The heat flow of the Moon: What do we know, and how do we measure it?

With the exception of the Earth, the Moon is the only terrestrial body for which the heat flow has been measured in situ. During the Apollo 15 and 17 missions, two probes at each landing site were inserted into the lunar regolith, and between 1971 and 1977 data concerning the temperature profile and thermal conductivity were collected. One of the more intriguing results of this experiment was that the derived heat flow at the Apollo 15 site appeared to be higher than that at the Apollo 17 site (21 vs. 16 mW m-2). In retrospect, the interpretation of the Apollo heat flow experiment data have turned out to be more complicated than originally acknowledged. Thermal conductivity estimates derived from two different techniques (a heating experiment and the measurement of the annual thermal wave) were found to be discordant by a factor of two. The Apollo-era studies neglected to consider the 18.6-year precession of the lunar orbit plane that acts to modulate the annual thermal wave by a non-negligible factor. The average temperature at a given depth was found to increase slowly with time, for which no good explanation currently exists. Finally, it is now known that heat producing elements in the lunar crust are distributed in a highly asymmetric manner, and if one would like to obtain the average heat flow of the Moon, more than two measurements would certainly be required. Our next big leap forward will certainly come with the acquisition of new data. While astronauts could emplace heat flow probes on the Moon, such stations would be limited to the number of human landing sites. A promising robotic alternative is the use of an electro-mechanical "mole" such as the HP3 that is being developed for ESA's ExoMars mission. After being deployed on the surface, such a device could hammer its way several meters below the surface, most likely deeper than any rotary drill could achieve. Thermophysical properties would be measured in the mole itself, as well as in a trailing payload compartment that could include a densitometer, thermal conductivity experiment, and electrical permittivity probe. Sensors embedded in the tether that connect the mole to the surface electronics box would perform long-term monitoring of the temperature profile.