We have conducted experiments to directly measure the heat transfer between molten lava and lunar regolith simulant (GSC-1). Regolith samples retrieved during the Apollo missions contained extra-lunar particles, including solar wind and solar flare particles, and the cosmogenetic products of galactic cosmic ray impacts. However, the surface regolith samples were oversaturated from prolonged exposure to the space environment. A paleoregolith that was exposed to space and subsequently covered by a lava flow could hold particles that, if retrieved, could give insight into ancient solar or galactic activity. The lava flow would provide protection of the underlying deposits and provide solid material to radiometrically date to determine exposure age. However, the lava flow would have heated the regolith, volatilizing many of the extra-lunar particles. We have performed numerical modeling and laboratory experiments to determine the depths to which a lava flow will heat the underlying regolith, thereby determining the minimum depth of regolith development needed to preserve ancient solar particles beneath a lava flow. The experiments were performed to validate our numerical model of lava–regolith heat transfer, and to derive better estimates of thermal conductivity of regolith simulant at high temperatures.
Experimental devices were constructed from sheets of 1-inch thick high-temperature, calcium silicate insulation with interior dimensions of 20x20x25 cm. The box was packed with a 15 cm layer of sieved and dried regolith simulant and then the whole box was further dried to remove moisture, before cooling to ambient temperature in an airtight container. As a mare basalt analog, basalt collected at Kilauea Volcano, Hawaii, was melted in a gas forge to >1200 °C, and then poured onto the simulant surface. System temperature was monitored internally by a thermocouple array and externally by a Forward Looking Infrared (FLIR) camera for the duration of experiment. The experimental data thus describe the heating and cooling of the system, and reveal the release of latent heat of crystallization within the cooling lava. Although not an exact analog for the lunar environment, our experiments provide the only direct measurement of heat transfer from a lava flow into a particulate substrate and one of a few attempts to measure simulant thermal conductivity.
Few data exist on the values of thermophysical properties of lunar regolith at high temperatures, so the values used in previous models were greatly extrapolated. Agreement between our model predictions and experimental data serve to validate the model, which allows for a more accurate quantification of the depth reached by the heat pulse as it penetrates the substrate. A quantitative understanding of maximum heat pulse penetration depth could aid in future expeditions to the lunar surface, during which ancient regolith deposits could be sampled through surface drilling to extract extra-lunar particles, revealing a history of the solar activity and galactic events not available on the Earth.