^1,2Mingyeong Lee,¹Minsup Jeong,^1,2Young-Jun Choi
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2024JE008406]
¹Korea Astronomy and Space Science Institute, Daejeon, Republic of Korea
²University of Science and Technology, Daejeon, Republic of Korea
Published by arrangement with John Wiley & Sons

Lunar regolith consists of unconsolidated grains with high porosity, called the fairy castle structure. It is closely linked to the lunar opposition effect, which is the effect where brightness sharply increases as the phase angle approaches 0°. However, owing to the Earth’s gravity, it is difficult to reproduce the structure to study the physical characteristics of the lunar fairy castle structure in the laboratory. We designed a lunar fairy castle structure model for 3D printing. These models had high porosity and were simplified to tree-like shapes. Various porous conditions of the surface were considered, represented by the number of trees, maximum trunk length, and maximum branch angle. In this study, a laboratory experiment was conducted to measure the reflectance of simulants with a fairy castle structure within a small phase angle range from 1.4° to 5.0°. The result is analyzed for the sample porosity with the tangential slope of the reflectance S(α), which denotes the strength of the opposition effect. In addition, the results of this study were compared with lunar observation data. The porous samples exhibited a relatively large S(α) value. The influence of branch length and attachment angle was very weak in this study. Samples with a porosity between 0.78 and 0.82 represent the similar S(α) values to the lunar observation data, a mean porosity of lunar regolith. In conclusion, our findings suggest a potential correlation between porosity and the opposition effect in printed samples, proposing a new research approach for understanding the lunar opposition effect.

¹Emma R. Stoutenburg,^2,3Razvan Caracas,⁴Natalia V. Solomatova,¹Andrew J. Campbell
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2024JE008525]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, IL, USA
²Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France
³The Center for Planetary Habitability (PHAB), University of Oslo, Oslo, Norway
⁴Laboratoire de Géologie de Lyon LGLTPE UMR5276, CNRS, Ecole Normale Supérieure de Lyon, Centre Blaise Pascal, Lyon, France
Published by arrangement with John Wiley & Sons

Iron hydrides are a potentially dominant component of the metallic cores of planets, primarily because of hydrogen’s ubiquity in the universe and affinity for iron. Using ab initio molecular dynamics, we examine iron hydrides with 0.1, 0.33, 0.5, and 0.6 mol fraction hydrogen up to 100 GPa between 3,000 and 5,000 K to describe how hydrogen content affects the melt structure, hydrogen speciation, equation of state (EOS), atomic diffusivity, and melt viscosity. We find that the addition of hydrogen decreases the average Fe–Fe coordination number and lengthens Fe–Fe bonds, while Fe–H coordination number increases. The pair distribution function of hydrogen at low pressure indicates the presence of molecular hydrogen. By tracking chemical speciation, we show that the amount of molecular hydrogen increases and the number of iron in Hx≥1Fey≥0 clusters decreases as the hydrogen concentration increases. We parameterize a pressure, volume, temperature, and composition EOS and show that the molar volume and Grüneisen parameter of the melts decrease while the compressibility and thermal expansivity increase as a function of hydrogen concentration. We find that hydrogen acts as a lubricant in the melts as the iron and hydrogen become more diffusive and the melts become more inviscid as the hydrogen concentration increases. We estimate 2.7 wt% hydrogen in the Martian core and 0.49–1.1 wt% hydrogen in Earth’s outer core based on comparisons to seismic models, with the assumption that the cores are pure liquid iron-hydrogen alloy, and we compare the small exoplanet population with mass-radius curves of iron hydride planets.