The SER3NE mission to hunt for water and other volatiles on the Moon

The Moon is known to uniquely reveal the inner Solar System history, and it is a natural laboratory for studying regolith formation and weathering on an apparently anhydrous airless body. Manifested in the diversity of lunar crustal rocks it exposes fundamentally the evolution of a differentiated planetary object and provides a window into the long-term thermal and compositional evolution of the Moon, while the lunar poles are special environments that bear witness to the volatile flux over (more) recent Solar System history. The Moon is also the only object besides Earth that humans set a foot on and will again soon. The SER3NE (Selene’s Explorer for Roughness, Regolith, Resources, Neutrons and Elements) mission is set out to:

• Unravel inner Solar System volatile origin and delivery processes using the spatial record and potential temporal variability of the volatile species on the lunar surface,
• uncover the geological processes that shaped the Moon and inner solar system bodies based on the spatial record and potential temporal variability of the solid species across the Moon,
• prospecting lunar resources for ISRU and at future landing sites using the same spatial record to assess ISRU relevant solid and volatile species,
• potentially we may be able to characterize gravitational to the moon bound neutrons at various altitudes, for constraining neutron lifetime estimate, and
• study the orbital evolution of the Earth Moon system using the tidal deformation record.

Early sample return suggested the Moon substantially lacks volatiles. We now know that the Moon is not entirely dry and holds a record of volatiles obtained throughout its history. Across the entire Moon, materials featuring hydrogen have been identified, allowing for speculations on how the Moon could replenish its water since its hot formation by celestial collision, and on the volatile transport mechanisms across the airless lunar surface. Neutron data indicate that concentration of H is enhanced at high latitudes (Feldman et al. 2001). Similarly, near-infrared reflectance (NIR) supports the presence of an absorption feature interpreted to relate to OH/H₂O-bearing materials (Pieters et al. 2009) which nature is equally unknown. Additional observations, e.g., by the Lunar Crater Observation and Sensing Satellite (LCROSS) mission (Schultz et al. 2010), support that water ice exists on the Moon in some lunar Permanently Shadowed Regions (PSR), (Hayne et al. 2015). The general lack of correlation between OH signatures in sunlit NIR data and neutron spectrometer H abundance data also suggests that the formation and retention of OH and H₂O could be an ongoing surficial process and point at heterogeneities at many scales that remain unresolved and unexplained. What they do reveal however is that the lunar polar regions could constitute the largest accessible volatile reservoirs of the Moon (see Lucey et al. 2022 for a comprehensive review).

It has been suggested that volatiles migrate in an active cycle and are trapped in PSRs near the poles, processes that occur at bodies with tenuous atmospheres across the Solar System. The physical form of the water is also poorly understood leading to uncertainty on the total amount of resources and questions its extraction. Although radar data restrict the presence of contiguous ice volumes for the moon several alternatives exist including hydrated minerals, adsorbed water molecules, pore-filling ice, and small ice grains mixed with regolith. To date however, little is known on the nature and size of such reservoir.

Finally, topography and surface roughness are often underestimated contributors to observable material properties, radiative behaviour, terrain accessibility, or geological processes that redistribute matter along slopes or other physical gradients (e.g., temperature) especially on airless bodies. Most prominently, topography creates the PSRs that can trap and accumulate volatiles through time, including water.  The SER3NE mission is dedicated to mapping the volatile and solids spatial and temporal variations and variability including the topographic context, having onboard GRiNS, the Gamma-Ray-including-Neutrons Spectrometer (by UiO), LIPS, the Lunar Infrared Point Spectrometer (by ROB/BIRA-IASB) and S3LA, SER3NE’s Laser Altimeter (by DLR).

The design of SER3NE can provide spatially and temporally correlated datasets for simultaneous elemental and mineral identification. This not only overcome the abovementioned current interpretational challenges, but also, since it is well known in the field of mining and resource extraction, that extraction methods are predominantly dependent on the exact mineral species that host element of interest. SER3NE mission is to provide a comprehensive assessment and characterisation of the forms of hydrogen compounds or water, and potentially the affiliation with other volatiles, globally. In detail, this mission shall be capable of describing volatile abundance, temporal variation, and the possible origin. This combination provides the necessary insights to a lunar water and more general volatile cycle. This information paves the way two-fold, scientifically it characterises the interaction of the Moon with its space environment and exploration-wise it prospects the lunar water and volatiles as a resource.