New Lunar Sample Density and Magnetic Susceptibility Measurements

Introduction: In order to make use of the flood of data on the Moon’s gravity and topography provided by missions such as Lunar Reconnaissance Orbiter and GRAIL [1] to constrain our understanding of the Moon’s internal structure, we have been developing a comprehensive database of lunar rock densities and porosities [2]. This database includes contributions from both lunar meteorites and samples collected directly from the lunar surface. Meteorites may sample a broader diversity of geologic types and a larger portion of the lunar surface, but specimens collected during the Apollo missions provide invaluable information about the geologic context where the rocks originated. In October 2013 we visited both the Apollo lunar receiving laboratory and the antarctic meteorite collection at NASA Johnson Space Center. During this visit, we conducted bulk density and magnetic susceptibility measurements on 19 Apollo samples and three lunar Antarctic meteorites. These samples exhibit a range of lithologies including anorthosites, highand low-Ti basalts, and impact melt breccias. Among the Apollo samples, each mission except Apollo 11 is represented. These data mark a significant addition to our existing database of 97 lunar samples and meteorites. Measurement: Our techniques are described in detail in [3] and [4]. All of our techniques are nondestructive and non-contaminating, and were performed on-site at NASA Johnson Space Center. We measure grain density by helium ideal-gas pycnometry, and bulk density by the archimedean glass-bead method. Magnetic susceptibility is measured with a ZHinstruments SM-30 meter, with a volumetric correction consistent with [5], and are reported as logarithmic units. Due to equipment problems, we were unable to measure grain densities during the trip in October 2013. We plan helium pycnometer measurements and possibly also 3D laser scanner measurements during a follow-up visit in early 2014. Both grain and bulk densities are necessary for determining sample porosity. For the Apollo samples, instead of our usual 700800 μm-diameter glass beads, CAPTEM requested that we use high-purity alumina (greater than 99.99% pure Al2O3) beads to eliminate any possibility of trace element (particularly Na) contamination. These beads were ~500 μm in diameter. Caveats: The small diameter of the alumina beads, and hence greater surface area to volume, coupled with the low humidity of the facility created a pronounced static electricity effect on them. Beads at the top of the container were literally jumping out, making it difficult to level the surface consistently. As a consequence, measurements of the bead-filled cup mass exhibited much greater variation than similar measurements with our usual glass beads, as exemplified by measurements on the Antarctic meteorites. This led to greater uncertainties in bulk densities. All of the Apollo samples discussed here are nonpristine specimens that were previously used in other studies. In many cases the available specimens allocated for measurement were less than the 10g minimum mass that we prefer to measure. Volumetric uncertainty is largely independent of the sample volume, and so for a small sample the error is proportionally much higher than for a large sample. This prevents us from reaching our stated goal of ±0.01 error in porosity for those samples. In a few high-priority cases, it may be necessary to measure larger fragments from the pristine sample collection to achieve uncertainties which are small enough to be useful in geophysical modeling. Another consideration associated with small sizes of samples is whether or not they are representative of 2.00 2.20 2.40 2.60 2.80 3.00 3.20 3.40