Lead author Seiichi Nagihara is an associate professor of geophysics at Texas Tech.
Many in the online media have reacted to the recent publication of a study in the Journal of Geophysical Research: Planets, which shows that Apollo astronauts' activity likely contributed to slight, local lunar surface warming. One of the study's lead researchers would like to clear up misunderstandings that have resulted, particularly in light of the millions of views these reports have since received.
The following is a question-and-answer interview with Texas Tech University associate professor Seiichi Nagihara, lead author on the study.
Q: To begin with, let's make sure I understand the story correctly. Two heat flow probes were placed on the moon at each of the landing sites of Apollo 15 (1971) and Apollo 17 (1972). Temperatures recorded at both of those sites gradually increased at all depths from the time of deployment through December 1974, which was the last data analyzed because data from January 1975-September 1977 went missing. Without this data, no one knew why the temperatures had increased.
I would phrase it a bit differently. To more fully understand the nature of the subsurface warming, we, the researchers, needed to examine the data for the full duration of the experiment. For example, if you look at the 1972-1974 data alone (Figure 3 of the paper), the deepest temperature sensors of Apollo 17 (the green dots) show only a hint of warming. But if you look at the data for the full duration of 1972-1977, it is very obvious that these sensors also warmed up.
Q: For this study, you and your collaborators tracked down and restored major portions of the missing data and analyzed it. You found that, after an initial cooling trend (which resulted from a heat spike due to the hole-drilling process), the probes at both Apollo sites recorded a gradual warming trend – for the Apollo 15 site, this lasted until instrument failure; for Apollo 17 site, until the experiment's end. You examined previous explanations for this warming and found that only the surface warming caused by astronauts walking on the moon's surface and disturbing its fine soil (ultimately making it darker so it absorbed more sunlight) properly explains the subsurface warming. You suggest that for future missions, a lander's thermal disturbance should be taken into consideration when deploying probes and measuring their data.
You said some media outlets may have misunderstood or misconstrued this news. At least one media outlet characterized this as global warming on the moon, and others have said the subsurface warming was the astronauts' fault and that they were “disrupting a celestial ecosystem.” What are your thoughts?
These comments really puzzled me. When I hear “global warming,” I think of warming climate. The moon has no atmosphere (it is extremely thin and practically the same as a vacuum). So, the moon has no weather or climate to speak of. There has been no life on the moon. So there has been no ecosystem on the moon.
Also, because of the lack of atmosphere, temperature of the surface soil of the moon changes dramatically between day and night. At noon time on the moon, ground surface temperature reached about 87 degrees Celcius (C) (189 degrees Fahrenheit (F)) at the Apollo 15 landing site. At dawn, ground surface temperature fell to -193 degrees C (-315 degrees F). The subsurface temperature at 1 meter depth is a lot more stable, roughly at the mid-point between the day and night (-20 degrees C or -4 degrees F). So the surface of the moon experiences extreme heat (and cold) every day. Again, that is because of the lack of atmosphere. It has nothing to do what the astronauts did on the moon. By the way, one lunar day is roughly equivalent to 29 days on Earth.
What our research suggests is that the long-term average temperature of the ground surface experienced a step increase of 1.6 to 3.5 degrees C (2.9 to 6.3 degrees F) when and where the astronauts walked on the moon, and that subsurface temperature slowly caught up with the surface warming in the following years. This is not a runaway warming. Also, this warming of the surface soil is confined to the places where the astronauts walked and drove. Because the moon has no weather, once the ground soil is disturbed, it stays that way for a very long time, until a meteor hits and disturbs it again, perhaps millions of years later.
Some of the people who made comments like “global warming” may be extrapolating these finding to Mars or other planets that have atmosphere. But that does not make much sense, either. You may have heard about the intense dust storms on Mars – that is what Mars' atmosphere does. So if, someday, astronauts land on Mars and leave their footprints, these footprints would be wiped out by dust storms or wind shortly afterward. That would reset any change in temperature the footprints may have caused.
Finally, I have enormous respect for the Apollo astronauts for their bravery and having gone through the rigorous training to get the moon, when the moon was literally an uncharted territory. They performed the experiments as they were instructed by the scientists on the ground, and they did a great job. I would not take anything away from their remarkable achievement.
