Apollo Samples Research Articles

Identification of New Lithic Clasts in Lunar Breccia 14305 by micro-CT and micro-XRF analysis

From 1969 to 1972, Apollo astronauts collected 382 kg of rocks, soils, and core samples from six locations on the surface of the Moon. The samples were initially characterized, largely by binocular examination, in a custom-built facility at Johnson Space Center (JSC), and the samples have been curated at JSC ever since. Despite over 40 years of study, demand for samples remains high (~500 subsamples per year are allocated to scientists around the world), particularly for plutonic (e.g., anorthosites, norites, etc.) and evolved (e.g., granites, KREEP basalts) lithologies. The reason for the prolonged interest is that as new scientists and new techniques examine the samples, our understanding of how the Moon, Earth, and other inner Solar System bodies formed and evolved continues to grow. Scientists continually clamor for new samples to test their emerging hypotheses. Although all of the large Apollo samples that are igneous rocks have been classified, many Apollo samples are complex polymict breccias that have previously yielded large (cm-sized) igneous clasts. In this work we present the initial efforts to use the non-destructive techniques of micro-computed tomography (micro-CT) and micro x-ray fluorescence (micro-XRF) to identify large lithic clasts in Apollo 14 polymict breccia sample 14305. The sample in this study is 14305,483, a 150 g slab of regolith breccia 14305 measuring 10x6x2 cm (Figure 1a). The sample was scanned at the University of Texas High-Resolution X-ray CT Facility on an Xradia MicroXCT scanner. Two adjacent overlapping volumes were acquired at 49.2 micrometer resolution and stitched together, resulting in 1766 slices. Each volume was acquired at 100 kV accelerating voltage and 98 mA beam current with a 1 mm CaF2 filter, with 2161 views gathered over 360deg at 3 seconds acquisition time per view. Micro-XRF analyses were done at Washington University in St. Louis, Missouri on an EDAX Orbis PC micro-XRF instrument. Multiple scans were made at 40 kV accelerating voltage, 800 mA beam current, 30 μm beam diameter, and a beam spacing of 30-120 micrometer. The micro-CT scan of 14305,483 (Figure 2) was able to identify several large lithic clasts (approx. 1 cm) within the interior of the slab. These clasts will be exposed by band-sawing or chipping of the slab, and their composition more fully characterized by subsequent micro-XRF analysis. In addition to lithic clasts, the micro-CT scans identified numerous mineral clasts, including many FeNi metal grains, as well as the prominent fractures within the slab. The micro-XRF analyses (Figure 1b,c) of the slab surfaces revealed the bulk chemical compositions (qualitative) of the different clast types observed. In particular, by looking at the ratios of major elements (e.g. Ca:Mg:Fe), differences among the many observed clast types are readily observed. Moreover, several clasts not apparent to the naked eye were revealed in the K:Al:Si ratio map. The scans are also able to identify small grains of Zr- and P-rich minerals (not shown), which could in turn yield important age dates for the samples.

New crater calibrations for the lunar crater-age chronology

Measuring the spatial density of craters emplaced on geologic units is the primary method used for remotely estimating ages of solar system surfaces. The calibration for this method, which enables conversion of crater density to absolute age, comes from Apollo and Luna lunar samples for which absolute radiometric ages have been determined. Researchers throughout the 1970s worked to establish the lunar chronology system based on that calibration, correlating crater densities with absolute ages. However, no uniform crater study has been conducted on all calibration terrains, a limitation that was previously unaddressed until this study. The latest lunar images from the Lunar Reconnaissance Orbiter were used here to re-map the eleven main sampling sites and new crater counts of those surfaces were conducted. These show significant differences for many sites' crater counts, in many cases having more craters than previously identified. These results, calibrated to the radiometric ages, show a revised lunar crater chronology that changes previously established crater-based ages by up about 1 billion years: Surfaces younger than ∼3.6 Gyr and older than ∼3.9 Gyr under the classic chronology are younger in this system, and those in-between are older in this new system.

