How Well Do We Understand the Geological Field Record on the Moon?

As it occurs on a rocky planetary object, one might think that conducting field geology on the Moon would be a fairly straightforward exercise and draw significantly on over 200 years of terrestrial experience. In fact, the nature of the Moon itself creates complications for field exploration. The impact-generated regolith covers almost all surfaces, creating a chaotic overblanket whose evolution and provenance are only partly understood. In place, unmodified bedrock is rare on the Moon, occurring as outcrop in limited portions of crater and rille walls in the maria. There is no bedrock exposed in the highlands, except as highly shocked, central peak material, brought up from great depth (and hence, no longer “in place”.) Moreover, the scales of heterogeneity on the Moon are not clearly understood, as it is difficult to place even the best-controlled, most thoroughly documented Apollo samples into their regional or local contexts. It is widely agreed that the mare basalts in the Apollo collections represent the surface flows of visited sites, but the provenance and origin of most highland samples is uncertain. A key objective of human lunar return must be to understand the geological style of the Moon, how the regolith forms and evolves, and how to draw conclusions about regional geology on the basis of statistical sampling. Such knowledge is crucial for the proper interpretation of lunar samples in the existing Apollo collection and those form future robotic and human missions.