Predicting water ice stability depths at the lunar poles by combining a microphysical thermal model with a 3D radiative flux model

Evidence for the presence of water ice at the lunar poles has been gathered using various methods, including radar (e.g., Nozette et al., 2001; Campbell et al., 2006), far-ultraviolet (e.g., Gladstone et al., 2012; Hayne et al., 2015) and laser (Zuber et al., 2012; Lucey et al., 2014) reflectance measurements, neutron spectroscopy (e.g., Feldman et al., 2001), as well as the LCROSS impact experiment (Colaprete et al., 2010). Another more indirect approach involves identifying potential water ice stability regions by combining three-dimensional illumination models of the lunar terrain with thermal models that predict surface and subsurface temperatures. Several studies have utilized this method (e.g., Paige et al., 2010b; King et al., 2020; Formisano et al., 2024), with the main differences between models relating to the level of detail in modeling the illumination source, radiative transfer, and subsurface heat transport.

In this work, we combine the one-dimensional microphysical model for the lunar regolith layer developed by Bürger et al. (2024), which more directly simulates regolith properties, such as grain size and packing-density stratification, with a three-dimensional radiative flux model including topography. This model applies the ray-tracing technique to describe the amount of incoming and reflected solar flux as well as the reflected thermal emission received for every surface element of the investigated area. The radiative transfer model is based on Potter et al. (2023), but the equations are modified to allow for an incidence angle-dependent albedo. Bolometric temperatures measured by the LRO/Diviner lunar radiometer (Paige et al., 2010a) serve as a reference for the modeled surface temperatures and it is demonstrated that the key thermal trends are accurately reproduced.

During the modeling process, we identified several key factors for a correct simulation. First, at high latitudes where the Sun remains near the horizon, it is crucial to model the Sun as a disk rather than a point source to avoid inaccuracies in temperature simulations during sunrise and sunset. Second, to more accurately capture the illumination conditions for the thermal model, the area in which ray-tracing is applied should extend beyond the region where temperatures are modeled. When examining the effect of an incidence angle-dependent albedo, we found that the steep increase in albedo at high incidence angles—required by previous thermal models of global regolith properties (e.g., Hayne et al., 2017; Feng et al., 2020; Bürger et al., 2024)—does not agree with the observed Diviner bolometric temperatures near the poles. Instead, a much weaker dependency, as adopted by King et al. (2020), results in a better fit. Finally, when assessing subsurface water-ice stability, it is crucial to account for the diffusion barrier that reduces sublimation loss rates, as described by Schorghofer & Williams (2020), yielding shallower stability depths. 

Two areas of interest are modeled in detail: First, the Shackleton crater, which is located almost exactly at the lunar south pole and therefore experiences a unique illumination pattern with the crater rim being continuously illuminated and the inner part being in permanent shadow. The second area investigated is the landing site of NASA’s CLPS CP22 mission located on the Leibnitz Plateau. Onboard this mission will be ESA’s PROSPECT instrument, which consists of a drill and a chemical laboratory designed to analyze volatiles (Trautner et al., 2024). Accurate predictions of water ice stability depths are crucial for this mission. Figure 1 illustrates the simulated surface temperatures at three different local times at the CP22 landing site.

At the conference we will present surface temperature and water-ice stability maps of these two regions and discuss the key lessons learned during the modeling process.

Figure 1: Simulated surface temperatures at the CP22 landing site on the Leibnitz Plateau. Illustrated are the surface temperatures at three different local times (left: ~4:30 h, center: ~10:30 h, right: ~16:30 h) with the direction of the Sun being indicated as well.

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