High-priority lunar landing sites for in situ and sample return studies of polar volatiles

Our understanding of the Moon has advanced greatly over the last several decades thanks to analyses of Apollo samples and lunar meteorites, and recent lunar orbital missions. Notably, it is now thought that the lunar poles may be much more enriched in H2O and other volatile chemical species than the equatorial regions sampled during the Apollo missions. The equatorial regions sampled, themselves, contain more H2O than previously thought. A new lunar mission to a polar region is therefore of great interest; it could provide a measure of the sources and processes that deliver volatiles while also evaluating the potential in situ resource utilization value they may have for human exploration. In this study, we determine the optimal sites for studying lunar volatiles by conducting a quantitative GIS-based spatial analysis of multiple relevant datasets. The datasets include the locations of permanently shadowed regions, thermal analyses of the lunar surface, and hydrogen abundances. We provide maps of the lunar surface showing areas of high scientific interest, including five regions near the lunar north pole and seven regions near the lunar south pole that have the highest scientific potential according to rational search criteria. At two of these sites—a region we call the “Intercrater Polar Highlands” (IPH) near the north pole, and Amundsen crater near the south pole—we provide a more detailed assessment of landing sites, sample locations, and exploration strategies best suited for future human or robotic exploration missions.

The formation of pure anorthosite on the Moon

The giant impact hypothesis for the origin of the Moon points to the creation of a hot, young Moon, likely with a global magma ocean. Such a magma ocean would produce a flotation crust of plagioclase crystals, or an anorthositic crust. Early calculations of the expected anorthosite content of the lunar highlands crust did not match initial measurements of Apollo samples, and more recently have not matched Clementine measurements or SELENE measurements, because they assumed interstitial melt would freeze. We consider a physical model of a magma ocean with an accumulating flotation lid, and find that significant escape of melt can take place for reasonable physical parameters and timescales of melt migration, even allowing for the inhibiting effect of plausible compaction viscosities. This model permits more nearly pure lunar anorthosites, consistent with observations. The model encounters some difficulty in explaining the expulsion of melt for the near-surface crust (up to five-kilometers’ depth) that presumably dominates the Apollo samples, suggesting that impact mixing and tidal heating are needed to explain the discrepancy.

Oral histories in meteoritics and planetary science-XXIV: William K. Hartmann

In this interview, William Hartmann (Bill, Fig. 1) describes how he was inspired as a teenager by a map of the Moon in an encyclopedia and by the paintings by Chesley Bonestell. Through the amateur journal “Strolling Astronomer,” he shared his interests with other teenagers who became lifelong colleagues. At college, he participated in Project Moonwatch, observing early artificial satellites. In graduate school, under Gerard Kuiper, Bill discovered Mare Orientale and other large concentric lunar basin structures. In the 1960s and 1970s, he used crater densities to study surface ages and erosive/depositional effects, predicted the approximately 3.6 Gyr ages of the lunar maria before the Apollo samples, discovered the intense pre-mare lunar bombardment, deduced the youthful Martian volcanism as part of the Mariner 9 team, and proposed (with Don Davis) the giant impact model for lunar origin. In 1972, he helped found (what is now) the Planetary Science Institute. From the late 1970s to early 1990s, Bill worked mostly with Dale Cruikshank and Dave Tholen at Mauna Kea Observatory, helping to break down the Victorian paradigm that separated comets and asteroids, and determining the approximately 4% albedo of comet nuclei. Most recently, Bill has worked with the imaging teams for several additional Mars missions. He has written three college textbooks and, since the 1970s, after painting illustrations for his textbooks, has devoted part of his time to painting, having had several exhibitions. He has also published two novels. Bill Hartmann won the 2010 Barringer Award for impact studies and the first Carl Sagan Award for outreach in 1997. Figure 1Open in figure viewerPowerPoint William K. Hartmann taken 2010 Aug 2 (Photo: Gayle Hartmann).

