LPSC 2016: The Moon Keeps on Giving
Posted by Ryan Clegg-Watkins
14-04-2016 8:04 CDT
There was no shortage of interesting lunar science talks at last month’s Lunar and Planetary Science Conference! I spent the majority of my time absorbing the lunar volcanism and South Pole-Aitken Basin talks, so this post will be slightly skewed towards those topics. I highly encourage anyone interested in all of the lunar talks to check out all the abstracts posted on the conference website!
One aspect of the lunar science that stood out to me this year was the way that scientists are integrating data sets from different instruments and missions. Few projects involved only one data set. Many incorporated remote sensing data from more than one mission, or combined sample data with remote sensing. The Lunar Reconnaissance Orbiter (LRO) and ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon’s Interaction with the Sun) are the only spacecraft currently collecting data at the Moon, but we are still making new scientific discoveries using many other robust data sets, including the Chandrayaan-1 Moon Mineralogy Mapper (M3) spectrographic data; Gravity Recovery and Interior Laboratory (GRAIL) gravity maps; Apollo sample analyses; Lunar Atmosphere Dust and Environment Explorer (LADEE) dust and exosphere observations; and Clementine multispectral images.
The first lunar session highlighted new results from recent lunar missions. Maria Zuber kicked the session off discussing how she is using GRAIL gravity data integrated with Lunar Obiter Laser Altimeter (LOLA) topography data to constrain the source depth of mass anomalies such as dikes, intrusions, faults, and other subsurface structures within the Moon’s crust. The high-resolution data from GRAIL and LOLA provide Zuber and her team with the ability to explore the shallow internal structure of the Moon in greater detail than any other planetary body, besides our own Earth.
There were a couple of talks about the lunar exosphere using LADEE data, but I had skipped over to the Mercury session for a while. So if exospheres are your thing, I direct you to Anthony Colaprete’s abstract on titanium, aluminum, and magnesium in the exosphere and to Cesare Grava’s abstract on integrating LRO data with LADEE and ARTEMIS data to detect exospheric helium.
The Results from Recent Lunar Missions session wrapped up with several updates from the Chang'e 3 mission. Zongcheng “Lewis” Ling, associate professor at Shandong University in China, discussed the discovery of a new type of lunar basalt at the Chang’e 3 landing site (a follow-up to the study discussed in his 2015 Nature paper). In their previous study, Ling et al. discovered an olivine-rich basalt at the Chang'e 3 landing site using mineralogical and compositional data from instruments onboard the Yutu rover. This study also found two types of rocks near the landing site – a light-toned rock type and a dark-toned rock type. Using Yutu Pancam data and M3 spectral data, Ling determined that the dark-toned rocks have higher olivine content and the light-toned rocks have lower olivine content, and he concluded that the basalt unit at the landing site must have experienced chemical differentiation to produce these two types of rocks. As a side note, the crater that the Chang'e 3 spacecraft landed next to now has a name – Zi Wei!
The next lunar session was on volatiles, which has been a hot topic in the lunar community lately. Shuai Li, a graduate student from Brown University, presented his work on constraining the volatile content of pyroclastics and silicic domes on the Moon. Li uses thermally corrected M3 data to determine areas of excess water signature on the lunar surface. Pyroclastics, materials deposited by explosive volcanic eruptions, exhibit excess water signature, and Li concludes that this excess water must be sourced from the lunar interior. Li also analyzed silicic domes on the Moon for excess water signatures, because they may contain indigenous water that was concentrated in the melts that ascended to the surface to form the domes. He found that only three silicic areas have enhanced water signatures - Compton Belkovich Volcanic Complex, the Gruithuisen Domes, and the Maria Domes. Li concludes that the silicic domes may have cooled more slowly than pyroclastic deposits, leading to volatile loss; or they may have different water contents in their magma sources; or their classification as being silica-rich may need to be reevaluated.
Continuing on the theme of silicic areas, the Lunar Volcanism session on Tuesday afternoon had quite a few talks on silicic volcanism! Most volcanism on the Moon is basaltic (the dark areas you see when you look up at the Moon are extensive basalt flows), but a few rare volcanic complexes on the Moon have silicic compositions. Silicic lavas produce lighter-colored rocks like granite and rhyolite. They have lower density, lower eruption temperatures, and higher viscosity than basaltic lavas, and are considered a "more evolved" form of volcanism that happened late in lunar history. The morphology, enhanced thorium concentration, high reflectance, and spectral signatures of these features indicate they are silicic in composition. Samples of granitic materials have been found in the Apollo sample suite, so we know that these types of rocks exist on the Moon.
Walter Kiefer, Staff Scientist at the Lunar and Planetary Institute, has been using GRAIL gravity data to determine the crustal density, and therefore constrain composition, at Hansteen Alpha and the Gruithuisen Domes. Both of these complexes have lower bulk densities than basaltic mare areas. The lower density could result from a different (silicic) composition, but it could also be caused by porosity from vesicles or pyroclastics, up to 10% porosity. Kiefer suggested that albite, orthoclase, and quartz are the most likely minerals at Gruithuisen Domes and Hansteen Alpha.
