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Vol. 60, n.3, September 2019
pp. 489-516

Lunar subsurface temperature profile modelling based on CE-1 and CE-2

W. Zhang

Received: 28 December 2018; accepted: 18 March 2019

Abstract

The subsurface temperature distribution of airless bodies across the Solar System can provide important clues to their formation and evolution. This paper investigates the lunar soil temperature profile using data from the recent Chinese lunar orbiting spacecrafts Chang'E 1 (CE-1) and Chang'E 2 (CE-2), to explore variations in the subsurface temperature of the Moon. These variations include heat flow information of the subsurface and the interior of the Moon. Before the launch of CE-1, the temperatures of deep layers of the Moon have only been measured at the landing sites of Apollo 15 and 17 by in-situ temperature probes. The CE-1 and CE-2 lunar orbiters were both equipped with a 4-frequency microwave radiometer (MRM) to detect the lunar surface brightness temperature (TB) and to retrieve data on lunar regolith thickness, temperature, dielectric constant, and other related properties. The MRM can penetrate to a nominal depth of 5 m in the subsurface with the 3 GHz channel. This research aims to develop a radiative transfer forward model for an airless body and then utilise MRM data to study an observed anomaly of 2 m deep TBs measurements in the Oceanus Procellarum region on the lunar subsurface. After initial validation of the MRM data and modelling of the lunar regolith parameters, a multi-layer radiative transfer forward model has been derived using the fluctuation dissipation theorem. The forward model calculates the radiometric contribution from several depths to the TB that would be observed by the MRM instrument around the Moon (at different frequencies), as a basis for an inverse method. Sensitivity analysis indicates that, as expected, mineralogy and density information are very important to the inverse calculation. The FeO/TiO2 distributions were also used to derive the bulk density of the lunar surface which was also incorporated into the calculation. The forward model was, then, used to invert the MRM measured TB data to generate 2-m depth subsurface temperature profiles. The provisional results show that, as expected, the 2-m subsurface temperature is potentially correlated to the distribution of radioactive elements such as uranium and thorium in the lunar crust. The 2-m subsurface temperature map was then converted to a lunar heat flow map, which was validated by the Apollo 15 and 17 measurements. Inspecting this heat flow map, abnormal high heat flow in the Oceanus Procellarum KREEP Terrain (PKT) region was noticed. The PKT is enriched with a high abundance of radioactive elements such as uranium and thorium. Hence, a heat flow model based on radioactive elements as well as internal cooling was built to investigate such a finding.



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