A wet, heterogeneous lunar interior: Lower mantle and core dynamo evolution

While recent analyses of lunar samples indicate the Moon had a core dynamo from at least 4.2–3.56 Ga, mantle convection models of the Moon yield inadequate heat flux at the core-mantle boundary to sustain thermal core convection for such a long time. Past investigations of lunar dynamos have focused on a generally homogeneous, relatively dry Moon, while an initial compositionally stratified mantle is the expected consequence of a postaccretionary lunar magma ocean. Furthermore, recent re-examination of Apollo samples and geophysical data suggests that the Moon contains at least some regions with high water content. Using a finite element model, we investigate the possible consequences of a heterogeneously wet, compositionally stratified interior for the evolution of the Moon. We find that a postoverturn model of mantle cumulates could result in a core heat flux sufficiently high to sustain a dynamo through 2.5 Ga and a maximum surface, dipolar magnetic field strength of less than 1 μT for a 350-km core and near ∼2 μT for a 450-km core. We find that if water was transported or retained preferentially in the deep interior, it would have played a significant role in transporting heat out of the deep interior and reducing the lower mantle temperature. Thus, water, if enriched in the lower mantle, could have influenced core dynamo timing by over 1.0 Gyr and enhanced the vigor of a lunar core dynamo. Our results demonstrate the plausibility of a convective lunar core dynamo even beyond the period currently indicated by the Apollo samples.

Organic Delivery Device

Planetary Science![Figure][1] CREDIT: K. L. THOMAS-KEPRTA ET AL., GEOCHIMICA ET COSMOCHIMICA ACTA (15 MARCH 2014) © 2014 ELSEVIER B.V. Several plausible mechanisms could produce complex organic molecules on the Moon, including synthesis by energetic UV radiation or impact shocks, delivery from meteorites, and even decay of biotic matter. However, Apollo samples have shown surprisingly low levels of carbon. Thomas-Keprta et al. have now detected complex organics larger than methane within lunar samples from Shorty Crater. They probed glassy beads tens of micrometers in diameter for fluorescence, revealing some with thin layers of carbonaceous material that were apparently embedded while the silicate surface was still molten. Chemical characterization with micro-Raman spectroscopy indicated the presence of macromolecular matter similar to kerogen, a compound that composes the bulk of stony meteorites. The authors favor a production scenario in which these abiotic organics were delivered via micrometeorites around the time that the volcanic glass beads formed. More substantial meteoritic accretion from cometary and asteroidal fragments also is thought to have occurred on the early Earth. The specific quantity and nature of these organics remain under investigation in the context of possible precursors for life in the solar system. Geochim. Cosmochim. Acta 10.1016/j.gca.2014.02.047 (2014). [1]: pending:yes

The production of small primary craters on Mars and the Moon

We model the primary crater production of small (D<100m) primary craters on Mars and the Moon using the observed annual flux of terrestrial fireballs. From the size–frequency distribution (SFD) of meteor diameters, with appropriate velocity distributions for Mars and the Moon, we are able to reproduce martian and lunar crater-count chronometry systems (isochrons) in both slope and magnitude. We include an atmospheric model for Mars that accounts for the deceleration, ablation, and fragmentation of meteors. We find that the details of the atmosphere or the fragmentation of the meteors do not strongly influence our results. The downturn in the crater SFD from atmospheric filtering is predicted to occur at D∼10–20cm, well below the downturn observed in the distribution of fresh craters detected by the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) or the Mars Reconnaissance Orbiter (MRO) Context Camera (CTX). Crater counts are conducted on the ejecta blanket of Zunil crater and the interior of Pangboche crater on Mars and North Ray and Cone craters on the Moon. Our model isochrons produce a similar slope and age estimate for the formation of Zunil crater as the Hartmann production function (∼1Ma). We derive an age of 35.1Ma for Pangboche when accounting for the higher elevation (>20km higher than Zunil), a factor ∼2 younger than estimated using the Hartmann production function which assumes 6mbar surface pressure. We estimate ages of 52.3Ma and 23.9Ma for North Ray and Cone crater respectively, consistent with cosmic ray exposure ages from Apollo samples. Our results indicate that the average cratering rate has been constant on these bodies over these time periods. Since our Monte Carlo simulations demonstrate that the existing crater chronology systems can be applied to date young surfaces using small craters on the Moon and Mars, we conclude that the signal from secondary craters in the isochrons must be relatively small at these locations, as our Monte Carlo model only generates primary craters.

