Size–frequency measurements of meter-sized craters and boulders in the lunar polar regions for landing-site selections of future lunar polar missions

The concentrations of volatiles, especially hydrogen (or water), at the lunar poles have been reported from remote sensing data in previous studies. Many future missions are targeting to investigate the permanently shadowed regions inside polar craters, where water ice can exist because the temperature is extremely low. Accordingly, landing sites are being planned around polar craters. We conducted a geomorphological analysis of the ejecta blankets of Shackleton Crater at the South Pole (89.63°S, 130.0°E) and Whipple Crater at the North Pole (89.14 N°, 119.5°E). Size–frequency measurements of the craters and boulders were derived from high resolution (~1 m/pix) image data obtained by the Narrow Angle Camera onboard the Lunar Reconnaissance Orbiter. The number of identified craters (diameter > 10 m) and boulders (diameter > 3 m) were 35,799 and 1273, respectively, around Shackleton and 17,074 and 3092, respectively, around Whipple. The number densities of boulders around fresh craters on Shackleton and Whipple rim are lower than at the Apollo and Luna landing sites, suggesting a low production rate of boulders in these regions. The number density of small craters is lower than expected from the formation ages of Shackleton and Whipple and is strongly correlated with regional topographic slopes, implying that small craters were eliminated by resurfacing with mass wasting from the continuous ejecta of Shackleton and Whipple. Ejecta blankets from small craters may cover and preserve past subsurface accumulations of water molecules. Based on the crater counting data, we propose feasible landing sites around Shackleton Crater and scenarios for exploring local subsurface water using excavators in future missions.