Pre-landing Assessment of the Surface and Shallow Subsurface Thermal Setting of Mare Crisium of the Moon for the Heat Flow Measurement Planned on the Blue Ghost 1 Mission

Abstract

In 2024, under NASA’s Commercial Lunar Payload Services initiative, a robotic lander will touch down in Mare Crisium of the Moon and deploy a heat flow probe. Ideally, the probe should penetrate the regolith deep enough (1.5–2 m) to avoid the influence of insolation, but it is a major challenge to excavate such a hole on a robotic mission. The present study assesses the insolation-induced temperature fluctuation in the shallower subsurface of the landing site in case the heat flow probe does not reach past that depth. The assessment was based on a newly constructed, one-dimensional heat conduction model, which accounted for the history of solar heat intake at the landing site for the past 100 yr. The thermal properties of the subsurface regolith of the model were constrained by the previous surface temperature observations by flyovers of NASA’s Lunar Reconnaissance Orbiter and the subsurface thermal measurement data from Apollo 17. The model showed that the amplitude of the insolation-induced annual subsurface temperature fluctuation is modulated by the 18.6 yr period precession of the Moon. The model also showed that the amplitude of this annual wave would be at (or close to) its minimum in 2024, when the mission would take place. Even though the thermal wave may be felt at depths greater than 1.5 m, because of its small amplitude, the thermal gradient in the depth range from 1 to 1.5 m would be within several percent of the gradient representative of the endogenic heat flow.