5:15 PM - 7:15 PM
[PPS08-P18] Lunar subsurface rock size-frequency distribution revealed by Chan’E-4 Lunar Penetrating Radar
Keywords:Moon, ground penetrating radar, Rock size-frequency distribution, Lunar Penetrating Radar, Lunar surface history
The rock size-frequency distribution (RSFD) on the lunar surface reflects the formation age and history of the lunar surface. RSFD has been investigated by camera observations (e.g., Di et al., 2016; Li et al., 2017), but camera observations can only observe the current lunar surface and cannot determine the past RSFD. We therefore focused on ground penetrating radar (GPR) observations. This nondestructive subsurface measurement technique uses radio waves and can observe multiple subsurface layers formed at different ages. Chang'E-3, 4, and 5 conducted GPR observations on the moon and found the paleo-regolith layers and old lava layers beneath the regolith (Liu et al., 2023). When radio waves propagate from a medium with higher (lower) permittivity to a medium with lower (higher) permittivity, an echo with the same (opposite) polarity as the transmitted pulse is obtained. In the case of GPR observations of rocks in the lunar regolith, the permittivity of regolith is typically about 3 (Feng et al., 2022), and that of rocks is about 6-15 (Chung et al., 1970), so echoes with the opposite and the same polarity of the transmitted pulse will be obtained from the top and bottom of the rock, respectively. If the rock is smaller than the resolution, the top and bottom of the rock can’t be distinguished, and mixed echo would be obtained. We hypothesized that it would be possible to estimate the subsurface rocks' size by examining the echoes' polarity. We aim to investigate the size of rocks in the regolith using Chang'E-4 Lunar Penetrating Radar (LPR) data.
We first simulated GPR observation using the FDTD method to investigate the relationship between the polarity of the echoes and rock size. We used gprMax (Warren et al., 2016) for simulation. A GPR transmitting a positive-peaked pulse with a center frequency of 500 MHz and duration of ~3 ns, the same as the LPR, was placed 0.3 m above the ground surface. A circular rock with a relative permittivity of 9 was placed at a depth of 2 m in a regolith with a relative permittivity of 3. The diameter of the rock was varied from 1 cm to 15 cm. As a result, we found that we can categorize rock size into three categories based on the observed polarity. When the diameter was less than 5 cm, only one echo with the same polarity as the transmitted pulse was obtained (Category 1) because the echo from the bottom of the rock was dominant. Although the two echoes could not be completely distinguished for the 6-8 cm diameter rock, a double-peaked echo with the same polarity as the transmitted one was observed (Category 2), indicating that the top and bottom echoes became more separated as the rock size increased. The fact that we can classify the size of rocks below GPR resolution is an important new insight we obtained. A pair of echoes with the opposite (from the top of the rock) and the same (from the top of the rock) polarities as the transmitted pulse was observed for rocks with diameters greater than 9 cm (Category 3).
The above rock size classification method was applied to the LPR data. Through basic data processing (Feng et al., 2023), we obtained data along the 1500 m rover path. We identified hyperbolic echoes in the uppermost regolith layer of several layers found in the data (Giannakis et al., 2024; Zhang et al., 2024) by hand, and 88 hyperbolic echoes were found. We examined the polarity of these echoes and classified them into three categories, and we found Category 1; 46, Category 2; 12, and Category 3; 4 echoes. For Category 3, the sizes were estimated to be from 15.0 to 52.5 cm, assuming a rock permittivity of 9. For the other 26, the three polarities obtained in the simulation were not applicable. For these echoes, we consider the possibility of echoes due to cavities in the regolith (Ding et al., 2021).
This study is the first trial to clarify the RSFD evolution in the lunar subsurface.
We first simulated GPR observation using the FDTD method to investigate the relationship between the polarity of the echoes and rock size. We used gprMax (Warren et al., 2016) for simulation. A GPR transmitting a positive-peaked pulse with a center frequency of 500 MHz and duration of ~3 ns, the same as the LPR, was placed 0.3 m above the ground surface. A circular rock with a relative permittivity of 9 was placed at a depth of 2 m in a regolith with a relative permittivity of 3. The diameter of the rock was varied from 1 cm to 15 cm. As a result, we found that we can categorize rock size into three categories based on the observed polarity. When the diameter was less than 5 cm, only one echo with the same polarity as the transmitted pulse was obtained (Category 1) because the echo from the bottom of the rock was dominant. Although the two echoes could not be completely distinguished for the 6-8 cm diameter rock, a double-peaked echo with the same polarity as the transmitted one was observed (Category 2), indicating that the top and bottom echoes became more separated as the rock size increased. The fact that we can classify the size of rocks below GPR resolution is an important new insight we obtained. A pair of echoes with the opposite (from the top of the rock) and the same (from the top of the rock) polarities as the transmitted pulse was observed for rocks with diameters greater than 9 cm (Category 3).
The above rock size classification method was applied to the LPR data. Through basic data processing (Feng et al., 2023), we obtained data along the 1500 m rover path. We identified hyperbolic echoes in the uppermost regolith layer of several layers found in the data (Giannakis et al., 2024; Zhang et al., 2024) by hand, and 88 hyperbolic echoes were found. We examined the polarity of these echoes and classified them into three categories, and we found Category 1; 46, Category 2; 12, and Category 3; 4 echoes. For Category 3, the sizes were estimated to be from 15.0 to 52.5 cm, assuming a rock permittivity of 9. For the other 26, the three polarities obtained in the simulation were not applicable. For these echoes, we consider the possibility of echoes due to cavities in the regolith (Ding et al., 2021).
This study is the first trial to clarify the RSFD evolution in the lunar subsurface.