Lunar feldspathic meteorites: Constraints on the geology of the lunar highlands, and the origin of the lunar crust

The composition of the lunar crust provides clues about the processes that formed it and hence contains information on the origin and evolution of the Moon. Current understanding of lunar evolution is built on the Lunar Magma Ocean hypothesis that early in its history, the Moon was wholly or mostly molten. This hypothesis is based on analyses of Apollo samples of ferroan anorthosites (>90% plagioclase; molar Mg/(Mg+Fe)=Mg#<75) and the assumption that they are globally distributed. However, new results from lunar meteorites, which are random samples of the Moonʼs surface, and remote sensing data, show that ferroan anorthosites are not globally distributed and that the Apollo highland samples, used as a basis for the model, are influenced by ejecta from the Imbrium basin. In this study we evaluate anorthosites from all currently available adequately described lunar highland meteorites, representing a more widespread sampling of the lunar highlands than Apollo samples alone, and find that ∼80% of them are significantly more magnesian than Apollo ferroan anorthosites. Interestingly, Luna mission anorthosites, collected outside the continuous Imbrium ejecta, are also highly magnesian. If the lunar highland crust consists dominantly of magnesian anorthosites, as suggested by their abundance in samples sourced outside Imbrium ejecta, a reevaluation of the Lunar Magma Ocean model is a sensible step forward in the endeavor to understand lunar evolution. Our results demonstrate that lunar anorthosites are more similar in their chemical trends and mineral abundance to terrestrial massif anorthosites than to anorthosites predicted in a Lunar Magma Ocean. This analysis does not invalidate the idea of a Lunar Magma Ocean, which seems a necessity under the giant impact hypothesis for the origin of the moon. However, it does indicate that most rocks now seen at the Moonʼs surface are not primary products of a magma ocean alone, but are products of more complex crustal processes.

Alternative models of the Moon’s origin

The idea that our Moon originated by collision of a large space body with Earth11. W. K. Hartmann, D. R. Davis, Icarus 24, 504 (1975). https://doi.org/10.1016/0019-1035(75)90070-6 has much shakier foundations than its almost universal acceptance might suggest. Before 2012 no published giant-impact model completely supported that hypothesis. All attempts to explain isotopic identity of Apollo samples and Earth’s mantle left too much alien impactor material in the resulting Moon.22. A. N. Halliday, Science 338, 1040 (2012). https://doi.org/10.1126/science.1229954In 2012 a new mechanism became the basis for the first successful giant-impact models.33. M. Ćuk, S. T. Stewart, Science 338, 1047 (2012). https://doi.org/10.1126/science.1225542 That mechanism allowed early Earth at the moment of impact to be spinning near its approximately 2.7-hour limit of rotational stability. With that added energy, the impactor could blast part of the mantle into an orbiting cloud, a future moon polluted only by acceptably small traces of the impactor. The new mechanism of lunar–solar tidal resonance transfer, a descendant of George Darwin’s original lunar origin by tidal interaction,44. G. H. Darwin, Phil. Proc. Roy. Soc. London 171, 713 (1880). https://doi.org/10.1098/rstl.1880.0020 then reduced angular momentum to near-modern values.The new models have the same initial conditions as fission models of the 1960s—namely, a very rapidly spinning, partially segregated, early Earth. Elimination of excess angular momentum by the new mechanism removes once fatal objections to fission hypotheses. In these revitalized models late-stage core segregation increased the rotation rate beyond stability limits55. D. U. Wise, J. Geophys. Res. 74, 6034 (1969). https://doi.org/10.1029/JB074i025p06034 to separate single or multiple parts of the mantle as lunar precursors. Subsequently a combination of tidal transfer; late-stage, backward-directed impacts; magnetic braking; drag in the solar wind; or escape of a silicate atmosphere,55. D. U. Wise, J. Geophys. Res. 74, 6034 (1969). https://doi.org/10.1029/JB074i025p06034 other volatiles, small debris, or other moonlets reduced angular momentum to near-modern levels.Distinctions between these contrasting hypotheses are already blurred. Authors of the new model note that it “blends aspects of the original impact hypothesis … and the fission hypothesis.”33. M. Ćuk, S. T. Stewart, Science 338, 1047 (2012). https://doi.org/10.1126/science.1225542 In reality giant impact is an unnecessary complication. Reinstated fission models could include possibilities for separation into two bodies, multiple moonlets, or equatorial fragmentation and reassembly. All avoid celestial dynamic baggage and impactor contamination while invoking the same rapid rotation and momentum-reduction mechanism as the recent giant impact models. Perhaps our quest to answer one of mankind’s oldest questions should expand to include a simpler hypothesis.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. W. K. Hartmann, D. R. Davis, Icarus 24, 504 (1975). https://doi.org/10.1016/0019-1035(75)90070-6, Google ScholarCrossref2. A. N. Halliday, Science 338, 1040 (2012). https://doi.org/10.1126/science.1229954, Google ScholarCrossref3. M. Ćuk, S. T. Stewart, Science 338, 1047 (2012). https://doi.org/10.1126/science.1225542, Google ScholarCrossref, ISI4. G. H. Darwin, Phil. Proc. Roy. Soc. London 171, 713 (1880). https://doi.org/10.1098/rstl.1880.0020, Google ScholarCrossref5. D. U. Wise, J. Geophys. Res. 74, 6034 (1969). https://doi.org/10.1029/JB074i025p06034, Google ScholarCrossref© 2014 American Institute of Physics.

