As part of the Apollo 15 and 17 (pictured here) missions, plenty of geological experiments and instrumentation were carried out and deployed on the lunar surface. MSFC/NASA

Astronauts lucky enough to have jumped around on the Moon encountered a bit of an enigma during their brief stays there: Namely, the surface was hotter than they were expecting. As spotted by AGU’s Lauren Lipuma, this mystery has now been solved thanks to some solid detective work – and as with all good mysteries, there’s a twist at the end of the tale.

The Apollo missions to the lunar surface were, quite clearly, not all about rubbing it in the nose of the Soviet Union. Plenty of science was, and still is, being conducted on chunks of the Moon, including on how heat escapes from its heart to the surface.

During the Apollo 15 and 17 missions in the early 1970s, probes were also placed into the ancient volcanic soil in order to see how our pale guardian was cooling down. This is probably more important than you think: after all, the cooling of Earth’s interior is the reason why we have continents, mountains, earthquakes, volcanism, and pretty much every surface process you can imagine.

The Moon’s a dead sphere, and it has been for millions of years; its volcanism has long since died out, and it certainly never managed to develop any tectonic plates. Still, it’s constantly chilling itself, and NASA wanted to know by how much.

Drilling a few holes into the ashen ground, astronauts on both these programs poked in their high-tech thermometers and noted the readings.

This wasn’t easy, mind you: they had to account for changes due to sunlight, and the heating created by the drilling itself. Eventually, though, long-term readings indicate that, according to the Lunar and Planetary Institute, the surface heat flux of the Moon is between 18 and 24 percent of Earth’s.

Well now that'll cause some surface disturbance: a Lunar Roving Vehicle, pictured here on the Moon during Apollo 17's mission. MSFC/NASA

Something was amiss, though: the heat probes registered a gradual heating of the Apollo landing sites long after the original measurements were taken. It was entirely unclear why, but it couldn’t be because of an internal process releasing more heat.

The data tapes clearly held the answers, but sadly, someone messed up. After these experiments ended in 1977, it seemed that scientists only archived the data from 1971 to 1974. The rest were, somehow, never achieved and were lost.