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Investigating the Latitudinal Dependence of Lunar Regolith Properties Using LRO/Diviner Data and a Microphysical Thermal Model

Presentation #207.02 in the session Moon & Earth II (Oral Presentation)

Published onOct 23, 2023
Investigating the Latitudinal Dependence of Lunar Regolith Properties Using LRO/Diviner Data and a Microphysical Thermal Model

Regolith is formed through weathering of the local rock by meteorite bombardment, space weathering (Pieters & Noble, 2016) and thermal erosion (Delbo et al., 2014). In the case of the Moon, the space weathering effects and diurnal temperature variations are reduced towards the poles. The aim of this study is to investigate whether the lunar regolith properties derived from the comparison of regolith temperatures measured by the Diviner radiometer (Paige et al., 2010) on board the Lunar Reconnaissance Orbiter (LRO) with simulated temperatures derived from a microphysical thermal model show a latitudinal dependence. The developed microphysical thermal model expands upon previous models by more directly simulating regolith properties, such as grain radius and volume filling factor.

By assuming constant regolith properties derived from the best fit at 0° latitude, we find that the Diviner regolith temperatures can be well described for low latitudes, but significant deviations begin at a latitude around 40° and increase with increasing latitude. The modeled regolith temperatures are too high at higher latitudes.

Earlier investigations involving LRO/Diviner data and thermal modeling showed contrasting results. The H-parameter maps created by Hayne et al. (2017) did not show a latitudinal gradient. In contrast, Yu & Fa (2016) found a strong latitudinal dependence of the derived surficial thermal conductivity, which they attributed to a variation of intrinsic regolith properties. One difference between the models of Yu & Fa (2016), Hayne et al. (2017) and this work is the albedo model used in each case. The albedo is a function of the solar incidence angle and increases dramatically at high angles. We will investigate whether the observed latitudinal gradient in this work can be removed by applying an alternative dependence of albedo on solar incidence angle. Qualitatively, a steeper increase of albedo with solar incidence angle leads to modeled temperatures that are colder at higher latitudes and therefore help to remove the latitudinal gradient. In addition, we will investigate the effect of unresolved surface roughness on the nighttime regolith temperatures as a function of latitude. An indication of the presence of this effect is the coincidence of the onset of the observed latitude gradient with the angle of repose of granular media.

Delbo, M. et al. (2014), Nature 508, 4466-4470. Hayne, P.O. et al. (2017) JGR, 122, 2371-2400. Paige, D. A. et al. (2010), Space Sci. Rev., 150, 125-160. Pieters, C. M. & Noble, S. K. (2016), JGR, 121, 1865-1884. Yu, S. & Fa, W. (2016), Planet. Space Sci., 124, 45-61.

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