Silicic volcanics are near and dear to my own heart. I presented my work using high-resolution Lunar Reconnaissance Orbiter Camera Narrow Angle Camera (NAC) images to study how silicic regions reflect light differently when viewed from different angles. This is a powerful tool for understanding the mineralogy and compositional and physical properties of planetary surfaces. I created maps of the light-reflecting properties, also known as single-scattering albedo, of two silicic regions: the Compton Belkovich Volcanic Complex (CBVC) and Hansteen Alpha. The single scattering albedo is strongly related to composition and mineralogy, and these maps allow us to see variations in albedo properties, and therefore composition, on 1-meter scales. In the map for a portion of the CBVC, pictured here, higher albedo values (yellow and orange) correspond to areas with less iron and titanium than areas with lower albedo values (greens and blues).
I have previously used reflectance data from spacecraft landing sites to determine the correlation between reflectance and composition, and I applied this correlation to silicic volcanic areas. Variations in reflectance among and within silicic complexes may be attributed to mixing of rock types with different amounts of reflective minerals such as silica; the presence of viscous lavas enriched with potassium, rare earth elements, and phosphorous (KREEP); and/or silicix pyroclastic deposits. Some large domes within the CBVC have albedo values that may suggest intermediate silicic compositions such as andesite or dacite.
Lunar granites are an example of evolved, silicic materials on the Moon and may be related to some of the volcanic features discussed above. Lucky for us, we have a small handful of these rare samples in the Apollo lunar sample collection! During Thursday’s lunar petrology session, Sarah Valencia, a graduate student at Washington University in St. Louis, presented her work analyzing an interesting granitic rock collected during the Apollo 12 mission. This rock, 12013 (pictured below), is the only large non-mare rock from the Apollo 12 mission:
Valencia studies the petrography and chemistry of 12013 using electron probe microanalysis and has found that 12013 is best described as a three component system with a granitic component, a mafic component, and an REE-rich (rare earth element) component. Her analyses have shown that the mafic and REE-rich components are substantially different. The mafic component is coarse-grained, and started out as a dark basalt or gabbro. The REE-rich component is likely an impact melt breccia, made of a type of rock called monzogabbro that later suffered the intense heating and shock of an impact.
The South Pole-Aitken Basin (SPA) was another widely discussed region on the Moon, which is not surprising because sample return from SPA is listed as a priority New Frontier’s class mission on the last Decadal Survey. The SPA is the oldest, deepest impact basin on the Moon and likely exposed lower crustal and/or mantle materials. Samples from SPA would allow us to date the age of the basin and expand our knowledge of the Moon’s differentiation and diversity of materials.
Dan Moriarty, a graduate student at Brown University, spoke about the composition and geology of the “Central SPA Compositional Anomaly” (SPACA), which he analyzed using M3 spectra, LOLA topography, and LROC images. Non-mare areas in the interior of SPA typically have compositional signatures consistent with magnesium-rich pyroxenes (norites). The SPACA, however, has a spectral signature that suggests the presence of more calcium-rich pyroxenes overlying magnesium-rich pyroxenes. Moriarty stated that the physical properties of the SPACA indicate resurfacing took place after the impact melt sheet solidified, creating this compositional stratigraphy. The composition, smooth terrain, and lack of large impact craters are all consistent with volcanic flooding and resurfacing that may have been triggered by the SPA impact event.
Noah Petro, Research Space Scientist at NASA Goddard, presented a study of a region in the central interior of SPA, pictured below, that contains the craters Bhabha and Bose, ejecta from Stoney crater, small-scale volcanic deposits, cryptomare, and the enigmatic “Mafic Mound”:
Continuing with the theme of data integration, Petro and his team are using mini-RF radar data, Diviner rock abundance data, LROC NAC and Wide Angle Camera (WAC) images, Clementine iron-oxide maps, and M3 spectral data to understand the compositional variations in this region and to determine the origin of non-mare material within the region. Petro discussed the diversity of materials in SPA, and concluded that the variation in buried and surface rocks are likely due to both ancient buried basalts and to an impact melt sheet underneath the thin regolith on the floor of craters. Obtaining samples from SPA is truly going to be the best way to determine the types of materials in the basin interior.
Harrison “Jack” Schmitt, Apollo 17 astronaut and the only geologist to walk on the moon, once again presented at LPSC, which is always a treat. Schmitt discussed his analysis of the intergrowth of various minerals from Apollo 17 samples 72415 and 76535. These mineral intergrowths provide strong evidence, both from their mineralogy and texture, that they originated from the lunar mantle and were possibly formed during the breakdown of garnet, which is a mineral that forms at high temperatures and pressures deep in the mantle. Schmitt's study of these mineral textures supports the hypothesis that late overturn of magma ocean cumulates occurred, at least below Procellarum. He actually compared these mineral textures to textures in Norwegian rocks, which he studied as part of his Ph.D thesis! Since I am not a petrologist, that’s about all I can say about that – but it was really neat hearing his stories of collecting the very samples he was analyzing."
The Moon has proved itself as the gift that keeps on giving. We are still analyzing Apollo samples that were returned 40+ years ago, and recent lunar missions are continuing to provide us with rich data sets that are unraveling the mysteries of the Moon. I am hoping that by this time next year, we are well on our way to another sample return mission!