Deducing Lunar Regolith Porosity From Energetic Neutral Atom Emission

Abstract

The surfaces of airless bodies are covered by a porous regolith, a loose ensemble of rocks and dust grains, due to a multitude of erosion and impact processes over billions of years (McKay et al., 1991). Its upper layers determine how those planetary bodies are observed as their surface morphology strongly affects optical properties (Hapke, 2008;Vernazza et al., 2012). The porous structure of stacked grains will also influence the interaction of any planet, moon, or asteroid with its environment or precipitating radiation. Especially the effect of porosity on thermal conductivity has been of recent interest (Ryan et al., 2022;Wood, 2020). The porosity of the upper regolith is also connected to the mechanical properties of the grain stacking (Kiuchi & Nakamura, 2014) as well as grain transport processes across a planetary surface (Schwan et al., 2017;Vernazza et al., 2012). While a large number of studies of lunar regolith have been performed, the porosity of the pristine upper regolith, defined as the ratio of voids to the total volume in the region near the surface that is accessible for precipitating radiation, is difficult to deduce from returned samples and requires non-invasive methods (Ohtake et al., 2010). Early investigations estimated a porosity value between 0.8 and 0.9 from reflectance measurements (Hapke & van Horn, 1963). Similarly, Ohtake et al. ( 2010) found a high porosity for the Apollo 16 sample site, which was confirmed by Hapke and Sato (2016), determining a porosity of 0.83 ± 0.03 for the upper lunar regolith at this specific site. This value for the upper regolith differs from the result of studies with returned samples of 0.52 ± 0.02 for the upper 15 cm of the lunar soil (Carrier III et al., 1991). Impacting particles, such as photons, ions, or electrons, however, have much smaller interaction regions on the order of millimeters (Hapke & Sato, 2016). It is thus questionable how applicable measurements of the porosity from returned samples are

Energetic Neutral Atom (ENA) Emission Characteristics at the Moon and Mercury From 3D Regolith Simulations of Solar Wind Reflection

“…With our model (Szabo et al, 2022b), we have previously shown that the total backscattering coefficient depends on regolith porosity, reproducing Chandrayaan-1 data for its angular dependence with fairy-castle like, highly porous regolith. Ultimately, the lunar regolith porosity at the surface could be constrained to 𝐴𝐴 0.85 +0.15 −0.14 .…”

“…We tested the sensitivity of the scattering function on regolith parameters by running one simulation with a lower porosity of around 0.5 and one simulation for irregular grain shapes, which were created with a fractal dimension f d = 2.6 using an algorithm proposed by Wei et al (2018) in the same manner as it was done in Szabo et al (2022b). Plots of these simulation results are given in Supporting Information S1, but the differences to the high-porosity, spherical-grain case presented here are minor.…”

“…However, more than 95% of the reflected particles have become neutralized during backscattering from the lunar surface (Lue et al, 2018). As previously discussed, this only has a minor effect on interpretation of the results considering other uncertainties in the simulation (Szabo et al, 2022b). We therefore neglect the charged component of the backscattered population and relate properties of all backscattered H from the SDTrimSP-3D simulations to ENA observations.…”

Establishing a Best Practice for SDTrimSP Simulations of Solar Wind Ion Sputtering

“…Because the cosine distribution of incidence angles approximates the variation in impact angles and corresponding yields expected for a spherical grain surface, it is considered the best approximation currently available within SDTrimSP for SW interactions with regolith. More recent research has also shown that an upcoming 3D version of SDTrimSP could provide another option to capture the granular surface profile (Szabo et al 2022b). Previous research has suggested that ion sputtering of regolith due to normally incident SW can be distinctly different than ion sputtering from a Figure 2.…”

New Compound and Hybrid Binding Energy Sputter Model for Modeling Purposes in Agreement with Experimental Data

“…In recent years there were several efforts to better constrain the erosion of rocky planetary bodies exposed to highly energetic solar wind ions. This includes investigating the effect of surface roughness (Biber et al 2022) and porosity (Szabo et al 2022b), performing ion irradiation experiments with mass yield measurements (e.g., Hijazi et al 2017;Szabo et al 2018Szabo et al , 2020aBiber et al 2022), and introducing a new surface and bulk binding energy (BBE) model from theory (Hofsäss & Stegmaier 2022;Morrissey et al 2022). In this work, we discuss the parameter of density and its inclusion in SDTrimSP (Mutzke et al 2019), as well as a new hybrid binding energy model that reliably recreates experimental sputter yields completely without the requirement to adjust input parameters.…”