The distribution of high-priority sample target materials on the currently unsampled lunar farside. Credit:  Moriarty et al., 2021.

The surface of the Moon serves as a geologic time capsule for early planetary evolution processes. Ancient samples returned from the lunar surface preserve a record of how the Moon differentiated into an interior silicate mantle and an outer silicate crust. Although many scientists agree on the broad outline of lunar formation and differentiation, the precise composition and structure of the Moon’s interior remain highly debated. Almost all of the samples returned from the Moon come from the crust. Only a few of the samples are potentially mantle rocks, and there is a lack of consensus on which these are. The most promising locations to look for exposures of mantle rocks on the Moon are in large impact basins and craters that may have penetrated through the crust, exposing and excavating samples of the underlying mantle. However, it is unclear which of these basins or craters have actually exposed the lunar mantle.

A review led by Daniel Moriarty III at NASA Goddard Space Flight Center summarizes these issues in light of recent experimental and spacecraft data. The review identifies some of the most promising sites for future exploration and sampling, focusing on the search for exposed mantle rocks. Based on their assessment, the unsampled farside of the Moon presents several opportunities to fill gaps in our knowledge. For example, large impact basins such as Mare Moscoviense and Schrödinger Basin contain rich outcrops of key mantle minerals olivine and pyroxene. In particular, the South Pole-Aitken Basin, the largest impact basin on the Moon, is the most likely candidate to have excavated material from mantle depths. Considering that our current returned samples are all from the nearside, samples from these areas on the farside will deepen our understanding of planetary differentiation on rocky bodies throughout the solar system. READ MORE