A recent study published in the Journal of Geophysical Research: Planets finally sheds light into a 40-year-old moon mystery that started with NASA’s Apollo 15 and 17 missions.

Back in the 1970s, when the two Apollo missions landed on the moon, the astronauts discovered that Earth’s natural satellite was going through a strange warming phenomenon.

One of their objectives was to monitor the moon’s core and find out the rate at which it loses heat. Known as the Heat Flow Experiment, their research involved probing the moon’s regolith — or mantle rock, the layer of disintegrated, loose rocks and soil lying on the surface.

But they encountered an anomaly that they were unable to explain. The moon was exhibiting a peculiar subsurface warming, which seemed to be more pronounced around the Apollo landing sites.

Missing Apollo Tapes

Their experiments on the moon ran from 1971 until 1977, years after the astronauts returned home. Although the scientists came back to Earth, their equipment remained on the moon, recording data on its surface and subsurface temperatures through probes placed in drilled holes up to 2.3 meters (7.5 feet) in depth.

The collected data was stored on tapes and archived at the U.S. National Space Science Data Centre, Science Alert reports. However, everything recorded after December 1974 never made it to the archive and was lost for more than three decades.

That is, until 2010, when researchers from Texas Tech University tracked down 440 of the lost tapes at the Washington National Records Center. Although the tapes were severely deteriorated and only represented less than 10 percent of the entire lost data (they covered just the period from April through June 1975), they still provided a good starting point for finally unwrapping the moon warming enigma.

After spending the last eight years restoring the tapes to recover the data — as well as combing through the logs of the Lunar and Planetary Institute in Houston, where they uncovered thermal data from the moon probes going from 1973 through 1977 — the Texas Tech team is finally ready to give us an answer to what made the moon get warmer.

Key To The Four-Decade-Old Puzzle

As it turns out, it was the Apollo astronauts all along — or, as Outer Places explains it, their influence on albedo, the solar light reflected by the moon’s surface.

It seems that while the astronauts were walking on the moon and driving their lunar rovers, they disturbed the whiter topsoil, which reflects the sun’s heat, and exposed the darker regolith, which does exactly the opposite and absorbs sunrays, eventually causing a small increase in the moon’s temperature.

In doing so, the astronauts inadvertently triggered a climate change process on the moon, which was revealed by the missing Apollo tapes. The recovered data showed that the moon was warmer closer to the surface than near the deepest probes, which suggested that the warming process was not triggered from the moon’s interior but from the surface.

By comparing the thermal data from the lunar probes with video recordings of the moon’s surface, taken during the Apollo missions, the Texas Tech scientists were able to establish that the astronauts were, in fact, responsible for the warming of the moon — a process that persisted until the Heat Flow Experiment was wrapped up in 1977.

“Recently acquired images of the Lunar Reconnaissance Orbiter Camera over the two landing sites show that the regolith on the paths of the astronauts turned darker, lowering the albedo,” the team wrote in their paper.

“We suggest that, as a result of the astronauts’ activities, solar heat intake by the regolith increased slightly on average, and that resulted in the observed warming.”

Study lead author Seiichi Nagihara, a planetary scientist at the Texan university, told Geo Space that explorers sometimes run the risk of contaminating or influencing the pristine environment that they’re trying to study.

“In the process of installing the instruments you may actually end up disturbing the surface thermal environment of the place where you want to make some measurements.”

However, as Nagihara pointed out, measures are being taken so that this kind of situation doesn’t repeat itself during future lunar exploration missions.

“That kind of consideration certainly goes in to the designing of the next generation of instruments that will be someday deployed on the Moon,” he said.

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