Western Oceanus Procellarum as seen by C1XS on Chandrayaan-1

We present the analysis of an X-ray fluorescence (XRF) observation of the western part of Oceanus Procellarum on the Moon’s nearside made by the Chandrayaan-1 X-ray Spectrometer on 10th February 2009. Through forward modelling of the X-ray spectra, we provide estimates of the MgO/SiO2 and Al2O3/SiO2 ratios for seven regions along the flare’s ground track. These results are combined with FeO and TiO2 contents derived from Clementine multispectral reflectance data in order to investigate the compositional diversity of this region of the Moon. The ground track observed consists mainly of low-Ti basaltic units, and the XRF data are largely consistent with this expectation. However, we obtain higher Al2O3/SiO2 ratios for these units than for most basalts in the Apollo sample collection. The widest compositional variation between the different lava flows is in wt% FeO content. A footprint that occurs in a predominantly highland region, immediately to the north of Oceanus Procellarum, has a composition that is consistent with mixing between low-Ti mare basaltic and more feldspathic regoliths. In contrast to some previous studies, we find no evidence for systematic differences in surface composition, as determined through X-ray and gamma-ray spectroscopy techniques.

Oral Histories in Meteoritics and Planetary Science-XXIII: Dieter Stöffler

In this interview, Dieter Stöffler (Fig. 1) describes how his interest in meteorites and impact craters dates from his Ph.D. studies at the University of Tübingen when it was learned that the Ries crater was formed by impact. A paper by Dieter's advisor, Wolf von Engelhardt, also triggered an interest in meteorites. After graduation, Dieter helped to establish a laboratory for high pressure mineralogy and he examined rocks from the Ries crater, which led to the concept of progressive shock metamorphism. The group also worked on newly returned Apollo samples and guided astronauts over the crater. A year at the NASA Ames Research Center taught Dieter about experimental impact research with a light-gas gun. After a few more years at Tübingen, Dieter obtained a professorship at the University of Münster where he created the Institute of Planetology, got involved in planning space missions including comet sample return, and continued high pressure mineralogy in collaboration with colleagues in Freiburg. Through several decades of research, Dieter and colleagues have documented the effects of shock on all the major rock-forming minerals and devised widely accepted schemes for the classification of shocked rocks. After the unification of Germany, Dieter became Director of the Natural History Museum in Berlin, during which he made much progress rebuilding the laboratories and the collections. Dieter also helped to create a museum and research center in the Ries crater. He received the Barringer Award of the Meteoritical Society in 1994 and several prestigious awards in Germany.

The Moon's Indigenous Water

Planetary Science Until 5 years ago, the Moon was thought to be essentially bone dry. Analysis of the samples brought back by the Apollo and Luna missions had not revealed any hydrous components. Recent studies, however, using improved analytical techniques, reported the detection of water in some lunar rocks, and observations from orbiting spacecraft showed that water is present in the lunar surface. Some of this water is thought to have been delivered by meteorites and through solar-wind implantation of hydrogen; another part is thought be indigenous. Using Fourier transform infrared spectroscopy, Hui et al. have detected water in two Apollo samples representative of the Moon's primary crust, crystallized from the lunar magma ocean that is thought to have once covered the Moon. Water in these samples is indigenous—with solar-wind implantation ruled out by the use of grains that came from the interior of the individual Apollo rocks—and it was present at very early times, before the crust solidified. The authors estimate that the initial water content of the lunar magma ocean was around 320 parts per million. This water could have prolonged the crystallization of the magma ocean. Nat. Geosci. 10.1038/ngeo1735 (2013).

