Remote sensing compositional data (from spectroscopy X, gamma, neutron, UV-VIS and NIR) and surface physical properties (from photometry), leading to the characterization of the chemistry, mineralogy, and physical state of surface materials, are fundamental for the study of planetary bodies. The Moon, because of its primitive nature, represents a unique object to reach these goals. In this proposal, the particular objectives are to investigate the nature and history of the lunar crust, the volcanism, the surface interactions with the space environment, and the water cycle on the Moon. Because of its small size, geological internal processes occurred only for a short time on the Moon, giving today access to the study of a primary planetary crust, relatively unaffected by subsequent magmatism. The Moon is thus an ideal place to understand how a planetary crust formed and was processed over time by meteoritic bombardment and early volcanism. A number of themes can be identified, but they are all closely interlinked, and should be viewed as strands of a unified project.
1. Remote Sensing of the Lunar Surface
Determining the composition (chemistry, mineralogy) of the lunar crust at global scale, using combined data from optical, IR, X, gamma, and neutron spectroscopy is a key objective. C1XS, the Chandrayaan-1 X-ray Spectrometer (Grande et al., 2009) provided superior X-ray detection, with spectroscopic and spatial measurement capabilities. The energy resolution, much better than 200 keV throughout the mission, was by far the best ever achieved by such an instrument. In order to record the incident solar X-ray flux at the Moon, essential for the derivation of absolute lunar elemental surface abundances, C1XS carried an X-ray Solar Monitor provided by the University of Helsinki. Mapping of the complete lunar surface was not possible due to the weak and sporadic solar X-ray flux in the current cycle. However, recently we were able to extract data on the uniformity of sodium for the first time (Athiray et al., 2014). The derived abundances of Na (2–3 wt%) are significantly higher than those derived from earlier studies, and demonstrate considerable unevenness, which, if confirmed, will have significant repercussions for discussions of magma ocean evolution. In this project, we will establish a cross-referenced map using X-ray and gamma data.
Multi-instrument multi-wavelength measurements allow us to reveal compositional variations both in surface (thus identifying broad geological units or provinces) and in depth (using impact basins and craters as probes). Knowledge of the lateral and vertical nature of the crust, along with its structure (e.g., thickness) determined from geophysical data, give constraints on the models of formation of the crust from a global magma ocean, as well as the conditions of magma ascent in the crust and its contamination. In the past, the combination of elemental maps (Fe, Ti, Th concentrations) derived from optical (Clementine) and gamma (Lunar Prospector) spectroscopy proved to be very useful to evidence, characterize, and map the major lunar provinces on the Moon (Chevrel et al., 2002). The integration of new elemental maps to this early work from other instruments, such as C1XS and M3 from the Chandrayaan-1 mission, as well as GRS on SELENE can greatly improve our knowledge of the nature of these lunar provinces.
An analysis of the Lunar gamma-ray spectrum as measured by the Lunar Prospector Gamma-Ray Spectrometer has revealed that 8–8.9 MeV gamma rays contain information about the elemental composition of near-surface materials. These high-energy gamma rays are found to be primarily sensitive to the total Fe and Mg content of the surface, although other elements also contribute. This information has been used to identify several regions with unique compositions, including the Hertzsprung and Orientale basins. A new method for deriving global Mg abundances from high-energy gamma-ray measurements has recently been described (Peplowski and Lawrence, 2013). This can be cross-validated with the nearside Lunar highland elemental maps derived by the X-ray spectrometer C1XS on Chandrayaan-1 (Carter, 2012).
2. Lunar Volcanism
Unlike more evolved planets such as Mars, Venus, and the Earth, the bulk lunar surface has been preserved more or less unaltered for a long time, and lunar volcanism can thus be studied over a prolonged period, right from its initial stages. According to current theory, the Moon underwent differentiation during the cooling of a global magma ocean. Plagioclase feldspar floated to the surface to form the outer crustal layer, while heavier minerals such as olivines and pyroxenes sank, creating the sources from which the mare basalts formed by partial melting. For this reason, understanding their composition and distribution is very important for understanding lunar mantle evolution and chemical history of the Moon.
Studies of returned samples in combination with remote sensing data have opened a window for us to look into the chemical evolution of the lunar mantle. Detailed chemical analyses of major, minor and trace elements in lunar basalts and other sample types (including meteorites) have been used by lunar petrologists to experimentally determine the depth and chemical composition of the lava sources and the processes by which the mare basalts and volcanic glasses formed. The distribution and mode of emplacement for the mare basalts spatially and through time are important for understanding the sources and the ascension of the lunar magmas in relation with the nature and structure of the primary crust and thermal evolution of the Moon. Apollo samples of mare material generally date from between 3.1 and 3.9 GY ago. However, studies involving crater counting suggest that volcanism may have occurred as recently as 2 billion years ago (e.g. Hiesinger et al., 2000), a fact confirmed by meteorite isotopic analysis (Fernandes 2000). Quite recently, evidences of basaltic volcanism have been detected within the past 100 Ma (Braden et al., 2014).
Most advanced remote sensing studies (Clementine, Chandrayaan-1, and others) showed, within each mare region, the occurrence of multiple basaltic units which differ in mineralogy, chemical composition and age, revealing complex modes of emplacement and evolution in the magmatic reservoirs.
Local and regional pyroclastic deposits are also found on the Moon. This explosive volcanism is the result of the presence of volatiles in the magmas. Although we cannot directly access to the nature of these volatiles, one can indirectly give some insight on their distribution and abundance in space and time.
