A new study sheds light on an old mystery surrounding strange lunar readings from the Apollo landing sites.

A new find may have solved an old mystery: what caused the anomalous heat readings from the instruments that Apollo astronauts left on the Moon?

In 1971 and 1972, the Apollo 15 and 17 missions deployed Apollo Lunar Surface Experiments Packages (ALSEP) that included a set of Heat Flow Experiment (HFE) monitors. The ALSEP packages, designed to probe the lunar interior over an extended period of time, were powered by a plutonium radioisotope thermoelectric generator (RTG) in order to survive through the two week long lunar night.

The astronauts placed the HFE monitors a few meters into the lunar surface to access soil undisturbed by the month-long day/night cycle. Lunar scientists planned to use the data to measure the flow of heat from the Moon's core to the surface, in hopes of characterizing geologic activity on the Moon.

Surprisingly, investigators noticed that the temperature of the regolith increased gradually at both the Apollo 15 and Apollo 17 sites, by 1.8° to 3.6°F (1° to 2°C). The warming continued until the sensors fell silent in 1977. The thermal gradient decreased at both at both sites to the same degree, and the warming was more apparent at shallow depths than deeper down. The HFE sensors on the Moon were placed far enough away from the ALSEP RTG that radioactive heat wasn't a factor — so what was warming up the soil?

A hint came from mission differences in sensor placement: Apollo 15 astronauts had a problem drilling into the lunar soil and weren't able to reach the targeted 2.5-meter depth for their HFE sensor. Apollo 17 featured an improved drilling mechanism, and the astronauts were able to place the sensor deeper. In fact, data from the Apollo 15 sensors was not considered valid until it was compared with Apollo 17 sensors years later. Astronauts installed a number of different sensors a multiple depths at both the Apollo 15 and 17 sites.

Remarkably, the shallower sensors saw the anomalous heating first — just what you would expect if the heating mechanism was coming from the surface instead of the core below.

Now, a new study published in the American Geophysical Union's Journal of Geophysical Research: Planets uses data recovered from the mid-1970s to demonstrate the cause of this anomalous heating: the astronauts themselves.

“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,” Seiichi Nagihara (Texas Tech University) related in a recent AGU blog post.

To Solve a Mystery . . .

The study wouldn't have been possible without some detective work. NASA had only archived data from 1971 to 1974, storing it on magnetic tape at the National Space Science Data Center at the Goddard Spaceflight Center in Greenbelt Maryland. But the data collected in later years had gone missing.

A breakthrough came when researchers discovered an additional set of 440 archive tapes at the Washington National Records Center in Suitland, Maryland. Then researchers came across a set of weekly logs with hundreds of heat-flow temperature readings that filled in the gaps from 1973 to 1977. These logs had been stored at the Lunar and Planetary Institute in Houston, Texas.

Heating the Moon

The additional data solved the mystery of the warming Moon: as the astronauts walked and drove the lunar rover around the site, they disturbed the Moon's smooth surface. The rumpled surface better absorbed heat from the sunlight.

For those who often look up at the Moon, this conclusion may seem odd at first. We know that even though the full Moon appears to be a bright pearly white, its albedo or reflectivity is actually quite low (12% on average), about the same as worn asphalt. Astronauts described Moon dust as comparable in color and texture to coal dust. The material darkens over time due to interaction with the solar wind, a process called space weathering. Beneath this dark surface coating is more reflective material — this is why bright ejecta rays surround newer craters, as the impact has excavated brighter material from underneath the surface. So one might think that disturbing the surface would expose brighter material.

However, even though it's more reflective, the regolith underneath the dusty surface is coarser — and pebbles can hold on to heat for longer than fine dust can. So as the astronauts disturbed the dust, they exposed this coarser, more heat-absorbent material, and warmed the surface.

"A key piece of information was the photographic images obtained by the Lunar Rconnaisance Orbiter (LRO) camera," says Nagihara. The LRO had photographed the Apollo landing sites from low lunar orbit, providing evidence that backs up the temperature readings. "The images show that the places where the astronauts walked and drove their rovers turned darker, absorbing more solar heat than brighter soil."

Implications for Future Missions

This finding will impact how experiments are placed on the Moon in the future. "Whenever we deploy or land something on the Moon, it alters the surface thermal environment of that vicinity," says Nagihara. "Scientists and engineers need to take that into consideration in designing the next generation of heat flow instrumentation for future lunar landing missions."

Planned missions show that we are far from finished with our exploration of the Moon: China launched its Queqiao Lunar Relay Orbiter recently in preparation for the first automated lunar farside landing later this year. NASA's current trajectory has also again shifted the focus of its crewed exploration program back to the Moon, which will begin with robotic missions on the lunar surface.

Nearly half a century after the Apollo missions, new science from the data they returned may affect the planning of these future missions.