Stratigraphy of Ice and Ejecta Deposits at the Lunar Poles - 2020

Astrogeology, Asteroids, Robots, Space Flight & Extraterrestrial Matters / Lunar Activities

Cannon and three co-authors reported in late 2020 that water ice has been delivered to the lunar poles from different sources over billions of years, but this accumulation was punctuated by large impacts that excavated dry regolith from depth and emplaced it in layers over the poles. Here, we model the resulting stratigraphies of ice and ejecta deposits in the lunar polar regions. Large polar craters were age dated, and their ejecta distributions calculated with standard scaling relations.

The authors then created a Monte Carlo model for ice deposition and ejecta emplacement. Typical model runs showed that deposits in older cold traps (>4 Ga) are divided into two zones: buried ice‐rich gigaton deposits and younger more gardened mantles. The latter are consistent with small crater morphometry measurements, but the existence of substantial ice buried at great depths is more difficult to confirm. Rare outlier model runs included Mercury‐like cases with significant deposition events in recent history (<200 Ma).

The authors concluded that in used a model of ejecta emplacement and stochastic ice deposition to simulate 10,000 possible stratigraphies in lunar cold trap environments. Typical model runs led to retention of >200 m of total ice in the oldest cold traps, with two distinct zones forming. The lower zone (“gigaton deposits”) consisted of thick ice‐rich layers with interspersed ejecta horizons, and the upper zone (“gardened mantle”) was thinner with lower implied ice fractions. Impact‐delivered ice exceeded volcanically derived ice by >400 times by mass. The majority of ice was deposited early, but a rare subset of model runs featured late, large impacts of hydrated asteroids leading to more Mercury‐like, thick surface deposits. Future exploration using drilling, trenching, and ground‐based remote sensing can be used to test predictions about ice deposits made here, and preserved microporous ice in these deposits could provide a robust record of past transient atmospheres around the Moon.