Comparison Between Copernican-aged Geological Activity On The Moon And Kuiperian-aged Geological Activity On Mercury

The Moon and Mercury are the two similar-sized bodies in the inner Solar System that resemble each other in the appearance. The two bodies share many common physical similarities, for examples both of them have silicate crust and airless surfaces, so that previous studies usually compare similar geological activity on the Moon and Mercury to better understand the basic mechanical rules of the geological activity.The lunar stratigraphic ages are well-constrained with the help of returned samples. The most recent geological epoch on the Moon is the Copernican age that started～800Ma represented by the Copernicus impact. All lunar rayed craters (i.e., craters with impact rays) formed during the Copernican age. On the contrary, the stratigraphic ages of Mercury were referred from those of the Moon and their absolute ages were not well established due to the lack of samples. The Kuiperian age is regarded as the youngest stratigraphic age on Mercury which was named after the Kuiper crater. The Kuiperian age corresponds to the Copernican age on the Moon and previous studies assigned an age of～800Ma for the Kuiperian age. All rayed craters on Mercury formed during the Kuiperian age. It is generally accepted that after～800Ma, endogenic geological activity has ceased on both the Moon and Mercury, and meteor impacts have been the dominant surface geological activity.Recently, the success insertion of the Lunar Reconnaissance Orbiter (LRO) to the orbit about the Moon and the MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) spacecraft to the orbit about Mercury has obtained huge amount of valuable scientific data, which have greatly promoted our understanding of the surface geological evolution of these two bodies. More importantly, analog studies of geologic activity on the Moon and Mercury provide new windows for understanding surface geological evolution across Solar System bodies. Using imagery, topography, gravity and reflectance spectral data returned from various spacecrafts (especially from LRO and MESSENGER), we find that some very recent geological activity has been occurring on both the Moon and Mercury. Among these discoveries, some have changed previous understandings about the thermal evolution and surface geological evolution of both the Moon and Mercury, such the Kuiperian explosive volcanism, activity of crustal volatile on Mercury, and widely-occurred Copernican-aged mass wasting features. Some others have improved our understanding of the basic mechanical rules for each geological activity, for example, analog studies for cooling fractures developed in Copernican-aged impact melt on the Moon and Kuiperian-aged impact melt on Mercury shed light on the development of columnar joints on different planetary bodies.In general, this dissertation targets the Copernican-aged and Kuiperian-aged geological activity on the Moon and Mercury, and also their indications. The abstracts for the discovered recent geological activity are the following:(1) This dissertation updates the absolute age scale for mercurian straitigraphies younger than Calorian. Global rayed craters and morphological Class1craters in the mid-low latitude regions on Mercury are collected. After testing the completeness of the database, the absolute model ages for these crater populations are calculated using the most updated crater counting technique and avoiding potential problems. The results suggest that the Class1crater population on Mercury has an average model age of～1.26Ga and the rayed crater population has an average model age of159Ma. Mercurian craters that have sharp rays yield a model age of40-60Ma.(2) A complex graben system that is composed of dozens of small graben is found in the southeastern continuous ejecta deposits of the Copernicus crater on the Moon. These graben verify that Copernican-aged extensional structures could form on the Moon although its global stress state has been compressional due to continuous global contraction. The graben system is located on a local high-relief area and no adjacent compressional features are visible associated with the graben. This area is located within a free-air and Bouguer gravity anomaly. After analyzing the morphology, geometry, and assemblage pattern of the graben system, we analyze the possible sources for the extensional stresses that formed the graben, especially about the reliability of the recently proposed shallow igneous intrusion model. Model calculation of the igneous intrusion model suggests that the gravity anomaly is not likely to be associated with the graben, and shallow igneous intrusion is not likely, or necessary in explaining the origin for the graben. The graben are most likely to form by a combined effect of activity of blind thrust faults and moonquakes, which is consistent with late-stage global contraction of the Moon.(3) Global-wide mass wasting movements have been happening on the Moon as seen in high-resolution images returned from the LRO mission. Although no surface water or atmosphere exists on the Moon, the mass wasting features highly resemble those on the Earth and some exhibit characteristics of low viscosity. Based on over300examples of various lunar mass wasting features, the morphology, geometry, slope angles, and ages of these features are included in a database. This dissertation finds that mass wasting is an important surface geological process in shaping regional geomorphology on the Moon. Mass wasting tends to decrease slope gradients over time. Nectarine and Pre-Nectarine terrains represent the final stage of surface topographic evolution on the Moon, where only regolith creeps occur. Mass wasting and impact cratering is the most important geological process in determining the thickness of regolith at regional scales, and mass wasting also affects the density of impact craters on slopes.