5:15 PM - 7:15 PM
[PPS08-P12] 3D Model Simulation of Water Molecules in Lunar Subsurface Caves
Keywords:water molecules, lunar caves
The presence of water on the Moon is critical for understanding the origin of volatile within the Earth-Moon system. Lunar vertical holes and their caves are of interest for long-term water residence. Temperature is stable inside caves and photolysis exposure is lower than on lunar surface. SELENE observation suggests that these holes might be skylights leading to lava tubes (Haruyama et al., 2009). Identifying these hole-cave candidates for water residence is beneficial for planetary exploration.
The existence of water inside caves remains unclear. Wilcoski et al. (2023) modeled temperatures and sublimation rates in polar caves and showed that water ice is stable only in caves within Permanently Shadowed Regions (PSRs) above 87.5° latitude. However, water could exist in subsurface caves at lower latitude as discussed in next section.
Desorption energy, the energy required for water to desorb from rock or water ice, is crucial for water persistence. Wilcoski et al. (2023) assumed ice-layer interactions, but actual cave surfaces are likely basaltic if these caves are lava tube. Hibbitts et al. (2011) provided a formula for adsorption time using desorption energy, and Jones et al. (2020) reported desorption energy for lunar mare samples. These studies allow consideration of adsorption time on basaltic rock rather than ice, potentially extending water residence time beyond Wilcoski et al. (2023).
Considering adsorption on lunar basaltic rock, this study developed a 3D simulation model to investigate whether lunar caves can retain water for over one lunar cycle. Water molecule flight-adsorption loop simulation was performed in a hole-cave system modeled for Mare Tranquillitatis Hole, or MTH (8.3°N, 33.2°E) which is the target vertical hole for UZUME future mission of 100 m, 1 km, and 10 km long. The model used 100 m as hole diameter and 60 m as cave height both based on Lunar Reconnaissance Orbiter Camera observations of MTH. Width was set equal to the hole diameter (100 m), and distance into caves was set to three different lengths of 100 m, 1 km, and 10 km, inferred from terrestrial lava tubes.
Results indicate that in 100 m distant caves, 30 % of the water molecules remain for one lunar day below 185 K while in 10 km long caves, 0.1% of water persists after one lunar day below 234 K. To estimate the latitude at which caves reach these temperatures, we used the radiation equilibrium temperature, a latitude-temperature relation considering solar influx and lunar heat loss (Haruyama et al., 2012). The equilibrium temperature of 234 K corresponds to 64.5° latitude. This estimated latitude is lower than Wilcoski et al. (2023) which suggested stable retention only above 87.5°. Our findings indicate water molecules may persist in mid-latitude caves, not only in PSRs.
Observations have identified three vertical holes above 65° latitude that may meet these conditions. Schomberger A1 is the highest-latitude vertical hole ever found at about 78°. We estimated total mass of water molecules in a cave extending from Schomberger A1 to distance of 1 km. Our scenario is that water molecules flow into the bottom of the hole on the way to transporting on the lunar surface. Diurnal variation of water ratio on the lunar surface at latitude of 80°S is approximately 250 ppm by weight observed by SOFIA (Jones et al., 2024). Over 1 billion years, water molecules possibly accumulate to form a layer several meters thick. However, once a certain amount of water is stored, it cannot accumulate to this extent because the desorption energy on water ice must be taken into account. At minimum, a monolayer of water ice should remain inside caves. Further research about developing process of water ice layer will be expected.
The existence of water inside caves remains unclear. Wilcoski et al. (2023) modeled temperatures and sublimation rates in polar caves and showed that water ice is stable only in caves within Permanently Shadowed Regions (PSRs) above 87.5° latitude. However, water could exist in subsurface caves at lower latitude as discussed in next section.
Desorption energy, the energy required for water to desorb from rock or water ice, is crucial for water persistence. Wilcoski et al. (2023) assumed ice-layer interactions, but actual cave surfaces are likely basaltic if these caves are lava tube. Hibbitts et al. (2011) provided a formula for adsorption time using desorption energy, and Jones et al. (2020) reported desorption energy for lunar mare samples. These studies allow consideration of adsorption time on basaltic rock rather than ice, potentially extending water residence time beyond Wilcoski et al. (2023).
Considering adsorption on lunar basaltic rock, this study developed a 3D simulation model to investigate whether lunar caves can retain water for over one lunar cycle. Water molecule flight-adsorption loop simulation was performed in a hole-cave system modeled for Mare Tranquillitatis Hole, or MTH (8.3°N, 33.2°E) which is the target vertical hole for UZUME future mission of 100 m, 1 km, and 10 km long. The model used 100 m as hole diameter and 60 m as cave height both based on Lunar Reconnaissance Orbiter Camera observations of MTH. Width was set equal to the hole diameter (100 m), and distance into caves was set to three different lengths of 100 m, 1 km, and 10 km, inferred from terrestrial lava tubes.
Results indicate that in 100 m distant caves, 30 % of the water molecules remain for one lunar day below 185 K while in 10 km long caves, 0.1% of water persists after one lunar day below 234 K. To estimate the latitude at which caves reach these temperatures, we used the radiation equilibrium temperature, a latitude-temperature relation considering solar influx and lunar heat loss (Haruyama et al., 2012). The equilibrium temperature of 234 K corresponds to 64.5° latitude. This estimated latitude is lower than Wilcoski et al. (2023) which suggested stable retention only above 87.5°. Our findings indicate water molecules may persist in mid-latitude caves, not only in PSRs.
Observations have identified three vertical holes above 65° latitude that may meet these conditions. Schomberger A1 is the highest-latitude vertical hole ever found at about 78°. We estimated total mass of water molecules in a cave extending from Schomberger A1 to distance of 1 km. Our scenario is that water molecules flow into the bottom of the hole on the way to transporting on the lunar surface. Diurnal variation of water ratio on the lunar surface at latitude of 80°S is approximately 250 ppm by weight observed by SOFIA (Jones et al., 2024). Over 1 billion years, water molecules possibly accumulate to form a layer several meters thick. However, once a certain amount of water is stored, it cannot accumulate to this extent because the desorption energy on water ice must be taken into account. At minimum, a monolayer of water ice should remain inside caves. Further research about developing process of water ice layer will be expected.