Accordingly, to fully understand volcanic deposits on the Moon, it appears essential to combine different spectroscopic and compositional datasets (for example chemistry and mineralogy derived from AMIE, SIR-2 and C1XS) with physical surface properties datasets from photometry (e.g., AMIE), especially in case of explosive pyroclastic volcanic deposits. For example, the combination of AMIE and M3 data on the Lavoisier lunar crater has been carried out to jointly characterize the physical and mineralogical properties of the pyroclastic deposits in this area (Souchon et al., 2013). This approach is very powerful as it emphasises the mode of emplacement of lunar volcanic units along with their distribution. The objectives are to understand how volcanism began and evolved through time on a simple planetary body, and to constrain the thermal history of the Moon. We need to understand why lunar volcanism varied in occurrence and composition over space and time, and how sources in the mantle supplied the magmas. Finally we must complete the inventory of volcanic rocks present on the Moon, and particularly ancient pyroclastic deposits from which we can retrieve information on volatile abundances on the Moon when it formed.
Lunar pyroclastic glass beads have been known about since the Apollo era, most notably the orange soil returned by Apollo 17. As early as the Clementine mission in 1994, over a hundred dark glass deposits were found over a wide range of areas on the lunar surface, and these are thought to indicate the locations of former volcanic vents where fire fountains rained molten, and subsequently quenched glass onto the lunar surface. Further deposits have been found with the M3 and Diviner datasets, but mapped at similar or lower spatial resolution to Clementine. The glass is of particular interest because: (1) it is an indicator of past volcanic vents, (2) it contains preserved volatiles from earlier times on the Moon, (3) the material in the glass comes from the deepest parts in the lunar mantle, several hundred km down, and so is a good way to probe the composition of the deep lunar mantle. Although some of the glass deposits were buried under lunar soil, subsequent impacts or surface disturbances have exposed some few tens of metres to km sections of these deposits. The dark glass in particular can be mapped at a higher spatial resolution than before, by examining imagery that was taken close to zero phase angle (when there is little shadow) and looking for material which appears darker than the existing already low albedo lunar surface. At higher phase angles it can also be detected, using multiple illumination or viewing angle, imagery of a particular area and looking for outliers in the general BRDF properties of different parts of the lunar surface.
3. Interaction with the Space Environment
The Moon is the ideal place to understand the mechanisms of interaction of a planetary surface with the space environment and to answer questions about the formation and evolution of a planetary regolith, which consists in a multi-component mixture of various rocks and minerals, surface chemistry and impact products. The importance of these processes throughout the Solar System is coming to be increasingly recognized. Via spectral and photometric data we aim to describe the mineralogical and physical (texture, roughness, maturity) properties of the lunar surface which are needed in geological studies to understand impact processes, ejecta blanket distribution, regolith production (soils) and reworking, volcanism processes (e.g., properties and mode of emplacement of pyroclastic deposits) and interactions with the solar wind. In connection with the space environment including the possibility of cometary bodies (nucleus and inner coma) impacting the lunar surface, the origin of swirl formation remains quite enigmatic and an opened field of investigation (e.g., Schultz and Snrka, Nature, 284, 1980; Pinet et al., 2000; Starukhina and Shkuratov, Icarus, 167, 2004; Garrick-Bethel and Pieters, 2011)(see also below section 2.4: water cycle on the Moon) that can be explored in a lot more detail by means of high resolution orbital multiangular imaging surveys.
In order to interpret the physical and mineralogical properties of a surface it is important to understand the physics that controls illumination, and the interaction of light with soil components. This can be achieved using photometric models with the critical support of experimental studies such as the laboratory spectrophotometric measurements made with ISEP (a wide-field multispectral and photometric imaging facility) implanted at IRAP in Toulouse, or in the Planetary Analog Terrain laboratory at Aberystwyth.
4. Water Cycle on the Moon
In recent years, our understanding of hydrogen (H), water, and other volatile materials on the Moon has changed dramatically. This new understanding has resulted from: 1) new measurements and analyses of enhanced H concentrations at the lunar poles; 2) new orbital measurements showing exogenic and endogenic enhancements of H2O/OH in non-polar lunar regions; and 3) new laboratory sample measurements revealing unexpectedly high amounts of water in lunar samples. Recent data and analyses from the Lunar Reconnaissance Orbiter (LRO) Diviner instrument are providing new understanding of previously collected data from the Lunar Prospector Neutron Spectrometer and has allowed a new map of bulk H concentrations in the lunar highlands to be derived (Lawrence et al., 2014). This new map will be an important tie point to the other reflectance, ENA, and sample datasets for understanding processes that relate global lunar volatiles.
Recent Chandrayaan-1/M3 data (Pieters et al., 2009) has shown that a sub-monolayer of water is found on the surface of the Moon at high latitudes. The Chandrayaan-1/M3 maps of water and hydroxyl deposits on the Moon indicate that their distribution is incredibly diverse. For example, some fresh craters from all across the Lunar surface show high volatile content, whereas nearby fresh craters show none. Similar results are noted for ancient craters. Furthermore, the extensive, dark mare basalts seem to contain less water than the surrounding light, lunar highlands. What drives the non-polar hydrogen/water distribution on the Moon? Do the surficial water concentrations observed with reflectance data extend to depth to be observed with bulk H values measured with other techniques? How do permanently shaded regions act as sources and sinks to the lunar water cycle? Are there critical new measurements that should be made to provide additional insight regarding the lunar water cycle?