Stratigraphy of Ice and Ejecta Deposits at the Lunar Poles

Abstract

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. We 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). Plain Language Summary The polar regions of Earth's Moon have topographic depressions that are never directly exposed to the Sun, so they are cold enough for deposits of ice to exist. Water can get into these regions by water-bearing asteroids colliding with the Moon, or from lunar volcanoes erupting gases that travel to the poles. At the same time, large impact craters that form at the poles eject an enormous amount of soil and rock that could bury existing ice. It is not well understood how these two processes work together to build up deposits that may have alternating layers of ice-rich and ice-poor soil. In this study, we used computer simulations to predict what these layered deposits may look like. We found it is likely that large amounts of relatively pure ice are buried at depth in the oldest deposits, covered with thinner layers hosting less ice. Impact cratering has been the dominant process affecting the lunar poles, but the effects of large polar craters on nearby ice deposits have not been previously addressed. Impact effects have been considered for micrometeoroids (e.g.,

Temperatures Near the Lunar Poles and Their Correlation With Hydrogen Predicted by LEND

“…Although aforementioned processes most certainly contribute to observe polar hydrogen abundance in general, as can be clearly seen by the fuzzy region of hydrogen abundance in Figure 8, we suspect that local asteroidal and cometary impacts are most likely the source of delivery to the regions with distinctively larger hydrogen abundance. This finding agrees with modeling results in the study by Cannon et al (2020). Here, we suggest that possible impactors mainly contribute to the immediate vicinity of the impact site for the same reasons as stated above; nearby PSRs would otherwise show enhanced hydrogen signals as well.…”

“…Despite the fact that they are in direct vicinity, Shackleton shows no distinct sign for hydrogen. Modeling results from Cannon et al (2020) show similar results and suggest that the relatively younger age of Shackleton could be the reason for the lack of significant water ice.…”

“…Modeling results from Cannon et al. (2020) show similar results and suggest that the relatively younger age of Shackleton could be the reason for the lack of significant water ice.…”

Secondary Impact Burial and Excavation Gardening on the Moon and the Depth to Ice in Permanent Shadow

“…Deutsch et al (2020) showed that spectral evidence of surface ice is concentrated within the permanent shadowed regions of craters older than the Copernican era, while Copernican craters are relatively ice-poor. Cannon et al (2020) showed that large ancient impacts may have produced layers of ice and ejecta to fill ancient craters and Needham and Kring (2017) showed how ancient volcanic outgassing may have provided large quantities of water to the lunar poles 3.5 billion years ago. If the majority of lunar ice is ancient (older than 3 Ga), then it will have been depleted by in-situ reworking at least as deeply as 3 m and pulverized in the degradation zone at least as deeply as 10 m over the last 3 Ga (Figure 6).…”

“…Impacts follow a negatively sloped power law; thus, numerical modeling of concurrent larger and smaller-scale processes requires exponentially more simulated impactors as the scale range increases. The simulation by Cannon et al (2020) assumed small-scale gardening like that we model here evenly processes the top meter in their simulations. Similar applications of our analytical gardening model to simplify more computationally expensive numerical modeling may be useful moving forward.…”