Lunar Heat Flow Simulation and Testing Chamber

Introduction: Interior heat flow provides essential constraints on the thermal and chemical evolution of planetary bodies. On the moon, heat flow instrument designs typically have to reach depths below the annual solar thermal wave for the Moon (2-3 m), or alternatively methods must be applied to be able to decouple the influence of the relatively large temperature gradients associated with the annual and diurnal solar thermal wave from the geothermal gradient. There are many challenges associated with precise heat flow measurements. Key challenges include: 1) accessing the subsurface to sufficient depth in various types of media (e.g. heavily compacted regolith and/or soils with variable in-situ rock distributions), 2) obtaining accurate measurements of the thermal conductivity and thermal gradient in an extremely insolating environment with extremely large surface temperature variations, and 3) calibrating out potential sources of measurement error including a) instrument thermal properties and instrument self-heating, b) local variations in thermal properties generated by the penetration of the instrument, c) thermal conductance of heat through the instrument to the surface, and d) strong potential differences in thermal coupling between the heat flow sensors and the in-situ medium in a vacuum environment associated with partially collapsed borehole walls or intermittent sensor contact with a borehole wall. To address these issues and provide a testbed for evaluating heat flow measurements and calibration techniques prior to flight, we have constructed a vacuum chamber capable of simulating the lunar heat flow environment.