Something strange happened on the moon during the Apollo missions of the early 1970s.
Probes deployed by the Apollo astronauts revealed that temperatures on the moon's surface and subsurface were inexplicably rising. Temps kept rising slightly during the six years that the probes were functional. For decades, scientists puzzled over what might have caused this increase in temperature. Could the phenomenon be ascribed to changes in the moon’s orbit? Was the moon being affected by excess radiation coming from Earth?
Now, as Nicole Mortillaro reports for CBC News, a group of researchers who spent eight years tracking down lost archival data think they have an answer to this enduring lunar mystery: The Apollo astronauts, as they walked and drove over the lunar surface, created disturbances in the regolith (also called lunar soil) that caused the moon to get hotter. The team’s findings were published recently in the Journal of Geophysical Research.
In 1971 and 1972, during the Apollo 15 and 17 missions, astronauts installed temperature-measuring probes near the moon’s surface and further down into its subsurface. The goal of the so-called “heat flow experiment” was to find out how much heat moves upwards from the moon’s core. Until 1977, when the experiment ended, the probes transmitted raw temperature data back to the NASA Johnson Space Center in Houston, where the data was recorded on magnetic tapes.
In 2010, Seiichi Nagihara, a planetary scientist at Texas Tech University in Lubbock, decided to try and find out, once and for all, why the lunar surface temperature started to rise soon after the Apollo missions started. But there was a major obstacle in the way of his research. Only the tapes that had been recorded between 1971 and 1974 were archived at the National Space Science Data Center, according to a press release describing the new study. The tapes from 1975-1977 had been lost.
Additionally, the tapes that were held at the National Space Science Data Center were not complete. According to a paper Nagihara and colleagues presented at the 2010 Lunar and Planetary Science Conference, these recordings “had been resampled from the original 7.2-minute measurement intervals to ~60-minute intervals.”
So Nagihara and his colleagues set out to find the missing data. They were able to track down 440 archival tapes, recorded between April and June of 1975, at the Washington National Records Center, part of the National Archives, in Maryland. The researchers also uncovered hundreds of weekly logs from 1973 to 1977, which recorded readings from the lunar probes.
“These logs helped the scientists reconstruct the temperature readings for the times not covered in the archival tapes–January through March 1975, and July 1975 through February 1976, when the instruments began to reach the end of their functional lives,” the press release explains.
The scientists spent several years extracting and analyzing data from the tapes. They fouund that the probes closer to the moon’s surface recorded greater and faster spikes in temperature than the ones further down, suggesting that the heat was originating not from the core but at the lunar surface.
With this new data in hand, Nagihara and his colleagues were able to formulate a theory. When the Apollo astronauts walked or drove over the lunar surface, they kicked up a type of light-colored rock called anorthosite, exposing the darker lunar soil underneath. “Darker soil absorbs more light from the sun, which makes it warmer, and the researchers suspect this is what caused the warming,” the press release explains.
The very process of installing the probes may have disturbed the surrounding environment, raising lunar surface temperatures by 1.8 to 3.6 degrees Fahrenheit in areas where the disruptions occurred.
To corroborate this theory, the researchers consulted photos of the moon taken by the Lunar Reconnaissance Orbiter Camera, according to Brandon Specktor of Live Science. The images revealed that areas around the Apollo landing sites were marked by dark streaks where the astronauts had moved about.
“You can actually see the astronauts tracks, where they walked,” Walter Kiefer, a senior staff scientist at the Lunar and Planetary Institute and one of the study co-authors, tells CBC’s Mortillaro. “And we can see … where they scuffed dirt up—and what it leaves behind is a darker path.”
The slight increase in lunar temperature that was observed in the 1970s likely won’t harm the moon. But the new study reveals how humans can alter the environment of other planets, just as they are changing the environment here on Earth. And as Nagihara says in the press release, “[t]hat kind of consideration certainly goes in to the designing of the next generation of instruments that will be someday deployed on the moon.”