Q: I know you were searching for the missing data for years. Can you tell me what it was like and how you felt when you finally found it?
I felt like I was a detective on a cold case. It was unlike any of the research projects with which I was previously involved. When we found the data tapes that were missing for decades, it was a relief, because we could now properly archive the data for future generations. It was also very exciting because I could finally analyze the data. It was almost like receiving data from a new space mission because some of the data on these tapes had never been analyzed before.
Q: I understand there's more on these tapes than just temperature readings. What else are you studying now that you've found them? What will happen to the data to make sure it isn't lost again?
The archival tapes we found contain data from all the scientific instruments deployed by the astronauts. My research group includes about a dozen people with different expertise. These collaborators are now working on data from other instruments such as seismometers and gravimeters. For example, my collaborators based at the NASA Marshall Space Flight Center in Alabama found a number of moonquakes not previously detected by analyzing the data from these tapes.
We also are in the process of archiving all the data from these tapes with NASA's data centers.
Q: What sort of problems did you have extracting the data from those tapes and then processing the information?
The open-reel magnetic tapes were 40 years old when we found them. So, their quality had degraded. We needed to work with companies specialized in extracting data from old magnetic tapes. That took a long time. Once the data had been extracted from the tape, we had to process them. The data, as extracted from the tapes, were recordings of the output of the various sensors on the instruments, but they were not scientifically meaningful numbers, like temperature in degrees C. They were more like voltage readings of the electronic component that was sensitive to temperature. In processing these data, we needed to follow the exact procedure used by the scientists and engineers involved in the Apollo program. It took us several years in recovering the relevant reports and memos and piecing together the data processing procedures because they were scattered in multiple locations and were not properly cataloged.
Q: Overall, how serious of a problem is this subsurface warming? Could it have serious consequences in the future, perhaps if someday there was more activity on the moon?
The surface affected is limited to where the astronauts disturbed the soil. Also, the effect is a small (1.6-3.5 degrees C), one-time temperature increase. As we said in our research article, the darkening of the surface soil caused by the disturbance is still well within the natural variation we see on the moon. Yes, there could be more astronauts going to the moon in the future, but remember that the moon is very big. So I do not think that their activities would cause any major change in the surface condition of the moon.
Q: Should the knowledge gained from this study be applied to future space exploration, or is it specific to the lunar surface?
The knowledge gained from this study would be applied to future lunar missions, especially if they are going to make heat-flow measurements. When you introduce something new (like a spacecraft) onto the surface of a moon or a planet, you are changing its local environment to some degree. It may disturb its surface soil. It may pose a shadow where there was none before. I am certain the people planning future planetary missions will consider all such scenarios by learning from the past missions such as the Apollo. Also, these scientists and engineers would try to minimize potential impact to the surface environment and take it into consideration in planning any scientific measurements or experiments.
Q: I know you're already working to design a heat-flow probe to study Jupiter's ice moon Europa. Will you be applying any of the knowledge from this study to that work?
Yes, to some degree. I've learned a lot about how to design scientific instruments for missions to extra-terrestrial bodies by going through the reports and memos left behind by the people involved in the Apollo missions. But Europa has its own complicated problems. In many aspects, it is a lot more difficult to make heat-flow measurements on Europa than on the moon.
Q: On a personal level, this has been a major focus of your academic life for years. How does it feel to achieve your goal?
Well, my goal has been only partially achieved. Yes, I think that we now have a reasonable explanation to the problem of the subsurface warming observed at the Apollo 15 and 17 sites, and that knowledge would be useful for planning heat-flow measurements on future lunar missions. I am happy about that. But my collaborators and I know that we have not found all of the data tapes that went missing. The search for these missing tapes will continue.
Q: What's next for you?
You may be aware that NASA is now planning to send robots to the moon, possibly within the next few years. NASA also is planning to return humans to the moon some time in the next decade. Discussion on what can be achieved scientifically through these future lunar missions is already underway among the U.S. planetary scientists. I am involved in it. It is an exciting time to be studying the moon.