3:30 PM - 5:00 PM
[PPS06-P05] Theoretical study on the lunar brightness temperature in the terahertz band
Keywords:Lunar Resource, Terahertz, Brightness Temperature
The presence of putative lunar water has been actively discussed based on theoretical and observational studies because, if it exists, it has significant implications for lunar science and potential resource utilization. One of the most convincing lines of evidence for the existence of water molecules on the Moon is the elevated hydrogen abundance within the depth of 1-2 m from the surface in the polar regions observed by neutron and gamma-ray spectrometers [1, 2]. Visible to near-infrared spectrometer observations provide implication for water molecule existence in the surface layer of few mm [3, 4]. On the other hand, the water ice is speculated to concentrate around the depth of several dozen cm from theories like the pumping effect [5]. Information at such depth would be significant in the respect of lunar water ice.
This work focuses on the surface temperature of the lunar regolith because it reflects the radiative and thermophysical properties of the near-surface regolith, which could provide useful information for estimating subsurface conditions. The brightness temperatures are observed globally by the Diviner Lunar Radiometer Experiment (Diviner) onboard NASA’s Lunar Reconnaissance Orbiter and the Microwave Radiometer (MRM) onboard Chang’E-2. Brightness temperatures depend on the observed frequency band, which controls the observable depth. Therefore, by measuring brightness temperatures at different frequency bands, we can obtain subsurface information about the presence of water ice. The TSUKIMI (lunar Terahertz SUrveyor for KIlometer-scale MappIng) mission planned to be launched in 2025 will obtain the brightness temperature at the frequency bands between MRM and Diviner (several hundred GHz). Therefore, we develop a method to estimate the brightness temperature variation with time and depth by first deriving the physical temperature at a given depth and converting it to brightness temperature at the observational frequency bands of MRM, TSUKIMI, and Diviner.
As previously suggested, the density profile of the near-surface regolith affects the brightness temperature. However, we find it to be relatively small when the most likely porosity structure; globally and at high latitudes, the difference is less than 12 K and 8 K, respectively. On the other hand, the presence of water leads to a brightness temperature difference of up to 15 K, which is larger than that caused by the variation of the regolith density structure. In addition, the diurnal curve becomes flatter and its peak is delayed when water ice is present in the regolith. It should be noted that this method makes several assumptions such as the form of water ice, where further careful studies are needed to establish a pragmatic method for water ice detection. Nevertheless, this study provides insight into the discussion on the existence of water ice in polar regions and contributes to the development of its detection scheme, which focuses on the behavior of brightness temperature.
Reference:
[1] Feldman, W. C., et al., 2000. Polar hydrogen deposits on the Moon. Journal of Geophysical Research-Planets. 105, 4175-4195.
[2] Sanin, A. B., et al., 2017. Hydrogen distribution in the lunar polar regions. Icarus. 283, 20-30.
[3] Colaprete, A., et al., 2010. Detection of Water in the LCROSS Ejecta Plume. Science. 330, 463-468.
[4] Li, S., et al., 2018. Direct evidence of surface exposed water ice in the lunar polar regions. Proceedings of the National Academy of Sciences of the United States of America. 115, 8907-8912.
[5] Schorghofer, N., Aharonson, O., 2014. THE LUNAR THERMAL ICE PUMP. Astrophysical Journal. 788.
This work focuses on the surface temperature of the lunar regolith because it reflects the radiative and thermophysical properties of the near-surface regolith, which could provide useful information for estimating subsurface conditions. The brightness temperatures are observed globally by the Diviner Lunar Radiometer Experiment (Diviner) onboard NASA’s Lunar Reconnaissance Orbiter and the Microwave Radiometer (MRM) onboard Chang’E-2. Brightness temperatures depend on the observed frequency band, which controls the observable depth. Therefore, by measuring brightness temperatures at different frequency bands, we can obtain subsurface information about the presence of water ice. The TSUKIMI (lunar Terahertz SUrveyor for KIlometer-scale MappIng) mission planned to be launched in 2025 will obtain the brightness temperature at the frequency bands between MRM and Diviner (several hundred GHz). Therefore, we develop a method to estimate the brightness temperature variation with time and depth by first deriving the physical temperature at a given depth and converting it to brightness temperature at the observational frequency bands of MRM, TSUKIMI, and Diviner.
As previously suggested, the density profile of the near-surface regolith affects the brightness temperature. However, we find it to be relatively small when the most likely porosity structure; globally and at high latitudes, the difference is less than 12 K and 8 K, respectively. On the other hand, the presence of water leads to a brightness temperature difference of up to 15 K, which is larger than that caused by the variation of the regolith density structure. In addition, the diurnal curve becomes flatter and its peak is delayed when water ice is present in the regolith. It should be noted that this method makes several assumptions such as the form of water ice, where further careful studies are needed to establish a pragmatic method for water ice detection. Nevertheless, this study provides insight into the discussion on the existence of water ice in polar regions and contributes to the development of its detection scheme, which focuses on the behavior of brightness temperature.
Reference:
[1] Feldman, W. C., et al., 2000. Polar hydrogen deposits on the Moon. Journal of Geophysical Research-Planets. 105, 4175-4195.
[2] Sanin, A. B., et al., 2017. Hydrogen distribution in the lunar polar regions. Icarus. 283, 20-30.
[3] Colaprete, A., et al., 2010. Detection of Water in the LCROSS Ejecta Plume. Science. 330, 463-468.
[4] Li, S., et al., 2018. Direct evidence of surface exposed water ice in the lunar polar regions. Proceedings of the National Academy of Sciences of the United States of America. 115, 8907-8912.
[5] Schorghofer, N., Aharonson, O., 2014. THE LUNAR THERMAL ICE PUMP. Astrophysical Journal. 788.