Using Boulder Tracks as a Tool to Understand the Bearing Capacity of Permanently Shadowed Regions of the Moon

by Sargeant, H. M.; Bickel, V. T.; Honniball, C. I.; Martinez, S. N.; Rogaski, A.; Bell, S. K.; Czaplinski, E. C.; Farrant, B. E.; Harrington, E. M.; Tolometti, G. D.; Kring, D. A.

Permanently shadowed regions (PSRs) are abundant at the lunar poles. They experience no direct sunlight and reach temperatures as low as 30 K. PSRs are of interest as evidence suggests that some may contain water ice (H2O/OH-), which could provide a record of the evolution of volatiles in the inner solar system. This water ice is also a critical resource for life-support systems and rocket propellant. A better understanding of mechanical properties of PSR regolith, such as its bearing capacity, will help optimize the design of future exploration rovers and landers. Thirteen boulder tracks were identified on the edge of, or inside, south polar lunar PSR enhanced imagery and used to estimate the strength of the PSR regolith at latitudes of 70 degrees to 76 degrees in sites with maximum annual temperatures of 65 to 210 K. PSR boulder track features are similar to those observed in highland, mare, and pyroclastic regions of the Moon, implying similar properties of the regolith. Measured features were used to estimate bearing capacity for PSR regolith at depths of similar to 0.28 to 4.68 m. Estimated bearing capacity values suggest that these PSRs may be somewhat stronger than highland and mare regions at depths of 0.28 to 1.00 m. Bearing capacity in these PSRs is statistically the same as those in other regions of the Moon at depths of 1.00 to 2.00 m. The results of this study can be used to infer bearing capacity as one measure for the trafficability of lower-latitude PSRs of the type measured here. Plain Language Summary The polar regions of the Moon contain areas that never experience sunlight, known as permanently shadowed regions (PSRs). These regions are thought to contain uneven distributions of water ice deposits. To access PSRs and their possible water deposits, we must first understand the strength of the soil at these locations to safely traverse them. Thirteen boulder tracks were identified on the edge of, or within, PSRs in the lunar south polar region using images taken by the Lunar Reconnaissance Orbiter. These images were processed to enable the measurement of boulders and their associated tracks. The tracks identified within PSRs have similar appearances to those identified outside PSRs in other regions on the Moon. The strength of soil within PSRs was estimated from the measurements taken and was shown to be at least as strong as highland and mare regions of the Moon at relatively shallow depths, although the studied PSRs show no evidence for the presence of water ice. Analysis shows that PSRs of the type measured here should be able to bear rovers at depths of at least similar to 30 cm. In situ measurements are required to confirm and better understand the mechanical behavior of PSR regolith at shallow depths.

Journal of Geophysical Research-Planets
2169-9100; 2169-9097