New lunar meteorite Northwest Africa 2996: A window into farside lithologies and petrogenesis

AbstractThe Northwest Africa (NWA) 2996 meteorite is a lunar regolith breccia with a “mingled” bulk composition and slightly elevated incompatible element content. NWA 2996 is dominated by clasts of coarse‐grained noritic and troctolitic anorthosite containing calcic plagioclase (An#~98) and magnesian mafic minerals (Mg#~75), distinguishing it from Apollo ferroan anorthosites and magnesian‐suite rocks. This meteorite lacks basalt, and owes its mingled composition to a significant proportion of coarse‐grained mafic clasts. One group of mafic clasts has pyroxenes similar to anorthosites, but contains more sodic plagioclase (An#~94) distinguishing it as a separate lithology. Another group contains Mg‐rich, very low‐titanium pyroxenes, and could represent an intrusion parental to regional basalts. Other clasts include granophyric K‐feldspar, disaggregated phosphate‐bearing quartz monzodiorites, and alkali‐suite fragments (An#~65). These evolved lithics are a minor component, but contain minerals rich in incompatible elements. Several anorthosite clasts contain clusters of apatite, suggesting that the anorthosites either assimilated evolved rocks or were metasomatized by a liquid rich in incompatible elements. We used Lunar Prospector gamma‐ray spectrometer remote sensing data to show that NWA 2996 is most similar to regoliths in and around the South Pole Aitken (SPA) basin, peripheral regions of eastern mare, Nectaris, Crisium, and southern areas of Mare Humorum. However, the mineralogy of NWA 2996 is distinctive compared with Apollo and Luna mission samples, and is likely consistent with an origin near the SPA basin: anorthosite clasts could represent local crustal material, mafic clasts could represent intrusions beneath basalt flows, and apatite‐bearing rocks could carry the SPA KREEP signature.

Workshop on Formation of the First Solids in the Solar System—Honoring Klaus Keil for his Distinguished Career in Meteoritics and Cosmochemistry

Workshop on Formation of the First Solids in the Solar System—Honoring Klaus Keil for his Distinguished Career in Meteoritics and Cosmochemistry

Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water

Remote sensing discoveries of hydroxyl and water on the lunar surface have reshaped our view of the distribution of water and related compounds on airless bodies such as the Moon. The origin of this surface water is unclear, but it has been suggested that hydroxyl in the lunar regolith can result from the implantation of hydrogen ions by the solar wind. Here we present Fourier transform infrared spectroscopy and secondary ion mass spectrometry analyses of Apollo samples that reveal the presence of significant amounts of hydroxyl in glasses formed in the lunar regolith by micrometeorite impacts. Hydrogen isotope compositions of these glasses suggest that some of the observed hydroxyl is derived from solar wind sources. Our findings imply that ice in polar cold traps could contain hydrogen atoms ultimately derived from the solar wind, as predicted by early theoretical models of water stability on the lunar surface. We suggest that a similar mechanism may contribute to hydroxyl on the surfaces of other airless terrestrial bodies where the solar wind directly interacts with the surface, such as Mercury and the asteroid 4-Vesta.

Study of complex dielectric properties of lunar simulants and comparison with Apollo samples at microwave frequencies

Laboratory characterization of dielectric properties of terrestrial analogues of lunar soil (JSC-1A) and comparison with lunar samples returned from various Apollo missions is made at different as well as normalized bulk density. Here measurements of dielectric constants and losses were made at four microwave frequencies such as 1.7GHz, 2.5GHz, 6.6GHz and 31.6GHz. Complex permittivity of lunar simulant was measured at temperature ranging from −190°C to +200°C using Wave-Guide cell method. Comparison of permittivity of JSC-1A with Apollo sample also has been done at similar microwave frequencies. The investigations reveal that dielectric constant and loss factor of terrestrial analogues of lunar soil are temperature dependent. As temperature is gradually increased both these parameter (storage factor and loss factor) also gradually increases. These temperatures were chosen because the Moon undergoes at that extremes level of temperature. It is scorching heat at 110°C during the day and freezing cold at −180°C during night. The measured value of ε can be useful for designing passive as well as active sensors.

