[P-PS09] Lunar Science and Exploration

[PPS09-P17] Study on the detectability of water ice using brightness temperature diurnal variation

*Koki Yoshioka¹, Hideaki Miyamoto¹, Ryodo Hemmi², Makito Kobayashi¹, Tomohiro Takemura¹ (1.The University of Tokyo, 2.National Institute of Information and Communications Technology)

Keywords:Moon, Brightness temperature, Water ice

The potential presence of lunar water has been discussed based on both theoretical and observational studies. If lunar water exists, it has significant implications for lunar science and potential resource utilization. Convincing evidence for the existence of water molecules on the Moon includes the elevated hydrogen abundance in the polar regions observed by neutron spectrometers [1, 2]. However, their spatial resolution is typically insufficient to evaluate water abundance on local to regional-scales. Visible to near-infrared spectrometer observations provide higher-resolution information [3, 4], but generally the observable depth is limited to the surface, and it is difficult to obtai the information at the cm-m scale depths where water ice is speculated to be concentrated [5]. We note that the diurnal variation in brightness temperature reflects the radiative and thermophysical properties of a subsurface regolith layer at a given depth by observing emission at different wavelengths. Regolith properties such as surface roughness and density structure influence the brightness temperature, which has been studied based on existing data and numerical models [e.g., 6, 7]. Similarly, the presence of water ice would also affect the behavior of the brightness temperature diurnal variation, and understanding the extent of its influence is required to constrain its subsurface distribution. Here, we focus on the diurnal variation in the brightness temperature at different observation frequencies and model the effect in the presence of water ice. By doing so, we aim to establish a scheme to constrain the vertical and horizontal water ice distribution based on brightness temperature. This would be beneficial for the TSUKIMI (lunar Terahertz SUrveyor for KIlometer-scale MappIng) mission which will fill the gap between the observation frequencies of existing brightness temperature data and aim to constrain the concentration of lunar resources such as water ice.
We consider a method to estimate the diurnal brightness temperature variation in the lunar polar region by first deriving the physical subsurface temperature using a 1D thermal model and converting it to brightness temperature at the given observational frequency bands using the commonly used weighting function. We compare the results with Diviner data to validate the model results.
When a 1 cm thick icy regolith layer with 5 wt% ice content exists on top of a dry regolith, the effect of higher thermal conductivity, albedo, and emissivity could (1) increase the amplitude of the surface temperature diurnal curve and (2) decrease the daytime surface temperature. These effects result in ~10 K difference between icy and dry settings. The Diviner bolometric brightness temperature data from Shoemaker crater and Haworth craters, both of which have relatively similar latitude, size, and illumination conditions, show qualitative agreement with the simulation results.

Reference:
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[2] Sanin, A. B., et al., 2017. Hydrogen distribution in the lunar polar regions. Icarus. 283, 20-30.
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[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. The Astrophysical Journal, 788(2), 169.
[6] Hayne, P. O., et al., 2017. Global regolith thermophysical properties of the Moon from the Diviner Lunar Radiometer Experiment. Journal of Geophysical Research: Planets, 122(12), 2371-2400.
[7] Bandfield, J. L., et al., 2015. Lunar surface roughness derived from LRO Diviner Radiometer observations. Icarus, 248, 357-372.