Heat Flow Probe Deployment Options for the International Lunar Network

Introduction: Knowledge of the Moon's thermal structure is fundamental in understanding its origin and internal compositional variation, both of which are intertwined with the origin of the Earth and the rest of the solar system (NASA Strategic Goal 3C). The flow of heat that originates from the lunar interior can be measured, and serves as a constraint to the thermal structure. That is why heat flow measurements were conducted during the Apollo missions [1] and are considered high priority for the International Lunar Network (ILN) missions planned in the next decade [2]. Heat flow is determined from two sets of measurements made in the subsurface: the thermal gradient in, and the thermal conductivity of, the depth interval of interest. A cylindrical probe is inserted into the subsurface for carrying out these measurements (Fig. 1). A heat flow probe typically contains a series of temperature sensors placed along its length. Temperature measurements obtained at different depths down the probe yield the thermal gradient. The probe also contains an electrical heater wire run along its length. After the thermal gradient has been determined, the wire emits heat into the surrounding regolith formation. The temperature sensors monitor how quickly or slowly the heat dissipates away from the probe at their depths. The information is used to calculate the thermal conductivity of the regolith [3-4]. The shallow subsurface temperature of the Moon is strongly influenced by the diurnal, annual, and prece-sional fluctuations of the insolation [1, 5-6]. Therefore , the best way to measure the internal heat flow is to insert the probe to a depth beyond the reach of the surface fluctuation. In order to avoid the 18.6-year-cycle precessions effect, the probe must reach 5-to 7-m depth [6-7]. Constraints and Options for Heat Flow Probe Deployment: A heat flow probe may be deployed in a number of ways on the Moon. However, for the ILN, very few options would fully meet both the constraints imposed by the small lander and the scientific objective of measuring the internal heat flow. For example, a mole can be an ideal tool for subsurface access for lander missions by meeting the mass and power constraints , but it is unlikely that it would reach the desired depth of 5 to 7 m into lunar regolith solely by internal hammering of the small mass. In addition, the hole dug by the mole would have variable diameters