spaceSpace and Physics

A Longstanding Mystery About The Moon Has Just Been Solved - And It Has A Twist Ending


Robin Andrews

Science & Policy Writer

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.

A team led by Texas Tech University decided to do some sleuthing back in 2010, and managed to locate the missing data tapes in a massive federal agency archival center.

Painstakingly restoring and recovering the data for the late-70s temperature readings, they gained a better understanding of the temperature changes: It appeared the temperature increase was detected by the shallower probes, before registering on the probes deeper down.


Indicating it was a surface-related cause, they cross-referenced their findings with images taken of the Moon’s surface at the time they were being made.

Writing in the Journal of Geophysical Research: Planets, the team explain that “images of the Lunar Reconnaissance Orbiter Camera over the two landing sites show that the regolith on the paths of the astronauts turned darker,” which lowered its reflectivity.

“We suggest that, as a result of the astronauts' activities” – walking around, doing science – “solar heat intake by the regolith increased slightly on average, and that resulted in the observed warming” as more sunlight was able to be absorbed at the surface.

So as is commonly the case these days, the temperature rise was our fault.


[H/T: AGU]


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