Apollo 17 Mission
Science Experiments - Heat Flow Experiment
The rate at which a planet loses its internal heat to space is an important control on the level of tectonism (faulting and folding of the planet's surface due to internal deformation) and volcanic activity on the planet. This loss of internal heat was measured by the Heat Flow Experiment on Apollo 15 and 17. This experiment was also attempted on Apollo 16, but failed due to a broken cable connection.
The Heat Flow Experiment involved drilling two holes into the regolith to depths of 1.6-2.3 meters. The temperature was measured at several depths within the hole. The rate at which temperature increases with depth is a measure of the heat flowing from the Moon's interior. The drilling caused some heating within the hole, although the effects of this heating decayed with time. Also, temperatures in the upper part of the regolith vary as the amount of incident sunlight changes throughout the lunar day and night. By monitoring temperatures in the drill holes over a long period of time, these effects can be accounted for, allowing for a determination of the average heat flow rate at the landing site.
The results of these measurements indicate a heat flow of 21 milliwatts per square meter at the Apollo 15 landing site and of 16 milliwatts per square meter at the Apollo 17 landing site. To place these numbers in context, powering an ordinary 60 Watt light bulb would require the collection of all the thermal energy flowing out of a region on the Moon that is 60 meters by 60 meters across. The Moon's heat flux is 18-24% of the Earth's average heat flux of 87 milliwatts per square meter. The small value of the lunar heat flow was expected, given the Moon's small size and the observation that it has been nearly dead volcanically for the last 3 billion years. Because the heat flow was measured at only two locations, it is not known how representative these values are for the Moon as a whole. However, because both measurements were obtained near boundaries between mare and highland regions, it is thought that the measured heat flows are probably 10-20% higher than the average value for the entire Moon.