The lunar surface as a recorder of astrophysical processes

The lunar surface has been exposed to the space environment for billions of years and during this time has accumulated records of a wide range of astrophysical phenomena. These include solar wind particles and the cosmogenic products of solar particle events which preserve a record of the past evolution of the Sun, and cosmogenic nuclides produced by high-energy galactic cosmic rays which potentially record the galactic environment of the Solar System through time. The lunar surface may also have accreted material from the local interstellar medium, including supernova ejecta and material from interstellar clouds encountered by the Solar System in the past. Owing to the Moon’s relatively low level of geological activity, absence of an atmosphere, and, for much of its history, lack of a magnetic field, the lunar surface is ideally suited to collect these astronomical records. Moreover, the Moon exhibits geological processes able to bury and thus both preserve and ‘time-stamp’ these records, although gaining access to them is likely to require a significant scientific infrastructure on the lunar surface

The varied sources of faculae-forming brines in Ceres’ Occator crater emplaced via hydrothermal brine effusion

Before acquiring highest-resolution data of Ceres, questions remained about the emplacement mechanism and source of Occator crater's bright faculae. Here we report that brine effusion emplaced the faculae in a brine-limited, impact-induced hydrothermal system. Impact-derived fracturing enabled brines to reach the surface. The central faculae, Cerealia and Pasola Facula, postdate the central pit, and were primarily sourced from an impact-induced melt chamber, with some contribution from a deeper, pre-existing brine reservoir. Vinalia Faculae, in the crater floor, were sourced from the laterally extensive deep reservoir only. Vinalia Faculae are comparatively thinner and display greater ballistic emplacement than the central faculae because the deep reservoir brines took a longer path to the surface and contained more gas than the shallower impact-induced melt chamber brines. Impact-derived fractures providing conduits, and mixing of impact-induced melt with deeper endogenic brines, could also allow oceanic material to reach the surfaces of other large icy bodies. The second extended phase of the Dawn mission provided high resolution observations of Occator crater of the dwarf planet Ceres. Here, the authors show that the central faculae were sourced in an impact-induced melt chamber, with a contribution from the deep brine reservoir, while the Vinalia Faculae were sourced by the deep brine reservoir alone

Unmixing of Laboratory IR Spectral Reflectance Measurements of Smooth Plains Analogs

The unmixing model used in this study has previ- ously been used for spectral unmixing of NASA RELAB data and lunar analog materials. In the framework of MERTIS it is applied to laborator mineral mixtures, including glasses and varying grain sizes [12-14]. These mixtures are prepared and analyzed at the IRIS (Infrared and Raman for Interplanetary Spectros- copy) laboratory of the Institut für Planetologie at the Westfälische Wilhelms-Universität Münster. Here we investigate a wide range of natural minerals, rock samples including impact rocks and meteorites, syn- thetic analogs, and glasses [3,7]. The results of these in- vestigations contribute to the generation of a mid-IR re- flectance database in the MERTIS-relevant wavelength range from 7-14 μm. This database enables the qualita- tive, but also quantitative interpretation of MERTIS spectra

The HAMO-Based Global Geologic Map of Ceres

The HAMO-Based Global Geologic Map of Cere

Timings of early crustal activity in southern highlands of Mars: Periods of crustal stretching and shortening

Extensional and compressional structures are globally abundant on Mars. Distribution of these structures and their ages constrain the crustal stress state and tectonic evolution of the planet. Here in this paper, we report on our investigation over the distribution of the tectonic structures and timings of the associated stress fields from the Noachis-Sabaea region. Thereafter, we hypothesize possible origins in relation to the internal/external processes through detailed morphostructural mapping. In doing so, we have extracted the absolute model ages of these linear tectonic structures using crater size-frequency distribution measurements, buffered crater counting in particular. The estimated ages indicate that the tectonic structures are younger than the mega impacts events (especially Hellas) and instead they reveal two dominant phases of interior dynamics prevailing on the southern highlands, firstly the extensional phase terminating around 3.8 Ga forming grabens and then compressional phase around 3.5–3.6 Ga producing wrinkle ridges and lobate scarps. These derived absolute model ages of the grabens exhibit the age ca. 100 Ma younger than the previously documented end of the global extensional phase. The following compressional activity corresponds to the peak of global contraction period in Early Hesperian. Therefore, we conclude that the planet wide heat loss mechanism, involving crustal stretching coupled with gravitationally driven relaxation (i.e., lithospheric mobility) resulted in the extensional structures around Late Noachian (around 3.8 Ga). Lately cooling related global contraction generated compressional stress ensuing shortening of the upper crust of the southern highlands at the Early Hesperian period (around 3.5–3.6 Ga). Keywords: Martian dynamics, Southern highlands, Extensional tectonics, Compressional tectonics, Age of structures, Buffer crater countin

In situ fragmentation of lunar blocks and implications for impacts and solar-induced thermal stresses

This study deals with an aspect of blocks observed on many rocky planetary surfaces: in situ fragmentation. Using LROC/NAC images, we characterized the morphology, morphometry and abundance of in-situ fractured blocks observed on the rim of six large impact craters of known emplacement age on the Moon. The relative number of disrupted blocks increases with crater-retention age of surfaces on which blocks are hosted, consistent with fragmentation post-emplacement due to impacts of small meteoroids. The type of break-up morphologies we observe appears to be independent of surface exposure age of the blocks. The inferred flux and size frequency distribution of projectiles responsible for disrupting blocks is consistent with expected lunar impact fluxes. Block fragmentation due to insolation-driven thermal stresses is subordinate to impacts. The possible effects of thermal stresses are evident as meridional cracks, which have preferred orientations in a young block population (~4 Ma), and as loose material (fillet) developing on top of surviving blocks in old populations (>800 Ma)

