Impacting Lunar Polar Ice

The regolith overturning or gardening process of the top meter of material on the Moon and Mercury. The deeper the material, the longer the time before the ice fully sublimates. For instance, a 1 meter ice deposit on the Moon would sublimate away in about 200 million years, while it would survive over 1 billion years on Mercury. The Moon receives about 10 times more impacts than Mercury, causing this difference. Credit: Costello et al., 2019, Figure 1.

The permanently shadowed regions (PSRs) at the lunar poles represent prime locations for finding high concentrations of water bearing material on the Moon. Even though PSRs have been remotely observed for the past few decades, much is still unknown about these areas. Key questions include the origin of the ice, how extensive it is, and how it changes over time. A recent model addresses some of these questions by predicting how long near-surface ice should exists before being sublimated away.

A presentation at the 2019 Lunar and Planetary Science Conference discussed a model that predicts the rate at which near-surface ice sublimates based on impact caused regolith turnover. Surface ice sublimates when exposed to warm temperatures and the vacuum of space. The sublimation process is dramatically slowed for ice under a few meters of regolith.

Object impacts cause surface regolith to get ejected and relocate deeper regolith towards the surface. This process is called impact gardening, mixing, or overturning. It can repeatedly invert material though large disturbances. The model presented suggests that the more impacts that occur, the more material mixing that occurs, and the more ice that is sublimated within the disturbed area.

The model uses Mercury as an analog environment, specifically the extensive surface ice deposits on the poles of Mercury. Despite having a very high daylight surface temperature, substantial surface ice exists within deep craters at the poles of Mercury. This is due to Mercury having an extremely low axial tilt (about 0.027 degrees; the Moon has an axial tilt of about 1.54 degrees), so the Sun never rises far above the horizon when observing from the poles. The Mercury ice deposits are unique because they have likely survived hundreds of millions (if not billions) of years.

The polar environments between the Moon and Mercury are very similar except for the number of impacts that occur. It is estimated that objects with diameters between 1 cm and 100 m impact the Moon ten times more than Mercury. This difference means that there is much more regolith turnover on the Moon than Mercury, causing disturbed ice deposits to sublimate quicker.

Depending on the initial ice deposit thickness, the longer the ice can exist before being sublimated away. To match the current observations of scattered ice, the initial ice deposit would have had to initially be between 5 and 10 meters thick to survive about 3.5 billion years. Credit: Costello et al., 2019, Figure 3.

The model predicts sublimation rates of ice deposits at variable depths. Based on a one meter thick ice deposit, the model predicts that it would be pulverized by the frequent lunar impacts within 200 million years. Meanwhile, a similar one meter thick ice deposit on Mercury would survive over 1 billion years.

Another interesting use of the model is predicting how thick an initial regolith ice deposit would have needed to be to survive from formation to today. Depending on how many impacts occurred (and regolith turnovers), the initial deposit would have needed to be between 5 and 10 meters in depth in order for 10% of the ice to remain today.

This model is relevant for understanding ice within the PSRs. It shows that frost ice on the surface within PSRs is unlikely to survive impact disturbances long term. This matches the scattered concentrations of surface ice observed today. Also, ice deposits deeper than ten meters under the surface are likely to still exist today with high concentrations of ice remaining.

While not direct proof of ice deposits, models like this help explain the observations we see today. Pure water ice probably won’t be conveniently waiting on the surface for us to collect, but models like this will help us learn where to find large quantities of ice.