(4) Several Mansurian-aged small-scale smooth plains that have formed from extrusive volcanic activity are found on Mercury. The plain material covers ejecta from adjacent morphological Class1craters, suggesting that extrusive volcanism occurred on Mercury after-1.26Ga. Moreover, some Kuiperian-aged volcanoes and pyroclastic deposits that have formed from explosive volcanism are found on Mercury. The pyroclastic deposits cover impact rays from adjacent craters indicating that melted material caused by partial melting of Mercury’s mantle retains a high content of volatiles during Kuiperian, and the crustal thickness and thermal state of Mercury are still suitable to form surface volcanism. These findings are dramatically different from previous believes. Moreover, normal volcanoes and pyroclastic deposits on Mercury have both a higher reflectance and a larger spectral slope at the UV-VIR wavelengths compared with the global average of the planet. On the contrary, some Kuiperian-aged volcanoes and pyroclastic deposits have reflectances lower than the global average and smaller spectral slopes than the older ones. This suggests that the Kuiperian-aged pyroclastic deposits have different compositions and/or physical properties, which is meaningful to understand the thermal evolutionary history of the interior of Mercury.(5) Previous studies believed that after the end of late heavy bombardment in the inner Solar System (-3.8Ga), compressional tectonic features no longer formed in the crust of Mercury. However, here some giant lobate scarps on Mercury that are dozens of kilometers long are found to have transected both morphological Class1craters and also potentially rayed craters. Some lobate scarps crosscut small Kuiperian-aged craters on Mercury, indicating that compressional features caused by late-stage global contraction of Mercury occurred at Kuiperian. Comparing the relative age between vast areas of smooth plains and giant lobate scarps, it could be certain that due to continuous cooling and global contraction, the thickening of Mercury’s crust have prohibited the extrusion of enormous amounts of melted material from the mantle during1.26-3.8Ga, and vast areas of extrusive volcanism has ceased during that time.(6) Numerous hollows and dark spots that are caused by loss of crustal volatiles are found on Mercury. Their morphology, geometry, global distributions, reflectance spectral, stratigraphic ages, potential compositions, and possible formation mechanisms are discussed. Both hollows and dark spots occur on various terrains on Mercury, except for high-reflectance smooth plains material. Bright haloed hollows are irregular-shaped rimless shallow depressions that are sometimes surrounded by high-reflectance material. The surrounding material has the highest reflectance on Mercury, and high-resolution images reveal that the high-reflectance surroundings are actually composed of numerous small pits that would grow larger to join the parent hollow. Hollows that have both bright interiors and exteriors may be still active and those without high reflectances might have ceased growing. Dark spots are thin surfacial dark material that occurs around hollows. The dark spot material has the lowest reflectance yet identified on the planet. However, not every hollow on Mercury has a surrounding dark spot. Dark spots may have formed from intense outgassing events that feature outgassing velocity exceeds～100m/s. Simultaneously, an embryonic hollow forms with the outgassing event that would steadily grow slowly to form bright haloed hollows. The dark spot material is very unstable on the surface of Mercury and its life time is smaller than that of impact rays on the planet. Therefore, all visible dark spots on Mercury must have formed at late Kuiperian. Materials forming dark spots and hollows are rich in volatiles that might have a high concentration of sulfur. The reason for the dramatically different reflectances between hollows and dark spots might be their different compositions and/or physical properties.(7) Impact melt vastly occur on floors and ejecta blankets of Copernican-aged lunar impact craters and Kuiperian-aged mercurian impact craters. Impact melt deposits on the Moon and Mercury mainly cool by thermal emission and the cooling process can form large enough thermal stresses creating extensional fractures. The morphology, geometry, and assemblage pattern of cooling fractures in impact melt deposits on the Moon and Mercury are studied and compared. The results suggest that the combined effect of depths of impact melt deposits, amounts of entrained solid debris in impact melt, and degree of vertical subsidence formed during the cooling process controls the development of cooling fractures. By comparing the cooling efficiency of impact melt and lava between the Moon and Mercury, it could be ascertain cooling rate caused solely by thermal radiation is not large enough to form columnar joints, and thermal convection and/or conduction is more important. Volatiles may be a necessary element in forming columnar joints on planetary bodies.(8) The median impact velocity of projectiles and surface gravity on the Moon and Mercury are different. We study the morphology and geometry of crater exterior structures for similar-sized fresh complex craters on the Moon and Mercury (including continuous ejecta deposits and continuous secondaries facies), the controlling factors during the impact excavation stage is analyzed using impact cratering scaling laws and comparative studies. This dissertation confirms a previous finding that gravity is a dominant controlling factor during the impact excavation stage of forming complex craters. We also find that impact velocity plays an equivalent role. Some craters on Mercury have more circular and isolated secondaries on the continuous secondaries facies than typical lunar secondaries. The most possible reason is that at some layers and positions on Mercury, the crustal material has special properties that have affected the impact excavation stage causing larger ejection angles and more circular secondaries. These target properties might be associated with low-reflectance material that may be caused by a higher content of volatiles compared with the average of the planet.(9) Central pits in impact craters on planetary bodies are supposed to be caused by the effect of target volatiles on cratering processes. Previous studies suggested that central pits would not form in impact craters on the Moon and Mercury due to the assumed low concentration of crustal volatiles. Here some impact craters on the Moon and Mercury are found to have both floor pits and summit pits (occur on central peaks) that are morphologically similar with those on Mars and icy satellites. Based on the database for the morphology, geometry, global distributions, and age of the central pit craters on the Moon and Mercury, central pit craters on different planetary bodies are compared. It is suggested that the central pits in impact craters on the Moon and Mercury do not need crustal volatiles to form, and forming central pits are related to an unknown mechanical process related to the cratering event.