MAPPING OF PLANETARY SURFACE AGE BASED ON CRATER STATISTICS OBTAINED BY AN AUTOMATIC DETECTION ALGORITHM

The analysis of the impact crater size-frequency distribution (CSFD) is a well-established approach to the determination of the age of planetary surfaces. Classically, estimation of the CSFD is achieved by manual crater counting and size determination in spacecraft images, which, however, becomes very time-consuming for large surface areas and/or high image resolution. With increasing availability of high-resolution (nearly) global image mosaics of planetary surfaces, a variety of automated methods for the detection of craters based on image data and/or topographic data have been developed. In this contribution a template-based crater detection algorithm is used which analyses image data acquired under known illumination conditions. Its results are used to establish the CSFD for the examined area, which is then used to estimate the absolute model age of the surface. The detection threshold of the automatic crater detection algorithm is calibrated based on a region with available manually determined CSFD such that the age inferred from the manual crater counts corresponds to the age inferred from the automatic crater detection results. With this detection threshold, the automatic crater detection algorithm can be applied to a much larger surface region around the calibration area. The proposed age estimation method is demonstrated for a Kaguya Terrain Camera image mosaic of 7.4 m per pixel resolution of the floor region of the lunar crater Tsiolkovsky, which consists of dark and flat mare basalt and has an area of nearly 10,000 km2. The region used for calibration, for which manual crater counts are available, has an area of 100 km2. In order to obtain a spatially resolved age map, CSFDs and surface ages are computed for overlapping quadratic regions of about 4.4 x 4.4 km2 size offset by a step width of 74 m. Our constructed surface age map of the floor of Tsiolkovsky shows age values of typically 3.2-3.3 Ga, while for small regions lower (down to 2.9 Ga) and higher (up to 3.6 Ga) age values can be observed. It is known that CSFD-derived absolute model ages can exhibit variations although the surface has a constant age. However, for four 10-20 km sized regions in the eastern part of the crater floor our map shows age values differing by several hundred Ma from the typical age of the crater floor, where the same regions are also discernible in Clementine UV/VIS color ratio image data probably due to compositional variations, such that the age differences of these four regions may be real

The Global Geologic Map of Ceres Based on Dawn HAMO Observations

The Global Geologic Map of Ceres Based on Dawn HAMO Observation

Mid‐Infrared Spectroscopy of Feldspars From the Bühl Basalt (Northern Hesse, Germany) Formed Under Reducing Conditions as Terrestrial Analogue of Mercury for MERTIS

AbstractThe MErcury Radiometer and Thermal Infrared Spectrometer instrument onboard the BepiColombo spacecraft is designed to investigate Mercury’s surface in the mid‐infrared (mid‐IR). Based on MESSENGER data and modeling, Mercury is thought to be evolved under highly reducing conditions (e.g., McCubbin et al., 2017, https://doi.org/10.1002/2017JE005367; Namur & Charlier, 2017, https://doi.org/10.1038/ngeo2860). The modeling also indicates that Mercury's surface is rich in feldspar. However, it is unknown if reducing conditions during the emplacement of volcanic melts have an influence on the IR properties of feldspars. Therefore, we investigated basaltic samples from the Bühl quarry in northern Hesse, Germany, that evolved under reducing conditions in the mid‐IR and compared the spectra with samples that experienced more oxidizing conditions during their formation. The Bühl samples are feldspar‐rich and contain metallic iron in some areas. Our investigations show that there are no differences between feldspars that formed under different oxidizing conditions. All spectral properties could be explained by well‐known factors that affect mid‐IR spectra of silicates.Plain Language Summary: ESA's and Japan Aerospace Exploration Agency’s spacecraft BepiColombo is equipped, beside other instruments, with a thermal infrared (IR) radiometer and spectrometer called MErcury Radiometer and Thermal Infrared Spectrometer (MERTIS). For the accurate interpretation of the data from the MERTIS instrument, laboratory analog material is necessary. This analog material must fulfill different characteristics, such as different chemical and mineralogical compositions. Another not yet studied property is the availability of oxygen during the formation of the minerals. Depending on how much oxygen is available, different minerals form. However, this is an important feature, because Mercury is thought to have evolved under highly reducing conditions, as opposed to Earth where nearly all material formed significant more oxidizing conditions. One phase that is strongly associated with reducing magma formation conditions is metallic iron. There are only few natural outcrops on Earth, were stronger reducing conditions were present so that metallic iron could be formed. One of these outcrops is the Bühl quarry in northern Hesse, Germany. From there we used different samples to analyze the effect of oxygen availability on mid‐IR spectra of plagioclase feldspars.Key Points: We present infrared spectra of basaltic samples from the Bühl, Hesse, Germany in preparation of the MERTIS experiment Comparison of feldspars formed at different oxygen fugacities showed no spectral differences This is an important result for MERTIS, which will investigate Mercury that formed under reducing conditions DLRhttp://bc-mertis-pi.uni-muenster.de