But the newly recovered tapes reveal that heat traveled all the way from the lunar surface to the bottoms of the boreholes, ruling out every explanation except for surface disturbance by the astronauts. As they explored the moon, their footprints and rover tracks compressed and darkened its surface. “It was the [absorption of] sunlight from where the astronauts were walking around that caused the moon to get hotter in those specific locations,” says study co-author Walter Kiefer of the Lunar and Planetary Institute in Houston. The results were published in May in the Journal of Geophysical Research: Planets.

The findings suggest that measurements taken at the start of the experiments—when the heat was still near the surface—were the most reliable, making the original data correct. “We now know we can trust those measurements in a way that we were not sure of a few years ago,” Kiefer says.