“Compaction Of Regolith On The Moon, Asteroids, And Other Airless Bodies: By Vibration Or By Thermal Cycling?”
The Apollo missions found that the lunar soil is highly compacted just a few centimeters beneath the surface, making it difficult to penetrate or extract core samples and giving it enough apparent cohesion to support vertical sidewalls when trenches were dug by the astronauts. An exception was found on the rims of young craters where the soil was not nearly so compacted. Although the geological compaction mechanism was never investigated in a published study, the understanding that developed (and has been passed along within the community by word-of-mouth) is that micrometeoroid tamping compacted the soil into this state. Cores returned to the Earth found the lunar soil to be layered with strata of varying thicknesses, with a mean of about 2 cm. In general the upper strata are less compacted than the lower ones, but compaction varies wildly and non-monotonically from one layer to the next in the column. Each layer was exposed to tamping on the surface for some random amount of time before a new ejecta deposit covered it over. The rims of young craters are a single, deep unit, and so their bulk was never exposed to tamping at the surface. Thus far the model seems to fit. Recent calculations of tamping energy, however, indicate that the effects of micrometeoroid tamping only reach 2 mm into the surface (double the depth of the “gardening zone” where soil is being overturned regularly), so not even the average uppermost layer should have been compacted by this effect. Another mechanism that operates on a longer length scale is needed. The thermal wave of a lunation reaches several tens of centimeters into the surface, so it has the appropriate length scale. Experiments were performed to investigate the effectiveness of thermal cycling as a compaction mechanism. Containers of simulated lunar soil with various overburdens to represent different depths in the lunar soil were subjected to either vibration on a shake table or to thermal cycling in an oven. While this was just a preliminary study, the results imply that thermal cycling is indeed the primary cause of lunar compaction, which implies that the soil in permanently shadowed craters will be in a different state than experienced during the Apollo mission. This was predicted prior to the LCROSS mission, the results of which indicate that the soil in Cabeus crater was probably fluffy to a depth of a couple meters. This has implications for missions attempting to visit regions with low insolation. Thermal compaction should apply to small, rotating bodies, as well, helping us to understand their physical state.
The presentation can be seen here.