Surface Strength of PSRs
Boulder tracks on the Moon. The left image is in Aristarchus (highland slope) and the right image is near Schrödinger (PSR). Regolith bearing strength can be inferred from the shape of boulder tracks. Credit: Sargeant et al., 2019, Figure 2.
Although multiple landers and rovers have touched down on the Moon, no vehicle has visited the Permanently Shadowed Regions (PSRs) at the lunar poles. Water ice has been observed within some PSRs, and therefore represents an ideal target for future lunar missions. An important unknown to study before the first PSR mission is understanding the surface environment, including how much mass the PSR regolith can support. It is critical that rover wheels and landing pads don’t sink into the regolith. To study this, a recent analysis used boulder tracks to evaluate the bearing capacity of regolith in these areas.
The Moon’s small axial tilt of 1.54 degrees away from the ecliptic causes some craters to experience complete and continuous darkness. These areas are called Permanently Shadowed Regions (PSRs). Without sunlight, these permanently dark craters have been observed of reaching temperatures as low as -238 C (-397 F). This extreme cold acts as a cold trap for water ice and other volatiles. Despite these challenges, the volatiles represent a prime opportunity for the first Moon based resource processing system.
The surface regolith on the Moon is often comprised on very abrasive dust with high porosity (empty space). Instead of sinking straight through it like quick sand, surface missions have observed that the regolith compresses onto itself, forming a traversable surface. There is still a risk that some regolith may not act this way, specifically at the PSRs.
Image from the LRO NAC instrument showing a boulder track in a permanently shadowed region (PSR). The left image shows the raw image, with the right image being stretched and filtered to bring out the details within the shadowed region. Credit: Sargeant et al., 2019, Figure 1.
Lacking direct measurements, inferences have been made about the strength of the PSR regolith via regolith porosity. The assumption is that the higher the regoligh porosity, the less force the regolith can support before shifting. Including the LCROSS impact, LRO DIVINER observations, and studies of analog material, all studies indicate that the PSR regolith has high porosity. To test whether the porosity vs force assumption is true, boulder tracks were used to study the bearing capacity of the regolith.
The deeper the regolith, the more bearing force it has. This chart shows the depth vs strength comparison across multiple regolith locations. Boulder track measurements were used to determine all data points. Credit: Sargeant et al., 2019, Figure 3.
Moonquakes cause materials to move and adjust periodically, creating rockfalls and abundant boulder tracks across the lunar surface. The boulder and the dimensions of boulder tracks can be used to infer the strength of the regolith via bearing capacity theory. For this study, specific PSRs were selected based on having diffuse sunlight reflected into them. This allowed estimated measurements to be made for the boulders and their tracks.
The analysis showed that the bearing strength of regolith at the observed PSRs was similar to regolith formed in pyroclastic regions, and was stronger than regolith found in highland and mare regions. This means that the PSR regolith has no unusual strength limitation, which is good for future PSR missions. Additionally, the bearing capacity generally appears to increase with depth.
The more slope the regolith is on, the less bearing force (strength) it has. This means that rovers and landers are more likely to sink into regolith on steep slopes than on flat terrain. Credit: Sargeant et al., 2019, Figure 4.
The regolith bearing capacity also decreases as the slope increases. This means that regolith in PSRs on slopes over 20 degrees has a bearing capacity about half that on flat areas. This will be important for rovers navigating on sloped terrain, especially when traversing into or out of PSRs. Based on these findings, these rovers would require large, dust capable wheels. Landers would also want to steer clear of dramatically sloped terrain since landing there would increase their risk of sinking into the regolith.
These results potentially contradict the assumption that high porosity regolith has a low bearing force. However, this is an early study and in-situ analysis will greatly expand these results. Despite the unverified results, these finding provide additional environmental data for designing systems that can explore and operate around PSRs. There is only so much remote observations that can be made before we must take the risk and send a mission to explore these environments.
References
Sargeant, Bickel, Honniball, Martinez, Rogaski, Bell, Czaplinski, Farrant, Harrington, Tolometti, and Kring (2019). ct Determining the Bearing Capacity of Permanently Shadowed Regions of the Moon Using Boulder Tracks. In: Lunar and Planetary Science Conference. [online] The Woodlands. Available at: https://www.hou.usra.edu/meetings/lpsc2019/pdf/1792.pdf