Magnetization of the lunar crust

Magnetic fields measured by the satellite Lunar Prospector show large scale features resulting from remanently magnetized crust. Vector data synthesized at satellite altitude from a spherical harmonic model of the lunar crustal field, and the radial component of the magnetometer data, have been used to produce spatially continuous global magnetization models for the lunar crust. The magnetization is expressed in terms of localized basis functions, with a magnetization solution selected having the smallest root‐mean square magnetization for a given fit to the data, controlled by a damping parameter. Suites of magnetization models for layers with thicknesses between 10 and 50 km are able to reproduce much of the input data, with global misfits of less than 0.5 nT (within the uncertainties of the data), and some surface field estimates. The magnetization distributions show robust magnitudes for a range of model thicknesses and damping parameters, however the magnetization direction is unconstrained. These global models suggest that magnetized sources of the lunar crust can be represented by a 30 km thick magnetized layer. Average magnetization values in magnetized regions are 30–40 mA/m, similar to the measured magnetizations of the Apollo samples and significantly weaker than crustal magnetizations for Mars and the Earth. These are the first global magnetization models for the Moon, providing lower bounds on the magnitude of lunar crustal magnetization in the absence of multiple sample returns, and can be used to predict the crustal contribution to the lunar magnetic field at a particular location.

Lunar meteorites from Oman

Abstract– Sixty named lunar meteorite stones representing about 24 falls have been found in Oman. In an area of 10.7 × 103 km2 in southern Oman, lunar meteorite areal densities average 1 g km−2. All lunar meteorites from Oman are breccias, although two are dominated by large igneous clasts (a mare basalt and a crystalline impact‐melt breccia). Among the meteorites, the range of compositions is large: 9–32% Al2O3, 2.5–21.1% FeO, 0.3–38 μg g−1 Sm, and &lt;1 to 22.5 ng g−1 Ir. The proportion of nonmare lunar meteorites is higher among those from Oman than those from Antarctica or Africa. Omani lunar meteorites extend the compositional range of lunar rocks as known from the Apollo collection and from lunar meteorites from other continents. Some of the feldspathic meteorites are highly magnesian (high MgO/[MgO + FeO]) compared with most similarly feldspathic Apollo rocks. Two have greater concentrations of incompatible trace elements than all but a few Apollo samples. A few have moderately high abundances of siderophile elements from impacts of iron meteorites on the Moon. All lunar meteorites from Oman are contaminated, to various degrees, with terrestrial Na, K, P, Zn, As, Se, Br, Sr, Sb, Ba, U, carbonates, or sulfates. The contamination is not so great, however, that it seriously compromises the scientific usefulness of the meteorites as samples from randomly distributed locations on the Moon.

Oral Histories in Meteoritics and Planetary Science - XV: John Wood

Abstract– John Wood (Fig. 1) was trained in Geology at Virginia Tech and M.I.T. To fulfill a minor subject requirement at M.I.T., he studied astronomy at Harvard, taking courses with Fred Whipple and others. Disappointed at how little was known in the 1950s about the origin of the earth, he seized an opportunity to study a set of thin sections of stony meteorites, on the understanding that these might shed light on the topic. This study became his Ph.D. thesis. He recognized that chondrites form a metamorphic sequence, and that idea proved surprisingly hard to sell. After brief service in the Army and a year at Cambridge University, John served for 3 years as a research associate with Ed Anders at the University of Chicago. He then returned to the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, where he spent the remainder of his career. At Chicago, he investigated the formation of the Widmanstatten structure, and found that the process informs us of the cooling rates of iron meteorites. Back in Cambridge, he collaborated with W. R. Van Schmus on a chondrite classification that incorporates metamorphic grade, and published on metal grains in chondrites, before becoming absorbed by preparations for the return of lunar samples by the Apollo astronauts. His group’s work on Apollo samples helped to establish the character of the lunar crust, and the need for a magma ocean to form it. Wood served as President of the Meteoritical Society in 1971–72 and received the Leonard Medal in 1978. Figure 1. John Wood. Download figure to PowerPoint