A paper published in the January 2020 issue of Icarus examines the relationship between the age of lunar craters and the abundance of surface ice. Surface ice at the lunar south pole is found predominantly in ancient craters, 3.1 billion years or older. Some smaller, newer craters also host surface water. But surface icy is “very patchy” in spatial distribution, suggesting high overturn or destruction rates by meteor and micrometeor bombardment.
The authors examined 20 south polar craters that host surface water ice and quantify the available cold-trapping surface area occupied by water ice, according to the abstract for “Analyzing the ages of south polar craters on the Moon: Implications for the sources and evolution of surface water ice.”
States the abstract:
“The majority of surface ice is contained in old craters ≥∼3.1 Gyr, where the majority of cold-trapping area on the pole exists. The ice [in] these ancient craters is very patchy in surficial distribution, occupying <11.5% of cold-trapping surface area available in individual craters. This patchy distribution of ice in old craters is likely to be due to impact bombardment and regolith overturn within the polar regions.
“Interestingly, surface ice is also located within smaller craters (<15 km in diameter), whose sharp crater rim crest morphologies suggest that they may be relatively young. Ice in fresh-looking craters suggests that ice has been delivered to the lunar surface more recently, perhaps from micrometeorites or through solar wind interactions with the lunar regolith.
“Finally, we also analyze a group of ancient craters that does not host surface water ice, even though these craters are present-day cold traps. These specific ancient craters would not have been thermally stable for the cold-trapping of water ice before the onset of true polar wander suggested by Siegler et al. (2016). If true polar wander did occur on the Moon, then the ages of ice-bearing craters presented here set an upper limit for the age of post-true polar wander hydrogen emplacement of 4.1 ± 0.1 Gyr.”
Notes to readers:
Gyr =1 billion years
“True polar wander” refers to the shifting of the poles due to changes in the rotation of a planet or moon.