When I was a graduate student at the University of Central Florida finishing up my dissertation on meteorite densities and porosities, one day my advisor Dan Britt called me into his office. He said, “I have a little side project for you.” He made me put on laboratory gloves and then handed me a rock. It was small, a few centimeters on each side, grey in color with a few lighter and darker inclusions. It didn’t look like most of the meteorites that I had worked with. “I want you to measure the density and porosity of that,” he told me.
And then he told me what I held in my hand. “That is an Apollo moon rock.”
He had been allocated five specimens from Apollo missions 12, 14, and 15, as the first part of a new study. Our collaborator in this research, Walter Kiefer from the Lunar and Planetary Institute, wanted to use data of lunar gravity anomalies to better understand the lunar crust, and for this he needed ground-truth data on the densities of lunar rocks.
As spacecraft orbit the moon, they experience variations in the strength of the gravitational pull, which causes the craft to speed up or slow down slightly. The change is small but measurable, and a careful record of this speeding up or slowing down can be translated into a gravity map of the lunar surface. These variations are the result of regions with greater or lesser overall density compared with the lunar average. One of the key factors affecting the density is the thickness of the lunar crust, so gravity anomalies can help one understand the crustal structure of the moon. But, to understand this, one needs to know the density of lunar rocks consistent with the geology of the region.
That’s where the moon rocks come in. These are field specimens; that is, they were collected by human beings from the surface of the moon, and the locations of where they were collected is very well documented. Therefore, they are not just “rocks from the moon,” but they are rocks with a geological story to tell. As part of our study, we also worked with lunar meteorites, which gave us a greater breadth of material to work with, but the Apollo specimens were best because we knew exactly where they came from.
The first few Apollo moon rocks that I worked with had been allocated from the “measurement suite,” a set of rocks that had already been extensively studied, and there was not really any problem with us working on them in my own laboratory.
In the work that I do, I measure two types of density: bulk density (the density of the whole rock, including any contributions from interior cracks or voids) and grain density (the density of only the solid component of the rock, excluding voids). The difference between the two gives the porosity, or the percentage empty space inside the rock. Grain density is measured with ideal gas pycnometry, where we place the object in a chamber that is pressurized with inert gas. A valve is opened that expands the gas into a second chamber. By comparing the pressure before and the pressure after, we can calculate the volume displaced by the rock. Since the gas penetrates interior space, this is only the volume of the solid component. Bulk density was, at that time, measured by the “glass bead” method developed by Br. Guy Consolmagno. By weighing a cup containing the rock and filled to the brim with tiny glass beads, it is possible to calculate the volume displaced by the rock.
We quickly finished the measurements on the five Apollo specimens that had been allocated, and dutifully returned them to NASA. But the research was not done. We had only scratched the surface of the moon. To complete the study, we needed a lot more rocks from all of the different landing sites. More requests went out for more specimens.
Requests to work with moon rocks are reviewed by a committee, called Curation and Analysis Planning Team for Extraterrestrial Materials, or CAPTEM for short. What made our proposal strong was that we did not have to remove all of the moon rocks from their home at NASA. All of our research equipment was portable, and so we could do the work on-site. This also made it possible to access a larger suite of specimens. And so it was that, as a newly minted PhD, I packed my research equipment in the trunk of my car and drove from Florida to Houston for the first of several visits to NASA.
NASA Johnson Space Center, in suburban Houston, is a magical place. After getting my credentials, I parked the car by Building 31, which houses the Lunar Receiving Laboratory where all of the moon rocks from the Apollo missions are stored. It is a fairly unassuming structure; from the outside little more than a big concrete box. One of the peculiarities of the space program is that the place where extraterrestrial specimens is stored and preserved in the most pristine of conditions happens to be in the middle of a hurricane zone. Hence the architecture: the building has to withstand hurricane conditions without threatening the integrity of the precious specimens stored inside.
The laboratory where I worked is not exactly a clean room, but is a very low-dust environment. Each time I entered, I had to don a lint-free smock and a cap (called a “snood” for some reason) and put plastic slippers over my shoes. All of the equipment had to be thoroughly wiped down before it could enter the space.
The place where the moon rocks are actually stored, on the other hand, is a much cleaner space. You go full bunny-suit to enter that area, and have to pass through an airlock with an air shower to blow off any dust that might cling to your suit. The moon rocks themselves are kept in glove boxes with a pure nitrogen environment. Nothing says “space rocks” like a good glove-box chamber. These glove boxes are in a hermetically-sealed vault with an imposing door.
The curation staff are wonderful people. The curator of the Apollo collection, Ryan Zeigler, is a jovial fellow who happens to have been at Washington University at about the same time I was. He made doing the research easy; instead of setting up bureaucratic obstacles to jump through, he did his best to smooth them over to make the research happen. Most of the other curation staff are NASA contractors, and are also a wonderful bunch of people. They have a friendly rivalry with “the other side of the building,” where the Antarctic meteorites are stored. (In my research, I have dealt with both sides of the building.) They often all go out to lunch at a local barbecue joint, and they welcomed me along when I was there.
After a couple visits, we went through another round of sample requests with CAPTEM. This time, someone on the committee expressed concern about possible trace element contamination from the glass beads. They recommended we use alumina instead of glass. Glass beads are cheap; you can get pounds of it for pennies. Alumina beads, on the other hand, are very expensive. We shelled out a pretty penny for a kilogram of the stuff, but as soon as we tried to use them for measurements, we discovered a problem. They develop a static electric charge, which causes the tiny beads to literally jump out of the container. This completely messed up our measurement. (It also messed up the lab. A cleaning crew spent several hours after the visit picking up all the beads that had flown everywhere.) That’s when Ryan mentioned, “I think there’s someone here who has a laser scanner. Maybe we can try that.”
Laser scanners are great. They create a 3-d model of the specimen in the computer, from which the volume can be calculated. There are no beads, and nothing touches the specimen except a low-intensity laser light. After a few tests to prove the concept, we became so enamored with the technology that we acquired the same model laser scanner for our own research. Today, it is my preferred method for measuring bulk density.
The visits were fun and productive. In total, we worked with almost 60 specimens from all 6 successful Apollo landings, and I have held over 1kg of these rocks in my gloved hands. Complementing the data with about 200 lunar meteorites, it provided a good data set from which to analyze the gravity data from Lunar Reconnaissance Orbiter, GRAIL, and other instruments.
It has already provided some useful insight. For instance, new data for the density of rocks from Apollo 14 suggests that our estimate of the depth of the crust in the Orientale basin has been underestimated by about 25%.
Unfortunately, I do not have plans to go back for more moon rocks. It was fun, but the project has been completed. I miss my visits to Building 31 and all of its occupants, whom I consider almost like family. (It was wonderful to have several of them, including Ryan Zeigler, come visit me last September for a curation workshop at the Vatican Observatory.) One day I hope to have the chance to go back to that magical place.
This week is marked by the commemoration of the 50th anniversary of the Apollo 11 moon landing. Undoubtedly it will also provide an occasion for the conspiracy theorists to once again make a claim that the astronauts never landed on the moon. Take it from me: they not only went there, but they came back with rocks to prove it. I have held these very same rocks in my hand.