteration in some lunar post-impact environments. While initial surface temperatures less than ∼110 K are adequate to maintain near-surface ice reservoirs at the lunar poles, when heated, pore pressures below a depth of ∼35 cm are potentially adequate to sustain liquid water. Mild near-surface environments (e.g., ∼5 °C) lasting a few decades, allow for aqueous alteration of sensitive minerals such as olivine, apatite, and glassy materials. Higher temperatures favor degassing of H₂O, but vapor-phase interactions may occur. The limited amounts of available water will likely result in reactions with only the most sensitive minerals such as glasses and Fe-metal. Over time, secondary mineralization would be mixed into the upper few meters of the lunar regolith through subsequent bombardment, assuming it escapes later intense heating events; however, surface exposures would be subjected to space weathering. Nonetheless, based on our modeling, future explorers should consider instrumentation capable of detecting minor to trace amounts of impact-induced chemical alteration in the upper few meters of the lunar surface.